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Marine Observer's Handbook

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Marine Observer's Handbook M.O. 218. METEOROLOGICAL OFFICE. MARINE OBSERVER'S HANDBOOK. al Cwmmtffee. LONDON: PRINTED UNDER THE AUTHORITY OF HIS MA.TKSTY'S STATIONERY OFFICE By DARLING AND SON, LIMITED, BACON STREET, E. To be purchased, either directly or through any Bookseller, from WYMAN AND SONS, LIMITED, 29, BREAMS BUILDINGS, FETTER LANK, F,.('., and 54, ST. MARY STREET, CARDIFF; or H.M. STATIONERY OFFICE (SCOTTISH BRANCH), 23, FORTH STREET, EDINBURGH; or E. PONSONBY, LIMITED, 116, GRAFTON STREET, DUBLIN-, or from the Agencies in the British Colonies and Dependencies, the United States of America, the Continent of Europe and Abroad of T. FISHER UNWIN, LONDON, W.C. 11)15. Priw Three Shillingx. 11 To the Captains and Officers of the Ships observing for the Meteorological Office. After sixty years of work we remain learners about the weather over the ocean ; and, as in 185-t when the work on the present plan began, so in 1914 it is upon the careful observations of Seamen that we must rely for the material for further advances towards a knowledge of its laws. Not that we have learned nothing. Within those sixty years the general characteristics of the revolving storm of tropical latitudes and of its milder relative the cyclonic depression of temperate latitudes have been made out, though their ultimate causes are still obscure. We know at least what the greater fluctuations of baro­ metric pressure mean for the Seaman, and where he is likely to find them. There are minor fluctuations, too, which we know about and which have a meaning, often a very incisive one, but we still have many questions to ask about currents in the water of the ocean, or in the air over it, their variations and their relations to ice, fog and other conditions. We have also learned the average distribution of pressure, temperature and weather over all oceans, and that has led to another advance by compelling us to regard the whole atmosphere as one, really and not only in name; so that marine meteorology, land meteorology, polar meteorology with its ice and snow, and the study of the upper air have become for all practical meteorologists simply aspects of the study of the atmosphere, of the meteorology of the globe. Our next step is to bring all these aspects into visible co-operation, and the various observations into juxtaposition. So in presenting to you a new version of the instructions for the guidance of marine observers, drawn up by one of your colleagues and of mine, we have not hesitated to ask you to think with us in terms of the meteorology of the globe, certain that the laws of weather for the sea cannot be disentangled from those for the land and the sky. The best expression for the obligation which I feel after your sixty years of co-operation with this Office is to appeal once more to your scientific interest, and ask you to continue to give us observations with all posMble accuracy. But it is not science only which prompts me to make that appeal. On the sea enterprise is hardly distinguish­ able from competition or rivalry ; and, other things being equal, that ship will certainly be most successful in peace or war whose officers know how to make the most of fair weather und the best of foul weather. He knows best who has learnt how to utilise the experience of others as well as his own. In the matter of weather, it is the task of this Office to do what it can to make the experience of all available in the mont useful form for all, and it is on that ground also that our continued appeal in Seamen is based. W. N. SHAW, Director. METEOROLOGICAL OFFICE, LONDON. llth December, 1914. M.0.,18, METEOROLOGICAL MERCURIAL BAROMETERS GRADU MILLIBARS. The graduation of the barometer is carried out in mfiliburx. 1,000 millibars = 29'53 inches of mercury, which is not far from the normal pressure of the atmosphere at sea-level, so that a reading of the barometer in millibars is the pressure of the atmosphere expressed as the number of parts per 1,000 of that normal pressure. In this respect the reading is very similar to that of a hydrometer, which gives the density of sea-water expressed as the number of parts per 1,000 in excess of the density of pure water. Instructions for the setting of the vernier an:l the reading of the scale are contained in the Marine Observer's Handbook of the Meteorological Office, pp. 20-24. An explanation of the meaning of the certificate from the National Physical Laboratory which is issued with the barometer is given on pp. 24-28 of that Handbook. On these pages a numerical method of correcting and reducing the readings to Mean Sea-Level is also given. For moderate heights, such as occur on board ship, these reductions can, however, be rapidly and accurately determined by means of a slide working alongside the attached thermometer, and in what follows arc contained instructions for the use of the slide provided. INSTRUCTIONS FOR THE USE OF THE SLIDE PROVIDED ALONGSIDE THE ATTACHED TlIERMOMETEU. The slide is intended to serve the purpose of the correction and reduction tables used for obtaining pressure at sea-level from the actual readings of the barometer and thermometer. The corrections are three in number : 1. The temperature correction. 2. The correction for the variation of gravity with latitude. 3. The reduction to sea-level. The slide effects the corrections with varying degrees of exactness : thus, correction (1) is allowed for exactly under all conditions if we assume that the reading of the attached thermometer really gives the temperature of the instrument, correction (2) is effected exactly only when pressure is nearly 1,000 mb., and correction (3) is arranged for a mean temperature of the air column of 285a. (53'6° F.). When the mean temperature differs greatly from this value the reduction to sea-level is not exact, especially if the height of the station approaches 100 feet, the maximum height provided for. The slide, which may be clamped in different positions by means of two milled head-screws, works between the attached thermometer of the barometer and a fixed scale which is graduated unequally according to the latitude of the place of observation. On the slide itself are two scales ; that on the left is a scale of heights from 0 to 100 feet above mean sea-level, that on the right is a scale of (2f>:ir>:t-iL'> wt, mr>r> 210 125 1/20 n.st. G. 2. corrections in millibars and tenths, whole millibars being figured and marked either + or . To use the slide : 1. Uuclamp and move the slide until the figure on the left-hand side of the scale corresponding with the height of the barometer above the level of the sea is in line with the figure on the fixed scale of latitude corresponding with the latitude of the ship 2. Clamp the slide in position. 3. Note on the right-hand scale of the slide and with the proper sign the reading in millibars opposite the end of the mercury column of the attached thermometer. 4. This reading is the complete correction required, and has merely to be added to or subtracted from the reading of the barometer. £t-ample. Barometer 142!). Latitude of ship 50° N. Height of i-istei-n of barometer above sea-level 40 feet. The Figure shows the slide set in the correct position. The end of the mercury column in attached thermometer is opposite the gradua­ tion + U-8 millibar on the slide. Consequently, the correction for temperature, gravity, and reduction to sea-level is + O8 mb. Beading of barometer = 1,012'4 mb. ,, ,, corrected and reduced to sea-level = 1,012-4 + 0-8 = 1,013-2 mb. NOTE. When sailing on an approximately East-West course (as, for example, from the British Isles to Xorth America), sufficient accuracy will be attained it the observer nets the slide to the mean latitude of the voyage ; but if sailing on an approximately North-South course he should reset the slide for latitude daily. The graduations of the fixed scale and of the scales on the slide are carried out in accordance with the instructions of the Tables on pp. 24- 28 of the Marine Observer's Handbook. Meteorological Office, South Kensington. S.W.7. January, 1920. TWO scales are now engraved on barometers issued "by the Meteorological Office, one reading1 in pressure units called millibars and the other in inches of mercury. Either can be used at the discretion of the observer. Observers are requested to use the millibar scale for the entries in the logs and forms of the Meteorological Office. 2695 Ill CONTENTS. PAGE INTRODUCTORY NOTICE TO CAPTAINS OP SHIPS ... ... ... - 1 CLASSIFICATION AND DESCRIPTION OF REGISTERS ... ... ... ... 8 PART I. CHAPTER I. The Barometer.—The choice of a Marine Barometer. The most suitable position in which to hang it. Handling the instrument. Defects of Barometers that can be remedied. Method of reading the Barometer. Effect of erroneous readings. Corrections of readings of the Barometer. The Aneroid Barometer. The Measurement of Baro­ metric Pressure in Pressure Units. Table of Equivalents. Graduation of Kew Pattern Barometer in Millibars. To take an observation. Correction and Reduction to Sea Level of Barometer readings in C.G-.S. units. The Sea Barometer. The Barograph... ... ... ... ... 6 CHAPTER II. The Thermometer*. Hydrometer, and Rain Gauge.— The principle, construction and graduation of thermometers.
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