WORLD METEOROLOGICAL ORGANIZATION

TECHNICAL NOTE No. 2

METHODS OF OABSERVATION AT SEA

PART I – SEA SURFACE TEMPERATURE

WMO-No. 26. TP. 8

Secretariat of the World Meteorological Organization – Geneva – Switzerland :-----~--- WORLD METEOROLOGICAL ORGANIZATION

TECHNICAL NOTE N° 2 METHODS OF OBSERVATION AT SEA

PART I• SEA SURFACE TEMPERATURE

PRICE: Sw. fT. 1.-

I WMO· N° 26. TP. 8 I

Secretariat of the World Meteorological Organization • Geneva • Switzerland 1954 METHODS OF:....Q~S:E.RVATION AT SEA. PART I - ~EA SURFACE TEMPERATURE

TABLE OF CONTENTS

Summary (French)

~ ~ 1.. Introduct ion_.._ n ••' ••" •••" , - ; _ _ •••;;_ ,..; _ ••; ,...... 1

2. Observations made by Dr. C.F. Brooks during trans-

atlantic .voyage s '- .._~";.,, .....~...~...... _~"'._. "'.wJ.m.....,"_...... __------:. m,.."''''~_._:....H_.':~ ....~...''': 2

). The President of the Commission for Maritime Meteo­ rology discusses bucket versus intake methods in reply .~" ~ to Dr .. Brooks _._.._._..w _.__ '" _ _._· _.__ m _ ,., _. • __••__.._ 4

4. Dr. Brooks I presentation to CIMO of analyses of observ- atiolliS of sea surface temperatur"e at different depths ...... -..... 5

5.1 United States of America _ :__ "'" __.._ .._ .-.__. 15 5~2 .Norway ._ , __ _ _ _.._ _ _._ ~._ .._ _ ~..__ __.._....;,.__. 16 5.3 Netherlands "._ _ _.."'."' ~ _~_.~ .._ ,.._ ...- _ _ .,_._.., 18 5.4 5.5 METHODES D.'OBSERVATION EN MER lere partie TEMPERATURE DE L'EAU A LA SURFACE DE LA MER

Resume

Cette note technique expose d'une ragan assez detaillee l'histo~ique et la situation actuelle de la question longuement controversee concernant la meilleure methode d'observer la tomperature. de Ileana la surface de la mer, fondee sur des avis d1experts et SlIT les recherches entreprises dans ce domaine.

D'abord M~ C,-,F .. Brooks du Blue Hill Observatory (Mass.),Etats-Unis, expose les avantages et les desavantages de la methode du IIseault , par oppo- sition a ceux de la methode du IT c·ondenseurn • ------,-----_----- L'etude 'passe ensuite en revue les travaux effectues par divers- pays en vue~e-dlierminer les merites respectifs de chacune des deux methodes, et---analyse'~un nombre considerable de donnees d I observation. Elle comporte des tableaux indiquant les'frequences et les valeurs moyennes des observa­ tions relatives aux differences de temperature entre la surface etdiverses profondeurs de la couche superieure de l'ocean~ Les observations comprennent notamment celles effectllees a ltaide'des methodes du seau et du condenseur, tant par Ie personnel des l'iavires meteorologiques que par des observateurs benevoles a bord de navires marchands. Jucune.·.·c.onclusion reellement definitive n la ete tiree de touts cette -"'....'4l'~..,-...,'<·-,·,·"",·,~.,~·,',"-·,.:"'_·~.c,,"_..,,,·~~~~~.~'"..'''''".-''''-~ ...... _... ,.--ll\-'- -" : . ..'. - • , ,, documentation quant a la methode qui se preteraitle mieux a un usage gene- ralise. Chacune des deux methodes presente en effet des avantages et des­ desavantages qui dependent dans une certaine mesure de la latitude, ainsi que des conditions meteorologiques et de la'saison danslesquelles'les obser­ vations sont faites~ 1

METHODS _OF OBSERVATION AT SEA

PART I. = SEA SURFACE TEMPERATURE

1. Introduction One of the major 'subjects which was discussed at the first session of both the Commission for Maritime Meteorology and the Commission for Instru­ ments and Methods of Observation was the making of weather observations aboard ship at sea a

These observations may be divided broadly as follows: 1. Sea surfac'e temperature 2~ Humidity including observations of both dry and wet bulb thermo­ meters 3. Barometric pressure 4. Cloud height 5. vJind 6. Rainfall 7. Visibility

The making of weather observations aboard ship at sea involves ~any difficulties which are not encountered by observers at land stations. ·These difficulties mainly arise from the following circumstances:

'(I) A vessel at sea is usually a heat source in its environment and the air in the immediate proximity of the ship is therefore un~ representative of the air over the open ocean;

(2) The motion of the ship creates eddies and currents which make ins­ trumental observations of such elements as wind and rainfall unre­ liable~

(3) The structure of a ship is such that it 'is impractical to install certain ob'servational aids such as cloud height projectors, ahd extremely difficult to find a position where the exposure for the usual instruments is suitable;

(4) In addition it must be remembered that all these observations have to be made by the Ships Officers on a voluntary basiso Although all such officers have, as a basic part of their navigational 2

training, some meteorological knowledge, they are nevertheless not professional meteorologists and the meteorological observations are normally made during their period of duty on the bridge of the ship and are additional to their normal ships duties.

It was therefore considered the task of the two Commissions mentioned to study the question and make recommendations as to the best methods of observation so that marine" observations might be niadeas nearly comparable with land observations as possible under the relatively difficult circum­ stances prevailing at sea.

In order to provide a basis for studying the matter the President of the two Commissions despatched circular letters with the object of collect­ ing information concerning the procedures in operation and experience gained by various Meteorological Services, together with new advice and ideas for the future.

The subject of this technical note is confined to method of observ­ ations of sea surface temperature since the material which has accumulated on it is fairly lengthy. Its content is derived from the replies received to the circular letters and from working papers submitted by various count­ ries in support of the relevant agenda item of the first sessions of the Commission for Maritime Meteorology and the Commission for Instruments and Methods of Observation.

An other technical note will be published shortly on methods of observ­ ation of the remaining meteorological elements listed above.

2. QQ§ervations made by Dr. C.F. Brooks during transatlantic voyages

Dr. C.F. Brooks of Blue Hill Observatory offered some interesting comments on methods of observation of various meteorological elements at sea based" on personal experience of voyages across the Atlantic. These comments were reproduced for the First Session of the Comroissionfor Mar­ itime Meteorology. With regard to the measurement of sea surface temperature he states: " Resolution XXII (ClMO, Toronto 1947) recommended that:

(i) Measurements of sea surface temperature be obtained by exposing the element of a mercury-in-steel thermograph in the condenser intake pipe of the sh~P9 where these or other thermographs are impracticable a second" method of installation of an ordinary thermometer in the condenser intake pipe at a convenient reading height is recommended;

(ii) When measurements by "buckets" are made, they be made from the ste~n of the ship in order to minimize errors due to evaporation; " 3·

(iii) '. Sea~waterthermometers :and thermographs' be gradua.ted to 1°F or ';0. 50 q. ,and that they be ohecked against the ice-point' or a stand­ .ard:thermome-ter a.t .least.' once : e~),'ch year. Not e : The abov'e,' resoluti'on:was' Iater"withdrawn by the' President. of the Commission for Instruments and Methods of Observation for further ·stu,dy.

MY recent experiences. in:20 days at sea on'Netherlands and British weather reporting merchant vessels would lead me to make some changes in the­ se re·commendations, namely, that the temperature be obtained by bucket -on all occasions. when the sun is even dLnly shining (or has been within the pa.st 4 hours) and the wind is Beaufort force 3 or less, that the bucket used be. an insulated one., and that the ·the::....mometer used have no· reservoir or hea.vy ~e~al; Gase (which in one case raised.the observed temperature to 4°C above the actual temperature of the sample (on·Volendam). With the use of an insul­ ated bucket and of intake temperatures in windy weather, there would be rio need, fO;l:"' .taking. bucket samples off ·the' stern (which is too long a. walk for qua~~~r..;.masters to take .from the bridge whenever a sea-temperature is wanted) •

. ' .'J?he ·intake temperatures used should be only those' that are .read 'at the· ~-',a ·'r.equest'is made by the Bridge•.The main 'intake is usually near t~e.: '..engine.er 's phpne, and the .. thermometer inserted i.n it·(a well in it} can'be read and reported back while the Bridge wa2ts on the phone. The intake tem~: erature .reported will almost always be slightly too high, 'but this error is, in the direction of the 'usual deviation of surface from. intake temperature when, ·there i·s a d,ifference. The reading of the intake. is too :high because (1)' the therm0meter is not immersed in the well to the top of the mercurY"· . in th~ stem (for which point it is calibrated), (2) there is heating of ~he" well.by conduction from the warm air in the engine-room, '(3}':tliere is heat­ ing.of ,t~eintake wt,lter before reaching the· well, and (4)'the' engineer usually withdraws the thermometer to read it, thereby subjecting it to the . heat 'of the. engine-room. (1) could be eliminated by calibrating' therrnomet'ers for intake use.only by submerging ..their bulbs and a.n· inch of stem in the'·· ca~ibr~tion bath. (2) and (3) are· 'probably' minor, especially (3) .··(2)coulCl be:reduc~d by the: use of a well about 2t in. deep. (4) cotildbe reduced 'and nearly eiiminate-d,by using a thermometer that is slow to 'respond. The import­ ance of hav.ing the engineer who is asked make::thereading of-temperature' him­ self was demonstrated very clearly on the MV' "Georgicu • The engineer 'was'" supposed to read the thermometer at frequent intervals, once an hour, and to write the temperature on a large blackboard outside his room, where the engineer who might answer the phone could read it at any time. I found: (a) that the intake temperature was about 10~ t()ohigh, .owing .to distanqe from the' sea-and "passage of the water- through,'t~e"'pump·hefore reacl1ipg' the thermometer• Also, the'pipe was rather'sniall, . which increased the"'6ha:nce of heating. (b) the engineer did not read the thermometer at regular. times al­ ways,· and when busy on some special task might omit an hourly reading. (c) the .calls from the Bridge might come just after a reading' one hour.~ and. .just befor'e a new reading the next, which would re'sult.in- a repetition at the 2'nd' ··hour of the temperature report at the' lst~ (d) the engineer might forget· . to change the' temperature on the blackboard; so the same temperature'would be repeated to the Bridge hour after hour. One day the reports were n45°F", 4 - .

while my observations with an·Ashford bucket showed variations to a few degrees below and above this temperature. When the ship rounded Nantucket Shoals and the sea temperature by Ashford bucket rose from 55°F to 63°F, the reports from the engine-room came through'at 57°F, 57°F. Yet with. that change in sea temperature the thick weather had changed to good visibility. The officer on the bridge sent a radiogram that the sea temperature was 57°F (based on the report from the engine-room), in spite of my telling 'him as he was'writing the message that the Ashford bucket showed. 63°F.

I. am afraid that the desire of the IMO to' have sea temperatures to the nearest O.2°F cannot be realized, I feel convinced that the best one can expect from engineers reading intake thermometers and quarter-masters re8rd­ ing bucket thermometers is O.5°F. If the engineers should be asked to read the tenths and give them over ~he phone, the chance of error in transmission would be greatly increased, by getting units and tenths reversed.

I might add here that the Ashford bucket is very difficult to get a catch ·with. When the bucket is let down into the sea, water will not flow in against the strong spring cover until the bucket has begun to trail a bit, and by then it is about ready to break out· of the sea. I have made a throw without getting a drop into the bucket. The bucket cannot be "trailed" fro:m. a fast ship; its bottom bounces along on the water• .3 or 4 dips usually would not bring up more than h~lf a bucket fUll, and this woUld be only half of the· volume of the inner metal container. If the mouth of the·bucket were widened, or equipped with a funnel, and if the spring to the lid were made only strong enough to keep it lightly shut, it would help. The inner con­ tainer' should be larger. Also, there should be no leak holes around its top (through which much of a sample can be lost). If the bottom were fitted with a valve held shut with a spring which would open only when there was more pressure than the weight of a bucket full, the desideratum of flushing the bucket would be met, where the present design fails to provide for this. i.e., the bucket when pulled through the water would have water flowing through it, but as soon as it left the water the last water to enter the bucket would be retained. The original design provided for a good flow, but the. change, to having the outer layer (between canvas and can) empty, virt­ ually eliminated this feature. The bucket has good insulating ability, changing only half way from sample to wet bulb temperature when exposed on a wind swept deck for half an hour. .

3. The President of the Commission for_Ma~itime Meteorology d!~cusses bucket versus intake methods in reply to Dr. Brooks

I am interested, and indeed, glad to note Dr. Brooks' recommendation. that an insulated bucket should be used in preference to the intake method . ¥nder certain conditions, and also his realization that taking samples of . sea water from aft is not really practicable nor desirable in a merchant ship. I gather, however, that he still recommends the intake method when the sea is rough. ~.. 5

, My personal·impr.ess.ion is, from practical.experience'and .·from rather exhaustive enquiries, that if an insulated bucket is used th~n there is no need, in ~ny circumstances, 1n the av~rageship .(unless ~he bucket is lost overboard)'to use ,intake readings. The only exception is in large, fast ships in which the use of a bucket for th~$.purpose is very difficult,and even dangerous, and in such ships the intake method would have to be used. Dr. Brooks seems to bear out my views about the liability to inaccuracy of intake readings, when he .quotes his experience aboard the ships he was in recently. It seems that the precautions which are .needed to ensure accurate readings, due to various practical reasons, are formidable, and may in cert­ ain cases give rise to a 'measure of friction between the Deck and Engine­ room departments. My personal recommendation is that an insulated bucket should be used 1n all cases except in large, fast ships in which the.intake method should be used, but this is only a personal opinion, and I :feel that before any decis10n could be made about such a point the views of the CMM 'should be sought.

To implement any method of using intake reaaings it would be necessary to carefully check the intake "thermometer and have a record of its siting (which 'varies from·ship to' ship) and the depth of the intake (.which varies from voyage to voyage), as well as to issue special instructions about the making of the observations. In some cases, the existing intake thermometer will have to be replaced and' this will involve expense. The divided re'spon­ sibility between Deck and Engine-room departments makes further complic­ ations. All these are reasons why I feel intake readings should only be' used when insulated bucket reading~ are clearly ~mpracticable.

I note that ·:Dr.· Brooks mentions : liThe engineer usually withdravrs the thermometer (from the intake) to read it". I suggest this is a very unusual practice because the intake thermometer is usually screwed firmly into the intake and is read in :situ. ·In other' cases, the temperature is taken by hold- ·ing·a thermometer'under a tap from the intake•.

4.~ !2i.., BrQok.§.~sentat1on to the'. Coinmission for Instruments and Methods of Oh~vat~on of analvsesof observations or sea intake tem~rature at different dsrp.t.u§.· . The summary of the situation by the President of the Commission for Instruments and Methods of Observation reads: .

"Resolution XXII '(c'IMO 'Toronto; 1947), made representation's 'fortaking sea surface water temperatures, but the resolution was withdrawn for further study afterdiscussions.which took place at the Plenary S~ssion of the Conference of Directors. The subject was also considered by the Commission.tor ~ritim.e .Meteorology, at its session in London,. in 1952, and will be considered ·further to find out if it is possible now to make definite rec6IDme~dations in regard to the method of taking these temperatures".. ~ . 6

Resolution XXII (CIMO Toronto, 1947) was made :on the basis"of ample experience that bucket temperatures were often appreciably "in error, that scientific comparisons of bucket and intake temperatures carefully obtained. revealed but an insignificant difference in temperature between surface and intake levels (except in sunny, quiet weather)~ and that engineer-read intake temperatures were usually less accurate than thermograph records of such temperatures.

The Commission for Maritime Meteorology, at its, session in Toronto, 1947, produced several resolutions on the question: Resolution XXIII said they did not believe that sufficient data were available to justify the specific recommendations in Resolution XXII (CIMO Toronto, 1947) and urged further consideration; Resolution XXXVII recommended detailed 'investigation of 'the temperature gradient in the sea. on at least one North Atlantic weather ship and in the tropics; Resolution XXXVIII rec­ ommended that services supply their selected merchant ships with small,' responsive, easily read, robust thermometers, -to use in rapid drying, insul­ ated, 4-6 litre capacity light and robust buckets (in place of unsatisfacto­ ry ordinary canvas bucket),' or condenser intake thermometers graduated to 1°F 'or 0.5°C, and Resolution XXXIX recommended that readings be made to O.2°F or O"~loC, to encourage accuracy.

The last two resolutions were adopted by the Corifarence of Directors, Washington 1947, as Resolutions 52 and 53 (CD Washington 1947) and the Confer­ ence "noted" Resolution XXXVII (CMM Toronto, 1947). The contents of the resolutions and additional points arising from the analysis which lead to the CMM resolutions provided the background for ,the Sea,temperature section of the, Guide.

In an exchange of letters between the Presiderit of CIMO, the President of CIMO's Sub-Commission on Station Instrumentation and Exposure, and Dr.H.U. 'Sverdrup, in December 1948 to Febru~ry 1949, Dr. Sverdrup wrote:

"Experience shows that the use of intake temperature is not satisfactory because in many situations there may be a difference of several degrees between them and at the surface itself. For this reason, the Bathyther­ mograph Section here (Scripps Institution of Oceanography) is recommend- ,ing to the Division of Oceanography of the Hydrographic"Office, that the weather ships be instructed to determine the sea surface temperat~ ures from bucket samplesn •

In February 1949, a 39 page manuscript: "The measurement of sea surface temperature for meteorological purposes: present inadequacies, improvements in progress, and recommendations for immediate and ultimate improvements in the accuracy of't1:lese reports", by the President of CIMO's Sub-Commission on Station Instrumentation and Exposure,was sent to the Presidents of CIMO and C:MM and to the USA and French members 'of GIMO's Sub-Commission on Stat­ ion Instrumentation and Exposure, along with copies of the above~mentioned correspondence. This paper included 6 pages of annotated bibliography and 11 pages of the full texts of all CIMO and CMM documents and resolutions on sea temperature". 7·

Wher~~to_get_~§.Presentatiy&"sal;l1I2J-~. ·~s the first pr,oblem;. ;ho~. to get it'fbllows'c :For i:rtImediate synoptic purposes, what is wanted.l~:·b.ottJ, t~e temperature'of the actual 'surface film and the general'temperatureof the wind~sii~redor potentially wind-stirred surface layer. The former is indic~t-· ive ':6f- the current tendency of the s~a to heat or· cool ~n,d. h~idify: the·' low­ est layer of the atmosphere; the latter is the ::lame, but what maY, be. counted on, despite even considerable sea disturbance, to prevail for some time. For cl-imaticpur.poses~' the surface temperature -is required..

\"1 . The temperature of the surface fi1m is obtainable, apparently, .only<"rith a radiation thermometerl ) Since, howe~rer, there is little p~ospect that sU~h apparatus will come 'into general use in the immediate fu:ture~ .ot~ermeans by which the surface temperat'L1X'e can be Fl,pproximated mus:t be cOl1s-id,er~d• . .. . When the surface is turbulent ·with breaking waves, as it aJjnost; .;invar... iably'is in latitudes where synoptic forecasting is important, th~tt'E! ':Ls.no . particl11a:r problem; any· sample fr.om the top 5 metres or so will,'have, assent':"' ia1'ly' the· same temperature • It. is when the wirld is insuffic;~e.nt/tq:·p~.yv,ert~·.' thermal stratification near the surface that sp~.cialcare must b~ ,:ta} ' ..'

Table I shows that in higher' middle latitll.des the sU~face·'·terhpefatJi.e .:" in spring and early summer averages from 0 to O~3°F warmer than that at 5 m. In the warm season (May-September) it is up to OQ6°F the warmer, though in August at one station there was already no difference, and in September the surface had. become. 002°r the colder... All, hO\ifev.er, were not. far.from,qoasts. In th€;) cold' sea'son j again, only a light-ship st'atiOl'l} the ..surface :avefaged f:r:onl,O.4°Fthe· coider ..in October';February to 0.., 2°F 'the' coide;!:'. in.·Mafch,..- In·. the 'lower.middle latitudes,' howeyer,· represented by qbservations'.·IO:':'20,·~le.s off the coast of southern California, the surface averaged in sUmmer about 102°F the warmer and for the year as a whole, ·O.6°F. Since this is''in a

1)·----- See references at end of publication~ coastal zone of upwelling cold water, however, it probably represents an excess not likely to be found far out to sea. There'are no data from the trop­ ics. It is to be noted that even in the extreme seasons and locations the difference is only of the order of 1°F, ,but that in the open ocean and in, higher middle latitudes the difference averages only O.to 0.3°F in the sun­ nier half year.

While these values seem'to indicate that, except' near coasts, sea temp-, eratures at a depth of 5 m may be accepted as a satisfactory substitute for actual surface temperatures, the frequencies of differences of various magnit­ udes are of niore 'interest to'the forecaster. These are shown in Table 2, based on the 296 observations in higher middle latitudes within the period April to July. The striking'fact is that, while differences of 0.2°F or more occurr~d in 21% of the cases, those of Q.5°F or more occurred in only 1% at 5 m depth.'Even at 10 m a difference of 0~5°F or more occurred in only 3% of the cases. Differences as great as 1°F were not observed at depths of 1 or 2 m,onlY once at 5 m, and only twice at 10 m. ,The extremes observed in the whole body of nearly 300 observations,were -0~5, + 0.8, + le2 and + 2.0oF at 1,2, 5 and 10 m, respectively.

T~ble 3, of the USA series of 151 sets, shows the distribution of differ­ ences according to Beaufort wind force. At 5 m depth there was U£ case of as much asO.5°F difference when the wind was Force 4 or more, and at 10 m only one .such:: The maximum differ~nce of + 1 .. 2°F at 5 m and + 2 .. 0oF at 10 m occurred at the end of a day (May 30) having wind of only 4.;.6 knots (Force 2) and 1 to 2 tenths of sky cover for at least 8 hours .. The infrequency of large or medium solar radiatiOn with light winds (Beaufort 1 and 2),- only 3% of the observations - seems to indicate that the use of intake samples in all winds and weather should serve in these higher middle latitudes, as, indeed, is the practice on U.S. Weather Ships, but that when the wind is Beaufort 1 or 2'and the solar radiation considerable a layering correction of + O.loF per hour of more or less continuous large solar radiation (rather clear skies after 09h and until 16h) might be applied. For May 30 such a correction applied to the temperatures at 5 m would have yielded IIsurface rr temperatures exactly right at 10h, .Oo4°F too high at 14h and 0.5°F too low at ISh, instead of the 001, 0.1 and 1.2°F, 'as observed~

Since, with the exception of the rather special case of southern Calif­ ornia, the above cited definitive. observations of the thermal gradient in the top 10 metres of the sea were made only ,in the higher middle latitudes it seems desirable to look into less satisfactory types of comparisons to ' obtain some knowledge of thermal gradients in lower latitudes. --~_.._---_.__.- ..- ..--...------.-----.------.---. ------.------"1 TABLE I- SUMMARY OF DIRECT MEASUREM:ENTS OF SEA TEMPERATURE AT VARIOUS DEPTHS (oF) ,

I SOURCE" INSTRUMENT LAT. LONG. SEASON Iffi .CASES SURFACE TEMPERATURE MINUS TEMP. AT DEPTH I ~-~_. 1M 5FT 2M 10FT _ 5M. 20FT 10M. 30FIJ.'1 1-'- ...... -.. . .----.--"-.-'.-....,,_._..:...;.;,...... --...-.---- Spring and Early Summer (April - July) .. '. '. ".. United Ba"thytherino-' 59.0N . 19 .OW . ·My.lb- Var. 98 - .001 -.002 -.02 -.09 K~ngdoml graph Je.1 1 52 3 Richter Reversing 55.5N 7.8E Ap.-Jy 08h 1,i330 +.18 +111'"{ thermometers 121- 135 4 U.S.A. Bathyther- 52.7N 35.5W Ap.1':"" Yare 151 +.01 +.02 .... +.06 +.• 15 mograph Jy.31 '49 Canada .' Thermistors 50.0N" 145W Je .1':"28 7 +.17 ' +.44 +.11 +.17 152 j''\D France5 Reversing 45.0N .16.0W, Ap.17­ Var. 00 +.18 +.32 +.• 29 . +.38 11 thermometers My.8'52 6 Smith Reversing 46~ 50W Ap .2p... 24 . +.04 thermometers approx.approx. Je.29'26 7 I MoEwen Reversin'g 10-20 miles Mar... +.5 or thermometers off S.Calif. +.7: ' I Notes:: 1 Compiled by C.F.·Brooks from Data in CMM r/noc. 55, London, 1952 2 ..C9~pi:l~,Q, .bY... O.F. ,Brooks from data in CMM r/noc. 88, London, 1952 3 J. Richter~: .Ann .·d;.Met·eorol'. v.4, 1951, PI>. 375-379 4 Compiled by C.F. Brooks from data obtain€~dby the U.S. Coast Guard, for,U.S~ ..N.•Jlydrog•. Office, ., 5 Compiled by C.FA Brooks from· data obtain€~d'onFregate "Le'Brix"', M. LeRoux, Capt. courtesy L.V.P. Revillon .. Obs. made frorq.,4.<;>ry 2 m to leeward of ship. Each series took about 1 hr... 6 Data supplied by·"Lt. Comdr. ('now Rear 'Adm. Ret.') Edw.H. Smith, from Ice' Patrol ships. Discussed by C.F. Brooks in Mon. Weather Rev., (USA) v. 54, PP. 252-253, 1926 . 7 G.F. McEwen, Jour. Washington Academy of Sciences, v. 18, 1928, p. 545 ._------.------...--.------.--•.------.....,.-- l I TABLE I- SUMMARY OF DIRECT MEASUREMENTS OF SEA TEMPERATURE AT VARIOUS DEPTHS (oF) (continued) ISOURCE INSTRUMENT LAT. LONG. SEASON HR CASES SURFACE TEMPERATURE MINUS TEMP. AT DEPTH 1M_.-.-2!'~,." ~OFT tt1~~~~.....,.....-.-~,~-~_..______...___..._____.....,_.__•• ._£.M 2!L,_?OF:T 10M 30FT Warm Season (May to September) Richter3 Reversing 55·5N 7.8E Aug.- OBh 465 o +.20 thermometers Sept. OBh 400 ~.23 ~.18 'Harvey8 .Reversing Wn.Eng1ish May- - 38 +.58 thermometers Channel Sept. Bigelow9 Reversing Gulf of ¥ay- - 4 +.60 (at 5 and 10 m) thermometers Maine Aug.

McEwen7 Reversing 10-20 miles Summer +1.1 thermometers off S.Calif. +1.4 I b Cold season (October - March) Richter3 Reversing 55.5N 7.BE Oct.- OBh 2,265 -.35 -.41 thermometers Feb ... Mar. aBh 465 - .16 -.27

Notes~

3 J." Richter, Ann.d.Meteorol. v.4, 1951, pp. 375-379 7 G.F.McEwen, Jour. Washington Academy of Sciences, v. 18, 1928, p. 545 8 H.W. Harvey, Jour. of the Marine Biological Assn. of the U.K., v. 13, pp. 683-681, 1925~ Summary in Mon. Weather Rev., ibid., p. 252 9 H.B.,Bige1ow supplied these data. They are discussed in Mon. Weather Rev., ibid. TAB~ - FREQUENCY OF DIFFERENCES OF SURFACE TEMPERATURE MINlIS THOSE AT VARIOUS DEPTHS (PER CENT) I I ::>. ':-O.loF >- ':-O.2°F :::> '::0 .. 5°F ::> .:t"1.0 oF Max. (oF)

1 m 2 m 5 m 10 In. 1 ill 2 ill 5 m 10,m 1 ill 2 m 5 m 10 m 1 m 2 m 5 m 10 ill 1 m 2 m5 m 10 m

United Kingdom - Bathythermograph - 59.0N 19 .. ow - May la-June 1, 1952 - Various hrsl>- 98 cases

1 8 45 76 o 1 6 17 o 001 o 0 b 0 -.1-.2 +03 +.5

U. S. A. Bathythermograph - 52.7N 35~5W - April-July 31, 1949 - Var. hrs. - 151 cases

18 28 54 70 ~ 1 8 20 40 o 0 2 5 o 0 <:1 1 +.3 +.4 +1.2 +2.0 l:3 France Reversing thermometer - 45.0N 16.. OW - April 17-May 6, 1952 - Var. hrs. - ,40 ,cases.

68 75" 12 82 68 70 60 75 2 0 0' 0 o ° 0 0 .... 5 +.,4 +.4 +.4

Canada - Thermistors - 50N l45W -. June I-June 28, 1952 - Yare hrs .. - 7 cases (Depths 5,10,20,30 ft)

86 100 57 86 57 100 43 72 0 29 14 14 0 0 0 0 +.3 +.6 +.8 +.5

All four sets combined - 45-59N 16-145W~. April I-July 31 - Yare hrs. - 296 cases

21 30 54 75 11 16 21 38 1 1 1 3 0 0 1 1 -.5 +.8 +1~2 +2.0 ~------.~----_. FREQUENCY OF DIFFERENCES OF SURFACE TEMPERATURE MINUS THOSE AT VARIOUS DEPTHS AT DIFFERENT t TABLE 3 I FORCES BEAUFORT, AT USA WEATHER SHIP STATION C, 52.7N 35.5W, APRIL 1 - JULY 31, 1949g I 151 SETS OF OBSERVATIONS (PER CENT)

BEAU- I! ::=> +. '> + o I FORT GASES - -O.l°F >- ':0.,2°F ~ ':0.5°F - --1.0 F Max 0 (oF)

10m 1m 2m 10m 1m 2m 5m 10m 1m 2m 5m 10m 1m 2m 5m 10m 1m 2m 5m 5m ----_._--_._.__.----_._ ....,_.....-- I ...... ---....-. _----_--_._._. ------~. ------,"------I 1 i 6 . 67 67 84 0 16 67 ' 67 84 0 0 16 33 0 0 0 0 +.3 +.4 +.5 +.6

2 10 30 40 90 90 0 10 30 70 0 0 10 10 0 0 10 10 +.1 -- •2 +1 .. 2 +2.0

+ + 3 28 29 29 64 89 a 14 29 57 0 0 4 7 0 0 0 4 -.1 -.2 +.5 +1.2 t-J iQ I + + + 4 39 15 23 54 62 0 8 20 39 0 0 0 0 0 0 0 0 -.1 -.2 -.2 +.4

+ 5-7 68 9 27 41 62 0 0 7 25 0 0 0 3 0 Q 0 0 +.1 -.1 ..... 3 +.6 - 13

The following are 'sUmmarized from C.F. Brooks' extensive treatment of 19520

Two examples of extreme surface heating'may be cited. At latitude 15° on a bright sunny day, with mostly light airs, a surface temperature by buck­ et ,dipped several feet from the side of the ship,. a temperature of 3.8°F ' higher than that at the intake,at 6 m, was observed by C.F 0 Brooks. At lat­ itude 41°, perhaps 20miles~ff the coast of Long Iso, N.Y. (USA), ona sunny (high, thin overcast) July day with wind of Beaufort 1, C.F. Brooks ob­ served a surface temperat~re .(by tnsulated bucket) at least 2.00 F and prob­ ably 2.6°F higher than the intake temperat:ure at about 6 m.

Several observations of bucket minus intake temperatures at different wind speeds'in sunny weather May 31 - June 3 on the western North Atlantic between 42° and 46°N by C.F. Brooks} showed average values of +06 +.2, and QOF with winds of Beaufort 1 and 2, 3 and 4-6:for 6,5 and 3 cases, respective-

~.' .

The daily range of surface temperature may be used as rough indication of the afternoon gradient from the surface to a depth of a few metre~~ In the. tropics it averages ,0.5 to Oo7°F, but on particular days it depends on sunshine and wind:

RANGE OF DIURNAL VARIATION OF SURFACE TEMPERATURE IN THE TROPICS' (AFTER SCHOTT) ,

1iind and Cloudiness T.~mperature Range (oF) Average Maximum Minimum

Moderate to fresh breeze Sky overcast 1.0 0.0 sky clear 2.0 0.5

C~lm or very light breeze ' Sky overcast 1.7 1.0 1.0 Sky cle'ar 2.9 304 2.2 14 -

From all the above and from other numerous studi.es, it appe'ars that:

(1) When the wind is Beaufort 4 ,or more the sea temperature at the surface and at 5 m depth are prac~ically identical at any season in any lat­ itude;

(2) When winds are light, Beaufort 0-3, and app:reciable solar radiation reaches the surface, in clear weather or with scattered or broken clouds, or even with a thin overcast, the surface temperature on the open sea becomes appreciably warmer ,than at 5 m, reaching a maximum in a calm or wind of BE?au­ fort 1 of about 4°F in the tropics and sub-tropics, 3°F in lower middle lat­ itudes, and 2°F in higher middle latitudes 9 and, therefore, that

(3) The use of a bucket or of an intake near the surface in quiet s~nny weather is necessary if a fair sampling of the surface temperature is to be obtained within the ,tolerance desired for meteorologic~l purposes, say 0.5°F, on every occasion. 'However, since such a combination of sunshine arid' quiet-' ness is so rare in the higher middle latitudes (only 4 cases 'in tHe 296 defin­ itive determinations) even in summer, the gain from using bucket observations seems hardly worth the extra expense for the eqUipment and the extra labour in making an observation. In the 151 sets of bathythermograph observations at lat. 52.7°N, considerable sunshine occurred in 14 of the cases, and medium in 32%, but most of these were with winds of Beaufort 3 or higher (which occurred 91% of the time). and so the solar radiation was impotent for produc­ ing a layer structure. In the lower middle latitudes and in the tropics, on the other hand, '-it would seem desirable to have the sea temperature observ­ ations (on vessels of less than 20 knots 'speed) made by bucket on all occas­ ions when the sun is even dimly shining (or had been within the past 4 hours) and the wind is Beaufort 3 or lesS G

In other words, though the ideal seems to call for sampling at the surf­ ace in quiet weather, we can get by with sampling at intake dept~s in,higher middle latitudes. Nevertheless, in the tropics and sUb-tropics, and in the more or less enclosed seas or coastal waters in middle or even high latitudes, bucket observations will be required in sunny, quiet weather if sea-§Y£face temper~tuIes are to be determined with the desired accuracy.

How to obtain the temperature of the water sample accurately involves getting a proper sample, minimizing the change in;temperature before it reaches the thermometer, and getting an accurate reading. It is of the great~ est importance that the sample be measured at a time close to that when the air temperature is taken. The "Guide to Meteorological Instrument and Observ­ ing Practicett details equipment and precautions. The requirement that the temperature be read to 0.2°F seems unrealistic, in view not only'of the numer­ misinherent errors in obtaining the temperature of the nsurfacelt but also of the difficulty of reading a thermometer as closely under ordinary observing conditions. Anything which del~ys getting a reading or which makes it difficult to remember what it was will increase the error of an observation or its rec­ ording or reporting. It would seem quite sufficient to have the temperature read in whole degrees Fahrenheit or 0.5 degree Celsius. 15

The Guide favors bucket construction which will 'reduce the 'error aris­ ing from a change in the temperature of the sample. When, however, the 'bucket is doubled or otherwise insulated the result is to increase the 'heat capac­ ity and' reduce the "Tater capac'ity. If sea temperatures are to be obtained usually from the intake and only in quiet weather by bucket, then the need for providing' against excessive wind effects on the temperature of the sample is somewhat reduced" If, however, the bucket still has fro.ill 4-6 quarts J capacity, some insulation is apparentlyadvantageous~When the initial temp­ erature ofa British Meteorological Office Mark III (Ashford) bucket was 3°F below sea temperature on2 oC9asions, 2nd dips were 0.3°F and .. O.2°F warm­ er tha:n first ones even after more or'less dragging~

Owing to the usual inaccuracies of obtaining sea temperature by the in~' .take method: relying· on the engineers" thermometers, installations' and pract­ ic,es, ~t will usually be necessary to'providespeclal thermometers:; installat­ ions and to get the engineers to adopt more scientific methods. An.inexpens­ ive and apparently workable installation is described by H.U. RoI12)~ A copper pipe 1 1/2 m lo~g carries sea water from the intake pipe to a small vessel into which the water flows over a precision thermometer. The vessel and ther­ mometer are·in a convenient spot for reading. By means of this device ·he eliminated the 0.5°F error found by C.F. Brooks and by WahTwhen the thermo­ meter was held under a faucet.

'Such an installat~on would still permit errors from careless reading and at times not on the synoptic hour. A resistance thermometer in the intake which can be read on the bridge would be better. W.R. Thickstun, U.S. Weather Bureau, in a letter of January 19, 1953, writes: nWe are des~~gl1ing new equip­ mentfor measuring sea-water ten:tperatures which wiil utilize a "'Teston resist­ ance thermometer sealed in a stainless 'steel well. The use. of a deflection bridge-type dial indicator designed so that 'indicators can be located at several points on the ship is being"considered. The data are expected to be accurate to less "!fan 0,5°FtI.

Various countries submitted co:mrnents on the measurement of sea surface temperature as follows:

5.1 UNITED STATES OF AMERICA

Th~ United States has made experimental tests on several of its ocean .stations to obtain sea temperature data by both bucket and condenser intake methods .. Results of over 1700 observations showed that the mean difference .between the readings obtained by these two met~ods was less than 0.5°F.

Investigation of the variation of temperature below the sea surface was also pursued, and temperature dato. from the surface, 1 m, 2 ffi, 5 m, and 10 m depths were obtained from 200 bathythermograph observations. The largest - 16 - difference found in any set of readings was 0.4°F. The mean difference was less than O.loF.

It follows from the foregoing that the condenser intake method ~s satisfactory. Variations in the depth of the condenser intake do not affect the accura.cy of the observation to any appreciable extent•.The bucket method, on the other hand, is impracticable on fast ships and often·dangerous on all ships. The usual situation of the condenser intake thermometers makes them difficult to read. The U.S. Weather Bureau is experimenting with a ;resist­ ance thermometer telemetered to the observer's quarters. It,~s mounted in the condenser intake pipe but could be mounted in the sea chest, of th~ sanit­ ary system in which a continuous flow of water is maintained regardless of whether the ship is underway. The thermometer is a deflection type resist­ ance bridge develOPed during the Second World War. It has a range of 20° to 120°F and is accurate to within Oo5°F.

5~2 NORWAY Measurements of sea surface temperature on board Weathership M (Polar­ front I and II) Thea~~ bucket and ~intake method_ by Audvin Amot

Measurements of sea temperature are made every day at 0900 GMT. The following data are obtained:

Tt = sea-thermograph reading Tb = bow reading ) Tsb= starboard reading ) bucket Tst= stern reading ) Tp = port reading )

At the same time temperatures are observed and water-samples collected from the depths 0 m, 1 m, 2 m, 5 m and 10 m. This preliminary report .only deals 'Hith the bucket and intake measure­ ments. The intake and output of water from the engine is on the port side of the ships, intake 3.5 m below sea-surface and output at 0.5 m. We shall first compare the four bucket readings to investigate the local effects from the ship itself. We have used the bucket measurements from February, March and April 1952 (total set of observations - 78). Taking the difference. between the highest and lowest bucket readings we get the following distribution: - 17 ...

Maximum d~fference (°91 Number of observations

0.00 - 0.09 59 0.10 - 0.19 15 0.20 - 0.29 2 Olt30 - 0.39 2

The four readings showing more than 0.2°C difference may happen to be an error of observation. One must therefore say that the sea ·surface temper­ ature from a ship station using the bucket method, may have errors of-c=0~'4c;>ai but usually about! O.loC. .

Taking the bow-reading as a standard value (the value which is probably less influenced by the ship) we get the following average values:

Tsb - Tb = O~OOO (oC) Tst -Tb =-0.005 Tp -Tb =+0.033 The averages of starboard and bow readings are equal, elsewhere the differences between the other readings are very small.

The stern readings are a little lower than the bow readings, obvious­ ly caused by the upwelling of colder water by the propeller. The port read­ ings are higher than the bow and starboard re~dings apparently caused by the output of warm water from the engine just below the sea surface. Even if the average differences are small, they show that a ship will have a local in­ fluence on the sea surface temperature close to the shipl1 At the time of observation the weather ship has the stern against the wind. On a fast mov­ ing ship the local influence will probablx be smaller, perhaps insignificant. As a conclusion one may say that a bucket reading from the bow or star­ board (on ships where the output from the engine is on the port) will give the most representative values of the sea ,surfa.ce temperature;, Now, the bucket samples usually are to be taken on the lee side of a ship, and thus it is not. possible to point out a fixed place for taking bucket samples.

'Our next step is to compare Tt and Tsbft The average of (Tt -Tsb) 0.08°C~ (January - March) is + The frequency distribution of (Tt - Tsb} is shown in figure 1. 72% of the observations show .(T -T b) > Oll The maxim- um frequency 32% is situated between 0.0 and +O.loC.t s . speed~ The average of (Tt -Tsb) for different wind is shown as follows: Wind speed Number of . Average of in knots observations T-T_t_~ __sb__(oc) -~-... _~---- _....--~-_ ..... _--- 00 04 7 +0.08 05 09 5 +0~09 10 - 19 30 +0010 > 20 37 +0.07 - 18 ....

The differences are too small to be ~ignificant. The detailed treat­ ment of the observations shows that the spreading of observations (the stand­ ard deviation) are largest in strong wind~ With wind speed less than 20 kts all the single values of T - Tsb are situated between -0.2 and +O~,3.o0 t (comPare the total frequency distribution in figtrre 1 J page 19)0 -

,Average values of Tt -Tsb for different cloud amounts are as follows: Cloud amount Number of Average of __i2__:__ ~1~ observations _tT-T",",_sb_(°0) 0-2 2 +-o~12 3 5 22 +0014 6 - 8 55 +0.06 -

The two cases with N =0 - 2 cannot give a correct average value of T Tsbe The average of T -T is less by N = 8 than by N t t sb 6 - =3 - 5, but the difference is too small to be reliable.

5.3 NETHERLANDS It does not seem possible to provide a selected ship with a bucket and a thermometer which are fool-proof~ The instructions to the observers seem the best way to improve the observations. Dr. Brooks cites a case aboard the Netherlands selected ship "V6lendamu in which the measured temperature was 4°e above the actual temperature as a result of the heating of sample and thermometer case by sunshine~ Of course, 'a thoughtless observer 'may attain such results but it is not sure that the same observer will do much better with the best instruments 0 It seems that Dr. Brooks was very unlucky in this case as may be seen from the fo11owing~

If the sun affects the observations, this must-result in a daily range which is too large.,~urthermore, the standard deviation of a single observ­ ation with respect to the mean will become larger in daylight compared with that during the night because the influence of the sun during the night is zero whilst by day the influence depends on the cloud amountc In "lable 7 the results of a test-computation for a one-degree square in the Atlantic and for a two-degree square in the Indian Ocean are given.

The table contains for each square, for each month and for the local' hours 0, 4, 8, 12, 16 and 20(),}) the mean sea. surface temperature, the stand­ arddeviation of a single observation with respect to the mean and the number of observations. 'In the Atlantic the maximum difference between the mean temperature by day and at night amolL."1.ts to 1.1°0 in the month of August, while in the Indian Ocean it is about 0.6°e in all months. The standard devi­ ation does not show any 'appreciable daily range in the Indian Ocean but in the Atlantic in the summer months, especially in August, it is, indeed,

(*) Only the data for Oh and 12h are reproduced in Table 7. - 19 -

.30

20

--t - - - 10 I t - - -- - I . - r-. I I i

Frequency distribution of (Tt - Tab) January, February, March 1952

Figure 1 ... 20 -

about 0.3°C larger by day than during the night. The largest standard deviation amounts to 0.9°0 in the Indian Ocean and to 1~7°C in the Atlantic. From these' figures we may find the' probability of the occurrence of large deviations from the mean.. It appears that a deviation of for instance 4°C, as mentioned by Dr. Brooks, has a probability of about 2% in the square in the Atlantic, while in the Indian Ocean the probability is smaller than 0&000010 The standard deviation of the sea surface temperature augments with increasing latitude, so in the North Atlantic, the region where Dr. Brooks observed this large deviation, the probability could be still larger. A test­ square in the North Atlantic on the Rotterdam - New York route, 49° ....50 0 N, 100-11oW shows that this is not the case, it gives the following results:

TABL!,2. - STANDARD DEVIATION OF SEA SURFACE TEMPERATURE 49°-500N, 100·-11°W [Month J F M A M J J A S o N D

I Mean °c 10.7 Ib.3 10.2 '10.8 12~1 14.1 15.0 16 .. 4 15e8 14~2 12.4 11.3 ISt.Dev.oC, 1.1 1.0 1.0 1.1 1.3 1.4 1.3 1.2 1.2 1.3 1.1 1.1 Number \439 469 535 540 566 468 422 .431 399 367 334 375 _~ __.l_..__~~__~_. .. . .__, ......

The probability of finding an observation deviating 4°C or more in the northern test-square is 1.1% at the most unfavourable hourll

Besides the possible influence of heating by the sun, which results in the observation being too high, another source of errors is produced by the wind, especially when the air te,mperature is considerably lower than the sea temperature and when the water samples are taken with an ordinary canvas bucket. In this case the measured sea temperature will tend to beco~ ing too low, the influence will grow with increasing wind velocities~

In the same test-square on the Rotterdam ~ New York route the correl­ ation-coefficient between sea temperature and wind force Beaufort was comp­ uted for all available observations in the months December, January and Feb­ ruary. It was fou-'1d to be -0.05 .!O. 03 so this' does not confirm the suppos­ itione If, however, only the occasions were taken with NW, mfW and N wind, the correlation~coefficientwas found to be· ...0.18.:±'0.07 with 194 observ­ ations. These are ·the cases in which the air temperature c·an· be expected to be lower than the sea temperature, so indeed there is reason to conclude that the sea temperature has been measured too low as a result of cooling and evaporation in unfavourable cases. In cases with S-SW winds and with NE-E winds no significant correlation-coefficient was found.

With respect to Resolution XXXVII (CMM Toronto, 1947), the foll~wing remarks can be made o The bathythermograph observations on Ocean Station C, furnished by Dr. Reichelderfer show in 27% of the cases an isothermal distribution, in 5% - 21

increasing temperature downwards, in 60% decreasing temperature downwards and in 8% irregul{lr temperature variations in the layer 0"- 10 m. According to D.M. Houghton3Jthe temperattu~e 'gradients down to a depth of 10 m are large­ ly influenced by the sign of the temperature difference between air and sea surface temperature. These signs not being given, it· is not possible to comp­ are these data with those of Houghton~

It is noteworthy that only in 16% of the cases the intake t~mperature was the same as one of the bathythermograph temperatures (or lying between the highest and lowest reading)" In 44% of the cases the intake temperature was higher than the bathythermograph readings at all depths,' in 40% it was lower. The depth of the condenser intake was .not give~., but it will certain­ ly not have exceeded 10 ll5 so this leads to the conclusion, that the intake temperat~ITe was not reliable in 84% of the cases, assumed that the b~thy­ thermograph readings were accurate to O.loF. However, the fact that 'i.n 70% of the CQses the figure of the intake temperature was zero behind the decimal point, gives rise to the supposition that the observations in these cases were:accurate only to whole degrees.

For comparison of intake and bucket methods a series of observations, also furnished by Dr. Reichelderfer, are available. Dr. Reichelderfer states that results of over 1000 observations show.ad that theflcombined mean differ­ ence fl between data obtained by both methods was less than 0.5°Fo He says that in 46% of the cases a bucket temperat~~e exceeded the intake temperature and in 32% the reverse, while in 22% of the cases there was no difference between them.

Before the war a large number of combined bucket and intak~ 9bservat­

ions were carried out on bOQ.rd Netherlands selected ship,s 0 These observations were compiled by T?H~ Kir0100n an average the intake tempe,rature was 1°F higher than the bucket temperature. In the same article A.H •. Gordon published the results of a compilation of simultaneous intake and bucket observations on board British weather ships. Gordon found also the intake temperat~e higher than ·the temperature measured in the bucketo "On the stati·onlJ he·found a significant average'd~fference of O.4loF. H. u. Rol12~ublished also r'esults of German comparative b~~ket and .in~ take temperatures. A specially designed bucket (Marine-P:Ci',tz) was used and in the engine-room a special apparatus was installed to ensure the intake temperature not being influenced by the engine-room heat. In 7305% of the cases the difference found between the two temperatures did not exceed 0.3°0. However, high wind veloci:ties (above 4 Beaufort) caused larger differences, the bucket temperatu;re b~ing belm'" the intake.

In the same issue (p~ 48l) Roll gives a table 'Vlith the change in temp­ erature of the water sample in a "Marine-PutzH during the first minute after the sampling, according to wind-velocity and air-sea temperature difference. This table is the outcome of experin~nts carried out in a wind-turu~el. The table is copied here: -22 -

TABLE 6' - CHANGE IN TEMPERATURE OF WATER SAMPLE IN FIRST MINUTE ACCORDING TO AIR-SEA·TEMPERATURE DIFFERENC;E AND WIND VELOC IT!

..Ii .. • .•: ~mop.s 2 4 6 8 10 13 16 19 Tai.:~se~

~ "ioao ~'l: .~ + 5.0 + 0,,1' O· 0 0 +0.15 +0.1 +0.2 +0.2

,. + 2.5 0 0 a a -0.05 -0.1 -0.05 -0.05 0 0 -0;,05 a 0 -0 .. 05 -0(11 -0.25 -0.2 - 2.5 0 0 0 -0.1 -0,,25 ·-0.25 -0~4. -0.7 ...... 5.0 ....0.05 -0.15 -0'.2 -0.2 --0.3 "';0,6 -0.8 -1~1 - lOtiO -0,,1 -0.3 -0.3 -0.4 -0.85 -1.0 -1.3 -1.7

In view of this table it seems desirable to stress once more the in­ struction for the observers to take the sea temperature very quickly and not to expose the water sample to the wind. Remembering vThat has been said about the accuracy of the intake temper­ atures and paying attention to the diverging results of the above-mentioned compilations, it does not yet seem justified to change over from the bucket to the intake' method. However, it will be recommendable to replace the simple canvas bucket by an insulated one as soon as a satisfactory model becomes available at a moderate price.

The mea~ computed from a large number of observations is sufficiently accurate for climatological purposes. In the synoptic messages only the air­ sea temperature difference is given accurate to a whole degree Fahrenheit (or half a degree Celsius)o So it appears that the observations are also sufficiently accurate for this purpose although, of course, an indifferent observer may report data which are erroneous by several degrees. But, as mentioned before, the same observer will not get much better results with the most accurate instruments~ There remains to be considered the demand for "representative" observ­ ations for scientific purposes, for example for the studying of the trans­ fer of heat between ocean and atmosphere. It is thought that the observ­ ations of the routine selected ships will never b?come accurate enough for -such purposes and it will be necessary to execute measurings with special instruments. The Ocean Weather Ships establish an excellent opportunity for such precision.observationsQ . - 23-

TABLE---_.-7 ~ MEAN SEA SURFACE TEMPERATURES

Local time Oh 12 h

,, Square Month, Mean St. Dev. Number Mean St.. 'Dev. Number °C °C °C °C

January 14.4 Ihl 230 14.7 Q. 1l 9 259 February 14,,1 1.0 228 1404 1.0 254 March 14·2 1.0 242 14.6 1.2 280 April 14.7 101 187 15.3 1.1 244 Atlantic Ocean May 15.8 1.2 249 1602 1.1 300 1~3 20B ' 17'.8 1.4 226 370 ... June 17.1 3?ON July 18.0 1.5 ' 238 18",5 1.6 242 90 - 100W August 18.3' 1.4 215 18.8 1.7 268 September 18,,7 1.6 225 19,,1 lif7 276 October 18~2 1.5 230 18.6 1.5 264 November 16.7 1.5 247 16.9' 1.3 '~265 December 15,,4 1.2 242 15.6 1.1 275 January 26.9 0.7 370 27.4 0.7 359 February 2701 0.7 340 27.5 0.7 336 March 27 .. 9 0.8' 384 28 9 3 0.8 372 April 28.7 0.7 430 2901 0.8 ,384 Indian Ocean May 29.0 0.8 312 29.4 0.8' 309 June 28c 3 0.7 208 28.5 0.7 210 f 'BO - lOON f July 27.3 0.7 IB5 27 .. 8 0.7 206 0 68° - 70 E August 27.1 ' 0.6 202 27 0 5 0.6 200 September 27.1 0.7 285' 27.5 0.7 242 I, October' 2703 0.7 381 27.8 0.8 376 November 27.4 0~7 367 27 .. 9 0.8 378 December 27.3 0<>8 360 ~706 0 0 7 330 1

5.4, CANADA

A study of basic features of the problem of obtaining correct sea sur- face temperatures requires attention to the following: '

(i) Where is the sample required from; (ii) What degree of accuracy is required; '{iii) Does a vertical gradient between surface and intake leveis (3 to 10 m ,down) actually exist, and if so under what conditions; (iv) , If no gradient exists, or if the gradient can be determ:Lned from known conditions, then either bucket or intake method is, satisfactory, in which,case attention shOUld be concentrated on the procedure and ap­ paratus used for both bucket and intake methods, for the type of ship determines which method is the more practical; - 24 -

(v) If a gradient exists, and it is found to vary with known condit~ons, then can the· correction to intake temperatures'be determined with reasonable accuracy; (vi) At what point in a~tempting to obtain greater accuracy would the weight of requirements tend to break down the co-operation of the observers and actually lead to hurried or undesirable practices. Sea surface temperatures are required for both forecasting and climat­ ological purposes. Weather conditions (i.e. air stability, turbulence, humid­ ity, temperature, pressure, "wind and visibility), over the sea, and even to 'some extent inland,are tq a varying degree due to the positive or negative differences between air and sea a~face temperatures. The accurate measure­ ment of these two temperatures and their difference is therefore of qonsider­ able importance. The' determination of the sea temperature' is of wide climatic importance and is of continuing interest to the climatologist. Further,. it is of immediate interest to marine shippers of perishable goods pecause of the effects of temperature and humidity on ship and cargo.

Comments on sub-paragraphs (i) to (vi) above: (i) Since the' atmospher~ is in contact with the sea surface, it is assumed that what is required is the temperature of the actual sea surface, although opinion on this point differs to some extent. In other words, is the bucket sample to be representative of the skin temperature of the sea or the average of the top foot or two. The design of the bucket is directly dependent on the decision. :1f the ~emperature of the sea skin surface is required, representative bucket temperatures would no doubt be preferred over the intake.

(ii) Air and sea temperature differences are often small; and, .since we are interested in knowing which is the warmer, even by small amounts, both temperatures-should be obtained as accurately as possible. The Conference of Directors in 1947 voted for readings of sea and air temperatures to O.2°F or D.loC, but indicated that the requirement was lito encournge accurate observingrl • Investigation indicates that it is unlikely that accuracy to this degree is possible. Are we justified then in setting our requirements beyond the abilities of the observers and instruments; and, if so, is a chnnge in requirement desirable ? .

(iii) The conclusions on the existence of a gradient must necessarily be based on an adequate number of representative observations of sea temp­ erature taken at the surface and at varying depths, both under varying weather conditions and at a sufficient network of places to determine' the contributing factors and their variation. Some of the related problems are: ' The effect- of various wind speeds and the degree of mixing, The effect of the ship movement on mixing in the upper few. feet, and consequent representativeness of bucket samples, 25

The method of obtaining sets of temperatures at varying depths, i.e., sui.tability of bathythermograph measurements, and any altern­ ate'methods such as a series of resistance thermometers using thermistors, The effect of surface advection with horizontal temperature grad­ ients, The effect of air temperature on water gradient, i.~~, when air is warmer than the sea and vice versa", , , The relation between' seas~ns ~rid the positive and negative air-sea differences and the resurta~t likelihood of related positive and negative gradients in the wate~.near the surface, The diurna:).. effect of sola~ radiation on sea surface temperatures, particularly with calm or. near-calm conditions, The possibility of errors due to the equipment used.

The accumulated data indicate that: Winds of force 3 or 4 produce .mixing .and uniformity of sea temper­ atures down to 5 m or more~ The movement of a ship, especially when large and fast, will mix the surface· water so that the bucket sample will be representative of the. top 1 or -2 m of water;'. A number of methods are in use for measuring the vertical gradient; a bathythermograph is most widely used; continuing· experiment with a variety of methods is advisable, in order to improv~ the instrum­ ents and procedure and obtain' sample temperatures in undisturbed water as accurately as possible;. . Advection is important only when the horizontal temperature grad­ ient is large; The vertical water temperature gradient near' the surface can be related to the air-sea temperature differences and in turn to their seasonal differences; A slight variation in sea surface temperatures can be attributed to the diurnal range of' solar radiation, so that a small correct­ ion could be devised to compensate for the effect; The size of the thermometer'bulb, the use of a metal shield, the type of bucket, the plabe of -i:rmilersion, the' procedure used in getting the temperature of the sample, the thermometer scale and calibration, and such factors, are all'contributors to small errors in the tabulated results on water gradientsG

(iv) The bucket method is in general use by most selected ships, whereas the condenser intake method is used by large ,. ,fast. sh.ips,. since the use of the bucket would be difficult and at times impossible.. Some of the factors to be studied 'concerning these two methods are: 26 -

Bucket method; The style of b~cket - whether single, double or triple valled,. with 'attached or removable lid; whether the second sample should be taken and used or not; the size of the bucket and its place of storage when not in use; all indicate a need for continuing attent~ ion to.' improve bucket design and standardization of, procedure; The' thermometer - IMO recommends that they "have small heat capac~ ity, quick response, and be robust and easy too readrt , and ttif they are provided with a cistern for holding a water sample this should have a large heat capacityn~ Errors are certain to arise unless the thermometer quiCkly takes up the temperature of the sample, before it becomes non-representative of the sea temperature. The type of graduations, ruggedness, the design of thermometers, dif­ fer from Service' to Service. There is e\"idence to indicate that the use of a cistern may induce an error of several degrees when adequate care is not taken to allow for the heat content of the cistern; The procedure - there is considerable variety in procedures used but some agreement that: The sample should be obtained sufficiently~forwardto be out of th~ effect of the ship as much as possible, The bucket should be trailed a sufficient length of time to allow it to come to the water temperature, The thermometer should be inserted as quickly as possible, be watched and read while in the water as soon as it has taken on the ~ater temperature (time depending on the type of thermometer), The sample should be stirred to ensure uniform temperature, The reading, should be observed to a reasonable accuracy (rno rec­ ommended O.2°F or O.loC), but there are comments that scales in half degrees ,are sufficient and that too fine a scale discourages care and induces reading errors9 Intake method: The position of the thermometer. The closer the thermometer is placed to the free outside water, the less possibility of warming of the intake water by the ship. ,The type of thermometer well varies, and there is a requirement for research and standardization of mounting. Investigation is needed into whether the well should be shallow or deep, the thermometer fixed or removable, standard type or mercury-in-steel distant reading, and whether the well should be enclosed, open to sea water or mercury filled; The procedure used. It is evident that the reading and reporting of the intake temperature are subject to human and technical limit­ ations. The acc~acy of the report depends on,the time taken in reading the thermometer, if it is removed; on the calibration of the thermometer; on the accessibility of the thermometer and like­ lihood of parallax errors in reading; on the timing and regularity 27 -

of observing the temperature and relation of these times' to the times of issue of the weather reporto

(v) T~e decision on whether a gradient actually exists or not is gradually being clarified by the accumulation of data and reports. Continuing study is necess~ry to determine the relevant factors, the weight of each and when they apply. The .errors inherent in present bucket and intake measurements retard the solution to the problem and indicate that improvement in apparatus and technique is of first importance. More data should be accumulated 'for further comparison of the two methods. At the same time an .extensive programme of multiple depth measurements, uS'ing b~thythermographs or other means found to be .equal­ ly or more suitable, should be ·carried outo (vi) The procedures 'authorized for use and their .degree of complexity should be carefully weighed against the t~me and interest factors aboard ship, so that the max:i:mum in accuracy and usefulness of the reports may be achieved.

5.5 UNITED KINGDOM In British selected ships the "bucket" method for observing sen surface temperature is normally used in preference to the condenser intake method. In large fast· ships, 'however, it is. difficult and sometime's hazardous, to obtain a sample of sea water with a bucket, and for that reason in ships of this nature the: condenser intake method is frequently used•. A note ·is made' in the; meteorological logbook as to which method is used, and the Port Meteorolo­ gical Officer, when recruiting or inspecting a ship, makes a note'about det­ ail of the method of reading condenser intake temperatures and checks" the .... accuracy of the thermometer.

Errors by the·buc-ket method will arise' unless care is taken.

(i) That the sample is taken far enough·.forward so as not to have be.en .: . contaminated by the ship, when the ship is making way through water. When the ship is lying stopped it is probably best to get the water sample from the lee side as far forward as practicable;

(ii) Not to collect the sample too quickly, but to keep the bucket in the sea for about a minute;

(iii) To read the temperature of the sample as soon as possible after collection, and to keep the sample out of the wind and sun. Very'ser­ ious errors would arise if the bucket ahd sample were exposed.to . strong winds .for more than two minutes and if the temperature. were read· more than five minutes after the sample had. been obtained. To get really accurate r~sults the temperat~e should be read within about one minute of' obtaining the sample; - 28 -

(iv) To read the thermometer immediately it has tnken up the temperature of' the sample. In instruments supplied by the British Meteorological Office this takes about thirty seconds; (v) To stir the sample vigorously be.fore reading. It has been found that errors up to icY" may arise if this is not done. All these errors, except the first, will be proportional to the differ­ ence. between air. temperature and sea temperature. When the air minus sea temperature difference' is la~ge there may be significant temperature gradient~ in the first few inches of the sea - \tJith winds up to about :rorce 4, and in these circumstances it is very desirable that the bucket method should be used in preference to the c'ondenser intake method, particularly in a slow ship. Errors due to the condenser intake are liable to occur

(i) Because of the varying depth of the intake according to the draught of the ship; (ii) The unknown nature of the gradient between surface and intake, which may be as deep as 30 feet in a big ship9 (iii) The inexact accuracy of the instrument used for recording intake temp­ erature;

(iv) The varying position of the intake thermometer in various ships; (v) The frequently inaccessible.position of the intake thermometer making parallax errors in reading very liable; (vi) The reading of the intake temperature has to be taken by the Engineer Officer on watch and telephoneq to the bridge. The Meteorological Office is trying to develop an "insulated't- bucket which is inexpensive, robust and easy to use, and investigations are being made in both selected ~hips and ocean weather ships. The following is an extract from a note on "Comparison of Intake and Bucket Methods for Measuring Sea Temperature" appearing in the Marine Observ­ er, January, 1952. "Two reports (A) a comparison, based on Netherlands data, of intake and bucket temperature aboard merchant· ships and (B) a comparison of intake thermograph and canvas bucket methods of measuring sea tempera.ture aboard British Ocean Weather Ships have been prepared in consequence of Commission of. Maritime Meteorology Resolution 37, Toronto, 1947, which recommended that the problem of the accurate measurement of sea surface temperature should be referred to the various meteorological services" The results of report (A) suggest that the intake temperature is of the order of lOF higher than the bucket temperature. The results of report (B) 29

suggest that the intake temperature is of the order of Oc-2°F higher than the bucket temperature"while the ship is under way and 0~4°F higher than the bucket "temperature while the ship is on stationd

(A) ~_~~~pa~~?o~_~~~~~_~~_~~~~~E!~~~~_~~~~b_~f_~~!~~~_~~~_~~£!~~ ~~~~~~~_~~_~~~~~~~~~_~~~._~~~E~E~~~~_~~~~r~_~~E~~~~~_~!:ip~ "'

1ntroductio.n An investigation has already been made into the recorded differences between bucket and intake measurements of sea temperature made in certain ,British ships during the war years. 'I'he results are limited by the fact that no index correction had been applied to the ships' intake thermometers.

Additional material in the form of Hollerith cards has been made avail­ able by the courtesy of the Netherlands Meteorological Service. These data include sea temperature measurements by the bucket method and by the intake method, the intake temperatures being corrected for index error. No inform­ ation is available as to how the intake temperature was measured and whether precautions were taken to avoid engine-room heating~

Data were selected" from those Marsden squares having:

(a) Observations available for each month during th~ year~

(b) An appreciable number of observations available o

Data from those squares satisfying these conditions were examined, the number of observations available in each square being shown in Table 8<;,

" ,.. Squares 003 004 039 074 "075 110 145 146 No. of observations 1182 323 1469 624 742 1556 957 ""458

§.eas"onal var:l~ Mean values Of the" quantity "Bucket temperature minus ;intake 'teniper~ at'Lire tl (B-1) were evaluated for each square during each month of" the yearo The results showed an appreciable amount of sca~ter, and to" reduce this a grouping of the monthly means into seasonal means was made.

The standard deviation of "a single observation throughout the year is shown in Tabl~ 9" 30

Squares 039.$ 074 145:~ 003 1 004 146 ---075, 110 ------Standard deviations: 0.69 0.81 0.92 "'--,...... ;...... ----_._-----_..------

These 'results suggest~ as might be expected, that the scatter of the . differences is a minimum in the equatorial region, where uniform conditions prevail, and increases toward the ten~erate zone where there is a much great­ er variety of weather· conditions.

A statistical test of the results showed broadly an absence ofrseason­ al change in the equatorial squares (003 and 004), but showed that there was a seasonal variation elSev.lhere which was minimum in summer and autumn ..and a maximum in winter•.

~ffect of wind

Within the s~asonal classification·a further sub-classification was' made to show the effect of wind. The classification of wind force into the groups 0-3, 4-6, 7 and over, is hardly satisfactory. The nU!nber of occasions of force 7' or over is too small to permit any firm deductionso Another factor which militates against any demonstration of the effect of wind is that any such effect may be due to the wind's action directly on the content of the bucket, in which case relative wind and not absolute wind would be the essent­ ial element. Any dependence of the quantity B-1 on relativ.e wind may to some extent be masked by the'rough classification adopted.

It was found that in squares 003, 004J there is no appreciable variat­ ion with wind. On the other hand, the results for the remalnlng squares suggest a definite relation, higher negative values of the quantity B-1 being associated with higher wind fqrceso

Variation with cloud amount

A classification was made in terms of total cloud amount. For this purpose the observations used were those of daylight hours only (taken bet­ ween the. times of civil twilight), it being considered that observations at night lacked the requisite degree of accuracy. The results suggest a general increase in the negative magnitude of the quantity B-1 with increase of cloud­ ines$. The. values averaged over the whole year'show a consistency which could .hardly be due to chance" .

Effect Qt: da;V,:Jj.ght ~ New seasonal means were evaluated for the period of daylight hours•. The smaller values of these means when compared with those previously given - 31 -

show that the factors which make for differences between the bucket and in­ take measurements are of miniInum effect during the daytime. These figures give evidence of seasonal variationsiffiilar to that already afforded by the full data.

Difference between air and s§.§:_:te.m12erature

If any part of the observed difference bet'Ween the bucket and intake measurements be .due to heat exchange between bucket and atmosphere, then the difference between air and sea temperatures should be significant. A classif­ ication in' terms of this difference showed that the largest negative values of. B-1 arE? associated with negative values, while small negative values of B-I occur when the difference betweeJ:). air and sea temperature is large and . positive.

Variation with depth ~:.IIIl'?l'_~_._•..••~.__ ... Mean values of B-1 were taken out for different depths of engine intake. Statistical tests showed:

(a) That in squares 003, 004, the variations of the mean with depth need not ne.cessarily be regarded as real but can b? accounted for by the scatter of the observations; (b) That for squares 039, 074,.075, 110, and also for squares 145, 146, mean at 3 metres depth is significantly different from the general mean but the variations at greater depths need not neces­ sarily·be regarded as significanto This result in itself does not demonstrate a real variation of temper­ ature with depthG All that can be deduced is that the discrepancy between bucket and intake temperat'ures is significantly less when the intake wate~ is drawn from a shallow depth (3 metres)~ This ,difference may arise from variations in the technique of observations. It. 'may also 'be due to the 'fact. that in this case both bucket and. intake methods are attempting to measure. t~e same thing, this no longer being so with gre~ter depths of intake.

Conclusion

There are three real factors which might account in some way for the above results. They are:

(a) Real differences of temperature with d.epth. There may be some evidence of this in the section headed "Variation with depthn • . . (b) Defects in the lfbtlCket fl method of· taking sea surface temperature when using an ordinary canvas bucket, e.g. errors due to loss.of heat from the bucket during the process of measurement, this loss of heat being due to the combined effect of heat exchange and evapo.rationc . 32

(c) Defects in the intake meth.odof taking sea temperatures, e.g. heating effects in· the ship itself during the process of measure­ mento

The -results are most readily interpreted in terms of (b)c The differ­ ence between air and sea temperature is obviously of great importance in the process of cooling which affect.s the' bucket water after be~ng d~awn from the sea. The effect of wind is of jJnportance in the same connection. Factor (c) would presumably be independent of wind)' and factor (a) wouJ.,d give. a resu~t in the wrong direction, for increased wind would entail increased sea distitT.'bance and hence smaller di.f:rere~1ces of· temperature with depth. Season­ al variations may be due to both factors (a) and (b)~ Cloud amount is o~

significance in two ways; first j because absence of cloud implies heating of the bucket by the sun, and secondly, because an increase of cloud a.mount is usually associated with an increase of wind,

The results support previous opinions that the bucket method is sub­ ject to appreciable error due to cooling, unless the bucket itself is suit­ ably insulated. No definite conclusions can be drawn regarding the accuracy of the intake method"

(B ) ~._~~~~E~E~~~~_~f_!~~~!~,_~!!~E~~~E~J?!2~!2~_~~!::Y:~~._~~~~~~_E~~~!!!~~ ~f....,se~._~~~E~~~~~~_~?~;:~~._~~~~.~~~_9s:~~~}i~~~!:~~_~~2:E~

Int:r.Q.dp.ctjg,n In a preliminary report prepared in the Marine Branch in 1946 ~ntitled UReport on Methods of taking Bea -temperaturell~ a large number of differences between observations of sea temperature measured by bucket and intake methods .aboard British m~rchant ships were analysed~ Frequency distribution curves suggested that the intake method gave results approximately half a degree Fahrenheit higher than the bucket method., Hm-Jever, it appeared from the irlide scatter of the values that errors not necessarily connected with any method obser~Tations of observation were inherent in the I) It emerged that tLE:s'e' errors might occur, firstly, in the mE?thodof measuring the intake temperat­ liTe, exact· details of which were often lacking but irThich clearly vari.nd from ship to ship, and secondly, as a result of cooling of the water ~n tll) buck- 'et by evaporation, and by conductive transfer of heat when the air was appre­ ciably colder than the sea (See report (A))~

!he c:les1g~~~~eri1ll-ep.t Although it may. not be possible to eliminate these errors entirely, the conditions under which the observations have ·been made aboard the Gbean Weather Ships have been designed to reduce their magnitude so that they no . longer hide real physical effects~ .

These conditions are:

(a) The new Mark III canvas bucket was used~ This bucket has been constructed with a double-walled copper vessel inside, the. space - 33

between the walls b~ing f~lled with wat~r. The water passes through the holes in the-bottom of the inner container, back up between the walls and out through the annular space under the lid. With this bucket, the error caused by the temperature of the wat­ er in the bucket changing before the reading is taken, owing to the processes of heat exchange and evaporation, is quite small. The error caused by the initial temperature of the bucket being different from that of the sea can be eliminated by towing for about 30 seconds. (b) The exact position of the thermograph element in the inta.ke pipe is known. (c) The Inean depth of the intake where the water enters the ship is 9 ft below the surface. ' - (d) All observations have been corrected for index error and the thermograph has been corrected once every 24 hours by an inspect­ or's thermometer placed in the "intake. (e) All observatioris were made by the meteorologists aboard the ships, whose duties are entir~ly of a meteorological nature.

Analysis of observations The differences between the values of sea temperature recorded by the intake thermograph and Mark III canvas bucket (first haUl) were tabulated from the records of three Ocean Weather Ships during the period March-October 1949. The observations from each ship were analysed separately according to whether they were made while the ship was on station or under way. The canvas bucket values were taken from the fil'"st haul, since mean-differences between first and second hauls were slight.

Mean differences (first minus second haul) for a large number of cases were: Weather Watcher +.04°F Weather Recorder +.09°F Weather Observer +.05°F Weather Explorer 00

Mean differences between inta~e thermograph and canvas bucket are shown in Table 10 for each voyage. All observations were read to the nearest ·tenth of a degree at the eight synoptic ~o~s. The mean differences are based on a total of about l70 observations while on station and about 40 observations while under way for each voyage. Statistical tests applied to the differences in Table 10 show: - 34 -

(a) The mean difference between bucket and intake while on station is significant to the 5.per cent level; (b) The mean difference between· bucket and intake while under way is not signific?-nt~

TABLE 10 - MEAN DIFFERENCE BET1JJEEN BUCKET AND INTAKE. METHODS OF MEASURING SEA TEMPERATURE

MEAN DIFFERENCE BUCKET MINUS INTAKE RATE OF FLOW VOYAGE INTAKE (oF.)

·On Under Station Way On Station Under. Way Ship:Weather Recorder 9 Mar .....14 Apr'. ITEM +0.01 +D.30 '290 tons/hr. 340 tons/hr. 2 -. 28 June ITEM +0.62 +0.61 260 340 14 Jul.-8 Aug. JIG -0.41 -0.55 280 360 24 Aug.-20 Sept. ITEM -0.11 +0.10 240 360 Ship: Weather Watcher 2 - 27 June JIG -0.65 -0.10 .644 g/min 2000 g/min 14 ~u1.-8 Aug. ITEM -0.97 -0.45 644 2000 24 Aug.-19 Sept. JIG -1.00 -0.50 644 2000 5 Oct.-l Nov. ITEM -I,ll -1.15 644 2000 Ship: Weather Explorer 14 - 25 Apr. ITEM -0.41 -0.45 1300 g/min· 1300 g/min 12 - 31 May ITEM -0.04 -0.00 1300 1300

MEAN -0.41 '-0.22

The results suggest that the intake method gives readings about half a degree higher than the bucket while on station and about a quarter of a degree higher while under way, although the latter difference is not a significant one.

The former difference may be caused by: (a) Transfer of heat from the engine-room to the intake pipe.. This explanation is borne out by the fact that the difference is less while under way when the rate of flow of water through the pipe is greater than while on stationG - 35

(b) Cooling of the bucket by evaporation and by transfer of heat on occasions when the air is substantially cooler than the sea. On the average the air is 1°F cooler than-the sea at JIG and 2.5° cooler at ITEM during the period covered. (0) Stratification of the water. There is a tendency for a vertical circulation to develop so that warm water is moving to the top and cooler water sinking. However, there might be occasions when the temperature of the water underneath the skin surface was warmer either as a result of a vertical or horizontal current or because the surface water is cooled by the air and vertical cir­ culation' has not developed' sufficiently to. equalize the difference. (d) Cooling of the surface water directly by evaporation due to the loss of latent heat.

These investigations seem to show that in a relatively small vessel such as a weather ship, in which the condenser intake is not too deeply immersed, the thermometer is mounted close to the ship's side, and the intake temperatures are read from a distant reading thermograph directly by the officer responsible for making the observations, then the intake readings will, when the vessel is under way, not be appreciably different from the readings taken by the insulated bucket method. In Report (A), a large number of different merchant ships of varying draughts were involved in which the intakes were at varying depths, and in which the temperatures were taken under varying circumstances, and the bucket employed was not of the insulated type. It is difficult therefore to form a real comparison between the two types of readings.n

LIST OF REFERENCES

1) H. Stemmel. W.S. von Arx, D. Parson and W.S. Richardson Rapid aerial survey of Gulf Stream with camera and radiation thermo­ meter, Science, Volume 117, June 5, 1953, pages 639-640).

2) H.li. Roll Annalen der Meteorologie, Volume 4, 1951, .pages 439-443.

3) g.M. Houghton Marine Observer, Volume XXII, 1952, page 43.

4) T.H. Kit,k Marine Observer, Volume XXII, 1952, page 53. I