BULLETIN OF THE AMEKICAN METEOROLOGICAL SOCIETY

Entered as second class matter at the Post Office, Worcester, Massachusetts, under Act of Aug. 24, 1912. Issued monthly except July and August. Annual subscription, $3.50; single copies of this issue, 35c. Address exchanges and business communications to CHARLES F. BROOKS, Secretary, American Meteorological Society, Milton, Mass. Address papers, reviews, notes and other items for publication in the Bulletin to ROBERT G. STONE, Editor, Dept. of Meteorology, New York Univ., University Heights, N.Y,

Vol. 24 April, 1943 No. 4

The Climate of the Mediterranean Region V. CONRAD Blue Hill Observatory, Harvard University, Milton, Mass.

SYNOPSIS In summer the well developed west-to-east I. In the first part the characteristic ob- pressure gradient causes three main cur- servational facts are given. Average com- rents directed toward the Persian Gulf. The fortable winter temperatures, hot summers, northern and southern currents, over the high and a dry period in summer are the significant pressure areas of the cool Black Sea and the features. Cloudiness is little, even in winter, Mediterranean, respectively, are lines of di- in the rainy period. vergence ; the middle current over the heated II. The physical explanation is based on south European lands is one of convergence. the pressure distribution and the main stream- The important wind systems are indicated by lines of the air. In winter, a "lake of low this representation. The rainlessness during pressure" above the Mediterranean Sea caused summer in the southern and eastern portions by the warm surface water is embedded in of the basin finds its explanation in the wind the huge region of high pressure from the systems, on the one hand, and in the stability center of action at the Azores reaching to the of the atmosphere caused by thermal condi- continental high pressure. The warm sea tions, on the other hand. intensifies cyclones invading from the Atlantic III. In the last part important local cli- and gives rise to cyclones in the three parts matic features are presented: excessive rains of the Mediterranean Sea: the Gulf of Lyon, in the desert; the bora and other cold winds ; the Adriatic Sea, and the Cyprus region. The winter easterlies of Palestine; peaks of tem- first and the last and Northwest Africa as perature ; great daily ranges of temperature well, are regions of convergence, as is seen in the deserts; and the airmass analysis of. from the streamlines of January. the sirocco.

I. THE STRIKING OBSERVATIONAL gion of Chile. FACTS 1. As far as Europe is concerned HE MEDITERRANEAN TYPE of cli- the high winter temperature is re- Tmate is characterized by a sea- stricted to the Mediterranean basin of son with more or less copious the sea. There the hinterland, especi- rain in winter and a season of drought ally toward the northeast is much in summer. Clear or cloudless skies colder, and a steep temperature gradi- in summer and partly cloudy skies in ent is the consequence in winter. The winter in spite of the rainy period are Balkans, inland beyond the western usual in this climate. mountain ranges, have low winter The beautiful blueness of the sky temperatures, some examples of of the Mediterranean region is pro- which are given in the following verbial. A very similar type is found table of January temperatures in the in America in southern California basin of the Danube and their tributa- and in the Valparaiso-Santiago re- ries:

Unauthenticated | Downloaded 10/08/21 10:28 PM UTC FIG. la. Actual January Temperature in Europe. (Adapted from B. J. Birkeland, N. J. Foyn, E. Alt, with modifications by W. Gorczynski). Reproduced by permission of W. Gorczynski; courtesy, Polish Institute of Arts and Sciences in America, New York City.

FIG. lb. Actual July Temperature in Europe. (Adapted from B. J. Birkeland, N. J. Foyn, E. Alt, with modifications by W. Gorczynski). Reproduced by permission of W. Gorczynski; courtesy, Polish Institute of Arts and Sciences in America, New York City. (*) (*) I wish to thank Dr. Ch. F. Brooks at whose suggestion I am inserting these maps in the place of sea-level isotherm mans. Unauthenticated | Downloaded 10/08/21 10:28 PM UTC FIG. 2. Isolines of the dates at which the curve of the annual course ex- ceeds the temperature of 70 °F. January temperatures in the east- temperatures indicated by the aver- ern hinterland of the Mediterranean age annual course were chosen on Sea:— account of the great variability of the Uzice (43.9°N, 19.8°E) 27°F extremes. Average temperatures Uskub (42.0°, 21.4°) 29° higher than 70 °F are taken as hot. Monastir (41.0°, 21.4°) 30° This threshold represents approxi- Banyaluka (44.8°, 17.2°) 29° mately the temperature of July at Giurgiu (43.9°, 26.0°) 27° Boston, Mass., St. Paul, Minn., and Bukarest (43.9°, 26.1°) 26° Los Angeles, Calif. In this way a Braila (45.3°, 27.8°) 26° threshold is obtained well imaginable Genova on the coast (44.4°N) has a at least for those who have experi- January temperature of 46°. enced the summer climate of the northern or western United States. The map (FIG. la) shows the iso- As the map (FIG. 2) shows, natu- therms of January and the steep rally, the temperature rises to above gradient, especially towards north- 70° earlier in the year the lower the east, as mentioned above. latitude is. The closed isoline of June 2. Period of high temperatures in 5 indicates the cooling and retarding summer, beginning, end, and dura- effect of the water body. Therefore, tion.—In summer the temperatures the temperature rises slower on the are high in the Mediterranean region. islands. Along the coast of Algeria (Fig. lb). Details of importance will the critical temperature is reached be mentioned later on. Here only a on June first. Inland and southward, rough survey is given of the dates at this date comes always earlier. In which the hot period begins and ends Tozeur, e.g. the critical temperature in the different parts of the Mediter- is exceeded by April 25. In Egypt ranean Sea. There is difficulty on the the same phenomenon is observed. one hand in deciding what tempera- The dates are: for Cairo (30°N) ture shall be considered hot, and on April 26, for Dakhla Oasis (26°N) the other hand, which temperatures March 20, and for Aswan (22°) are to be considered average or ex- March first. treme. For this investigation the The dates when the average daily

Unauthenticated | Downloaded 10/08/21 10:28 PM UTC FIG. 3. Isolines of the dates at which the average daily temperature drops below 70°. temperature falls below 70° are weeks, and in Aswan 39. :shown on the map, FIG. 3. 3. Precipitation and the duration FIG. 4 represents the duration of of the dry period.—Northward, the the hot period of the year (in weeks). Mediterranean climate merges with These isolines are of course more the continental type (summer rain smoothed than the preceding. Natu- maximum) of Central Europe. In the rally the duration increases with de- transition zone, the maximum of rain creasing latitude. Observations ex- occurs in fall and spring; summers tended over Mesopotamia (Iraq) are poor in rain, but not rainless, would show that an increase exists and the precipitation falls then in too with increasing easterly longitude. the form of occasional downpours Along the European coast the dura- and thunderstorms. tion is about 10 weeks, on the North On the African coast the true Medi- African coast approximately 25 terranean climate is restricted to a

FIG. 4. Isolines of the duration in weeks of an average daily temperature of 70° or higher.

Unauthenticated | Downloaded 10/08/21 10:28 PM UTC FIG. 5. Isolines of the number of months with an amount of rain ^ 0.2 inches. small coastal strip, wide enough, central portion of the Mediterranean however, to have been very important region (Libya, 30°N) 7 months are in the North African campaign. rainless (April through October). At Hanson W. Baldwin, summarizing in Malta, 36° N, 4 to 5 months are rain- the New York Times, wrote: "The less, at Sicily, 38 °N, 4 months. first Allied rush for Bizerte and Tunis The number of rainless months, in- last year was nearly successful and creasing from west to east, can be might have been completely so, had it seen from the following data:

Candia 25.1 °E Long, rainless Jn to Au — 3 months Famagusta 34.8°E " " My to Au = 4 " Haifa 35.0°E " " My to Se = 5 " Jericho 35.5°E " " Ma to Oc = 8 " not been for the extraordinarily heavy According to the most recent data, rains of late November and Decem- especially those given by E. Alt and ber,—the heaviest for many years by the Physical Department in . . . The rains . . . were so heavy, Cairo2, the duration of the rainless the earth so muddy that tanks, jeeps, period is a little diminished even if a trucks, bogged down and many were monthly amount of 0.2 inches passes lost." for "rainless". These new computa- Inland, the climate changes to that tions are represented on FIG. 5. An of dry steppes and the deserts. It isoline dividing the Iberian Peninsula was necessary to terminate the cam- into approximately equal parts, run- paign in that portion early, for it is ning along the axis of the Mediter- hard to believe that even modern war- ranean Sea, rounding Italy, crossing fare can be successfully waged in the Greece towards the south coast of the summer of the Sahara. Summer Black Sea, is about the northern limit threatens with death as much as war of the region with a period, rainless itself. according to the definition given The rainless portion of the year above. Especially in the Nile valley increases from north to south and 2Climatological Normals for Egypt and the from west to east. (FIG. 5.) In the Sudan, Cyprus and Palestine, Cairo, 1938.

Unauthenticated | Downloaded 10/08/21 10:28 PM UTC BULLETIN AMERICAN METEOROLOGICAL SOCIETY the length of the rainless period graphic rain, it falls ln summer. increases rapidly with decreasing lati- Gebeit at 2625 feet, only 37 miles west tude so that at Assyut, 27° N, the rain- from the coast of the Red Sea, has less period covers the whole year. 0.59 in. rain in winter, 4.09 in sum- The amounts of winter rain also mer. Gebeit and Suakin are only 31 decrease more or less from west to miles apart. east, and from Tripolitania to Egypt The rainless region of the Sahara the desert encroaches more and more Desert represents the transition zone on to the coast. The rapid decrease from the winter rain in the Mediter- in rainfall from the coast inland is ranean region, to the summer rains in apparent in following data from the the tropical belt. Nile valley: In Palestine similar conditions pre- vail: Lat. N Inches per year 31.4° Rosetta 1.38 Long. °E Inches per year 29.9 Helwan 1.22 35.0 Haifa 24.61 27.2 Assyut 0.20 35.2 Jerusalem 16.06 25.5 Dakhla 0.00 24.0 Aswan 0.00 35.5 Jericho 4.92

It should be emphasized that Jeru- The same phenomenon is observed in Libya: salem is located 2490 feet high on the ridge of the West-Jordanian High- land, while Jericho lies beyond the Lat. ° N Inches per year ridge in the Jordan depression 820 32.9 Tripolis 15.87 feet below the sea level. The extreme 31.9 Nalut 7.72 dryness of the latter plafee is ex- 30.1 Ghadames 0.83 plained by the rain shadow caused by the highlands. The lack of rain The Mediterranean and the Sahara in Jerusalem is, therefore, the more climates merge into one another in remarkable. many regions in an imperceptible 5. Distribution of the genuine Medi- way. Rainless summers extending terranean climate.—These examples to nearly rainlessness the year round show the transition of the Mediter- are found east to Turkestan and Iran. ranean climate to those of steppe and The southern border line of the desert. It is hardly possible to draw winter rains is approximately at a sharp line between these climates. 20 °N. An example is given by the It is rather more practicable to make following pair of stations: a distinction on the basis of the dis- tribution of the specific Mediterran- PRECIPITATION (inches) ean flora. In earlier years, Theobald Oct.-May Apr.-Sept. Fischer made an excellent and suc- Suakin 19.1° N 6.46 0.67 cessful attempt in this direction. FIG. Gebeit (2625 ft.) 0.59 4.09 6 represents the distribution of one Merove 19.6° N 0.04 1.18 of the most characteristic Medi- terranean trees, the olive tree. Except The three places are still in the dry for the fertile coast region of Cyre- zone but, nevertheless, there is the naica, which, jutting into the sea, region of transition from winter to gets ample winter rains, the cultiva- summer rain. If there is any oro- tion of the olive tree does not extend

Unauthenticated | Downloaded 10/08/21 10:28 PM UTC FIG. 6. The culture of the olive tree in the Mediterranean basin. (Black areas are the regions of most intensive culture; Dotted areas those of less intensity) (After Theobald Fischer; taken from M. Rikli, Lebensbedingungen und Vegetationsverhaltnisse der Mittelmeerlander und der atlantischen Inseln, p. 80, (Jena 1912).

FIG. 7. Mean Isobars of the Mediterranean Region in January. beyond the longitude of 15 °E on the II. PHYSICAL EXPLANATION OF THE African Coast. MEDITERRANEAN CLIMATE This fact is in closest harmony 1. Isobars, cyclones and their tracks, with what has been said about the secondary depressions in winter.—The desert encroaching more and more on average air pressure distribution is the coast from Tripolitania eastward. basic for a physical explanation of

Unauthenticated | Downloaded 10/08/21 10:28 PM UTC FIG. 8. Average temperature (°C) of the coldest month ( ) and amount of rain (mm) in the mor Lth with maximum of precipitation ( =====) on the west and east coa sts of the peninsulas. I, 1 = Rome; II, 2=Chieti; III, 3 = Durazzo; IV, 4 = Kavala. Average latitude = 41.6°N. the Mediterranean climate. West of within the region of the sea itself. the Mediterranean Sea is the Azores Even the average distribution of pres- "high" which shifts from 30° N and sure shows that the "trough" is split 35° W in winter to 35° N and 32° W into some secondary depressions (see in summer, when it becomes intensi- FIG. 7) as a consequence of the dis- fied. East of the Mediterranean Sea tribution of land and water, i.e. of over the continent another high pres- the peninsulas which separate great sure center which develops in win- basins from one another. Over the ter is linked with the Azores High. interior of these peninsulas, much Only the Mediterranean Sea itself colder than the sea, pressure is some- represents a "lake" of relatively low what higher. As a result of these pressure, a trough of low pressure facts, warm, moist, and rainy south- in consequence of its relatively high erly winds prevail along the west temperatures. Thus the conditions in coast of the peninsulas, while the winter favor the maintenance and east coasts have colder and drier transmission of invading depressions winds. The effect is clearly seen in and also the development of cyclones FIG. 8.

FIG. 9. Cyclone tracks of the Mediterranean region. (From W. G. Ken- drew, "Climate").

Unauthenticated | Downloaded 10/08/21 10:28 PM UTC The trough of low pressure over the Mediterranan Sea is invaded and crossed by depressions from the Atlantic Ocean as shown in FIG. 9. The average tracks on which these move, are numbered 1, 2, 3, etc. Other cyclones develop over the warm Mediterranean Sea itself, above all in the Gulf of Genoa, the Adriatic Sea, and the Cyprus region. The depressions of the Gulf of Genoa, the Adriatic Sea, and the Cyprus region follow paths 4, 6, 7. Track 7 (see FIG. 9) is split into two tracks, nos. 8 and 9, by the penin- sula of Asia Minor so that generally FIG. 10. Frequencies ( % ) of cyclone tracks no. 8 ( ) and no. 9 ( -) in the the highlands of this region are not Mediterranean region. crossed by depressions and the winter and spring are 64% and greater portion of the interior does 79% respectively, of autumn 29% not receive much precipitation in and 16%, and of summer, only 7% winter. and 5%. A sharp frequency maxi- "Only Egerdir Lake has light mum occurs in winter and a very green, milky, fresh water and many low minimum in summer. Thus the fish, whilst in the salty-bitter water winter rains are explained primarily of the Burdur Lake (ca. 38°N, 31 °E, by the thermal properties of the 3200 feet) fishes cannot live. The Adshi-Tuc-Gol, near the Burdur Lake, half dries up in summer and its water is salty-bitter, as the name indi- cates." (P. Zistler, Die Temperatur- verhaltnisse der Tiirkei. Leipzig, 1926, p. 66). These facts are the proof of the insufficient amounts of rain in these parts of the highlands of Asia Minor. Evaporation exceeds precipitation because of deficient cyclones, the cause of rain. 2. The annual course of frequency of cyclones as explanation of the char- FIG. 11. The important singularities of a field of flow: acteristic distribution of rain over the 1. The flow of air in a "neutral point". year.—In this connection the annual Two air currents meet each other from oppo- site directions and give way to each other. course of the frequency of cyclones This case of a singularity plays a fundamental role in frontogenesis; see V. & J. Bjerknes on the different tracks is of interest. and collaborators, Physikalische Hydrodyna- FIG. 10 represents the seasonal fre- mik, pp. 703 ff. and R. Wenger, Meteorol. Zeits., 1920, p. 242. quencies (%) of cyclones on tracks 2. Point of divergence at the core of a pressure maximum. 8 and 9, which belong to the most 3. Point of convergence at the center of a eastern portion of the Mediterranean low pressure region. 4. Line of divergence caused by a wedge or Sea. The trend of the two curves is ridge of high pressure. 5. Line of convergence produced by a trough impressive. The frequency-sums of of low pressure.

Unauthenticated | Downloaded 10/08/21 10:28 PM UTC FIG. 12. Streamlines of the Mediterranean region in January. (Period 1878-1913).

FIG. 13. July mean isobars of the Mediterranean region. (Fig. 7, 12, 13* 14 from L. Weickmann, Klima der Tiirkei, Miinchen 1922)

Unauthenticated | Downloaded 10/08/21 10:28 PM UTC FIG. 14. Streamlines of the Mediti ranean region in July. (Period 1878- 1913). huge water body nearly 2400 miles of convergence, represented by lows long and with a surface area of about which appear in the map of the pres- 1.2 million square miles,3 including sure distribution (FIG. 7) the west the Black Sea and the Caspian Sea. Mediterranean "low", the east Medi- For a closer understanding of the terranean "low", and the tropical characteristics of the Mediterranean "low" southwest of the Red Sea. climate the knowledge of the aver- As a result of these conditions the age streamlines of the air above this northern coasts have winds with region is indispensable. These lines northerly components while on the give the picture of the air motion at African coast southwest winds pre- a certain moment, for the middle of vail. Westerly and southerly winds January (FIG. 12) and the middle of predominate in southern Palestine and July (FIG. 14). winds with easterly component in its To facilitate the study of the fol- northern portion. In Central and lowing maps of streamlines, FIG. Upper Egypt northerly winds are 11 introduces some of the important prevalent even in winter because of singularities of a field of flow. the convergence region southwest of 3. Streamlines of January.—The the Red Sea. streamlines of winter (FIG. 12) are U. Pressure distribution and stream- characterized by three main regions lines in summer.—The summer condi- 3The Gibraltar threshold is at a depth of tions (FIGS. 13, 14) are much simpler- about 1600 feet only, so that the cold bottom water of the Atlantic cannot penetrate into The movement of the air is then con- the Mediterranean basin. Therefore, the trolled by the Azores High over the bottom water of the Mediterranean Sea has a temperature of about 55° F which equals the North Atlantic too. In this season it winter temperature of the air above the sur- face. is farthest north and the pressure

Unauthenticated | Downloaded 10/08/21 10:28 PM UTC has fallen east and south of the Haifa (22° N), this northerly air cur- Mediterranean region. The continen- rent meets the big current starting tal high pressure has changed into a from the tropical high over the Sudan "low" while the High of the Azores and flows northeastwards to the Per- is intensified. A gradient, more or sian Gulf. less even, from west to east has devel- oped which causes NW and N winds over the entire region. (See FIG. 13, representing the pressure conditions and FIG. 14, the map of the stream- lines in July.) 5. The three main currents.—The west-east gradient results in three main currents which are fed by the Azores High and stream to the Ara- bic-Asiatic low. The moist northern current no. I (map 14) coming from NW touches the south coast of the Black Sea, crosses the Caspian Sea and the Karakum. An immense wind stream connects far Turkestan with the Atlantic Ocean. This first main current of the Mediterranean wind system also includes the Etesian winds of summer. These blow over Greece, the Aegean Sea and Asia Minor with an accentuated northern component as prevailing winds of great steadiness. They diverge from the first main cur- rent (I) (FIG. 14) and flow towards the second main current, (77) in the map. This represents the great con- vergence line passing along the Adri- atic coast, rounding Greece, and cross- FIG. 15. Isobars and prevailing winds at 8 a.m., Egypt; averages of July. (Reprod. from ing the Bay of Cyprus and Mesopota- "Climatological Normals"4). mia. Its "goal" is the low over the In the summer, resultant wind direc- Persian Gulf. tions with northerly components pre- The third large summer current, 777, vail along the entire Mediterranean follows the axis of the Mediterranean coast of Africa and penetrate far into Sea. Then it flows along the coast of the valley of the Nile, as before stated. Egypt, crosses Palestine, and ends too Thus Lower and Central Egypt receive in the Low over the Persian Gulf. northerly winds the year round. This summer current is the second line The survey of the distribution of of divergence and decisive for the cli- pressure and of the streamlines of the mate of the North African countries. Mediterranean and the adjacent re- Northerly winds branch off this line of gions was necessary for a deeper un- divergence into Algeria and Libya. A 4See the monthly maps of isobars and pre- big air current branches off towards vailing winds in "Climatological Normals for the Nile valley and a secondary Low Egypt and the Sudan, Candia, Cyprus and Abyssinia." Ministry of Public Works, Phys- over the Red Sea. South of Wadi ical Department, Cairo 1922.

Unauthenticated | Downloaded 10/08/21 10:28 PM UTC der standing of the climate in ques- heated land surfaces. The dew-point tion. The general distribution of is much decreased so that even in air pressure with a west-east gradient which must ascend a few thousand the year round is highly modified by feet no condensation occurs. FIG. 16 the thermal properties of the huge gives a significant example of the dry- water body. A trough of relatively ing effect of the hot land surface low pressure over the warm water upon the air masses which flow in- between cold land surfaces favors the land. invasion and formation of depressions which cause the winter rains of these regions. The northward-shifting, sub- tropical High joins the relatively high pressure above the cool water sur- face between hot lands in summer. 6. Stability of the atmosphere above the Mediterranean Sea in summer. The drying power of the land sur- face.—The maps of isobars and of streamlines show how northerly winds result for the greatest portion of the Mediterranean littoral of Africa in summer and these northerly winds penetrate deeply into Egypt. Pales- tine has westerly winds in summer which even have to ascend the west Jordanian highlands; nevertheless no FIG. 16. The variations of temperatures and rain falls there from May through dew point in July from the coast of Egypt September, and the October rain is southward. almost negligible at most places. The 1. Alexandria and Port Said, 31° N; 2. Qurashiya 30.8°; 3. Qasr el Gebali 29.3°; effect of these rains is more of a psy- 4. Assyut 27.2° ; 5. Esna 25.3° ; 6. Wadi Haifa chological value than a physical cool- 21.9°; 7. Merove 18.6°; 8. Atbara 17.7°; 9. Khartoum 15.6°; 10. Mongalla 5.2°. ing and wetting. As a matter of fact, nearly six months are practically Up to Wadi Haifa, about 700 miles rainless. In Jericho in the Jordan south of the coast, a clear and simple Valley, virtually no rain is observed fact is seen from the graph. The from March to October, as has been temperature rises nearly proportion- discussed before. ally with the distance from the coast Now the question arises: how is it by about 14 F° per 700 miles. The possible that winds which blow over a dew point is decreased by 25 F °. At sea surface for about two thousand the coast the dew point is 9 F° below miles, do not produce any rain, even the air temperature; 600 to 700 miles if they cross the coast? The main southward, the difference is 49 F°. reason for the lack of summer rains These data are illustrative of the is the stability of the lower atmos- cause of the rainlessness of the phere above the cool water surface Sahara. Approximately in 18° N combined with the anticyclonic ten- northerly winds turn eastward and dencies of the general pressure dis- meet the air masses laden with moist, tribution. This causes the northerly cooler air from the SW which origin- winds, which transport cool maritime ate from the inner tropics (see FIG. air masses towards the strongly 15).

Unauthenticated | Downloaded 10/08/21 10:28 PM UTC 140 BULLETIN AMERICAN METEOROLOGICAL SOCIETY [Vol. 24

III. SOME IMPORTANT CLIMATIC melted away like lumps of sugar".5 FEATURES AND DETAILS This disaster was the consequence of 1. Certain peculiar local winds of a rainfall of only 1.7 inches on Jan- the European coast are noteworthy. uary 17, 1919. —The depressions mentioned as well 3. Midwinter east winds in Pales- as the frequent secondary depressions tine.—It is above all the wind which of the Aegean Sea cause northerly makes the regional differences in cli- winds at their western side, which mate with the exception of the inner transport air masses from the cold tropical belt and some regions in the interior of the continent toward the other parts of the tropics. As far very warm Mediterranean Sea. Be- as Palestine is concerned it should cause these air masses have to de- be added that in winter there is a scend from highlands down to the spell of fair weather including the sea one should expect them to arrive second half of January and the begin- relatively warm at the coast, heated ning of February. In this short sea- by the foehn effect. The temperature son east winds are very frequent, gradient, however, is so great that caused by the Cyprus depression. On these winds appear as cold fall-winds the other hand, we know that there is as a result of super adiabatic gradi- a long summer period of west winds. ents. On the French coast these This shifting of the wind by nearly winds are called "Mistral", in the half the compass has caused some northern Adriatic Sea we find the authors to speak of a "Palestine Mon- name "Bora", and "Vardarac" at the soon." This expression is not justi- coast of the Gulf of Salonika. The fied. Cloudless skies and rainless analogy between Mistral and Varda- droughts are contradictory to the idea rac is striking. Also Novorossisk, on of a summer monsoon. The more so, the NE coast of the Black Sea, with as the east winds of the short winter the spur of the Caucausus as hinter- period just mentioned, do not appear land, has heavy bora winds. Each of so clearly in the observations at the these winds is characterized by high surface, but far more in those of the gustiness and frequently great in- upper winds. The layer of the pre- tensity so that they cause damage vailing west winds in summer has a nearly every year. Navigation at sea thickness of about 8000 feet. and in the air are endangered by them; High diversity of winter tem- the thickness of the cold layer is, how- peratures in the eastern basin.—The ever, only a few thousand feet. winter temperatures over the sea and 2. Floods in the Sahara.—As was in the coastal regions are delightfully said before, vast regions of North warm, as was said before (see FIG. 1). Africa are rainless the year round. The absolute extremes of winter However, it would be wrong to think temperatures vary within wide limits. that rain never occurs there. In the coastal regions of Egypt and Pictures and photos show that at Palestine highest individual tempera- these same places downpours occur tures reach 91° in winter. Minima which cause the more damage as no approach the freezing point. On the precautions have been taken against slopes of the Jordanian highlands the such very rare events. "There were temperature can fall somewhat be- boats in the streets of Cairo, the tram- low the freezing point. ways were sunk in mud to their win- 5. Summer temperatures in the dows and in the Manchiet el Sadr eastern basin, especially in North quarter, the houses of unbaked brick ®Gautier-Mayhew, "Sahara," p. 11.

Unauthenticated | Downloaded 10/08/21 10:28 PM UTC Africa.—The summer temperatures dously high temperatures, if one con- are exceedingly high, especially south siders that average maxima, exceed- and east of the Mediterranean Sea. ing 104° F occur everywhere even at These features are also clearly repre- the coast. sented in the foregoing maps (FIGS. 6. The world's highest temperature. 2 to 4). In July the isotherm of 79° Effect of desert winds strengthened runs along the African coast, turns by foehn effects.—The world's record to the north beyond the Nile Delta, highest temperature is held by the and returns westward at the latitude Mediterranean region, at Azizia (Asi- of Cyprus. The rapid increase of siah), Tripolitania (ca, 32Ms0 N, average summer temperatures in- 13%° E) September 13, 1922. This land in Egypt may be seen from FIG. temperature was: 17. 136°F (about 58°C) and was taken by an incontestable instrument and observer. Such enor- mously high temperatures are con- fined to the subtropical belt and can never occur either in the wet tropics nor in the temperate zones. This high temperature is the more remarkable, as this place is only 37 miles from the coast. The fact also may be emphasized that this peak temperature occurred nearly three months after the solstice. The sur- face of the soil in this region is dark so that the high absorption increases the surface temperature and conse- quently that of the air. In spite of this fact these very high temperatures appear to be a problem which can be explained by the geographical situa- tion only. The place is located in the Djefara plain, separated from the coast region by a chain of dunes. In the south of Azizia the highland rises in a steep scarp, 2000 feet high. The troughlike relief favors, more- over, the superheating of the lowest layers. Winds with southerly compo- FIG. 17. Isotherms (°C) of July for Egypt. nents have to descend this slope. The (Reproduced from "Climatological Normals."). temperature of these air masses is very The warmest month in Egypt, per- high because they cross huge areas haps also in other parts of North of the hot surface of the Sahara and Africa, is August, with 78.6° F in descending the 2000 feet they are adi- Alexandria (31.2° N), 84.9° in Suez, abatieally warmed a further 11 F°. (29.6° N), and 91.2'° in Aswan Thus, it is the "foehn" effect that (24.0° N). The absolute maxima in- explains the exceedingly high tem- crease from 111° at Alexandria (1901- perature in Azizia and also the 1934) to 122° at Aswan. Tremen- 113 °F which occurred simultaneously

Unauthenticated | Downloaded 10/08/21 10:28 PM UTC at the nearest part of the coast line. 9. Very low humidities.—A decisive 7. High surface temperatures.— and advantageous feature of the The high altitudes of the sun in the Mediterranean summer prevails also southern Mediterranean regions com- in Mesopotamia. In Babylon the aver- bined with cloudless skies and an ab- age monthly humidity is 13% in July solutely dry, often dark sandy soil, to September at 2 p.m. The average produce very high temperatures at monthly minimum is 5% from May the surface of the ground. In Pales- through October. In summer the wet tine 140 °F, in the dunes of the Sahara bulb is on the average about 36 F° 158°, in West Africa, 176 °F have lower than the dry bulb. been observed. 10. Sirocco.—On the occasion of From the standpoint of war climat- the Khamsin of April 2, 1922, the ology these high surface temperatures meteorological observer in Cairo re- can be highly consequential. "In the marks in his register: "I never met battle of Metarfa which was with a day like this special afternoon. fought in the dunes, the native sol- The hot wind blows. The sky turned diers, incapable of holding a prone yellow. The small stones are carried position for firing remained standing by the wind. Life is hated in such despite orders and were all killed".5 days." Also the winds with easterly 8. Daily range of temperature with components in Palestine influence examples from Mesopotamia.—The human beings in a bad way. It is very daily range of temperature is greatest significant when we read in the Bible: above the loose sandy and dry soil of (Jonah, IV.8) : "And it came to pass the deserts. The heat conductivity of when the sun did arise, that God pre- the soil is exceedingly small and the pared a vehement east wind; and the radiation is therefore used for heat- sun beat upon the head of Jonah that ing a very thin top layer to a high he fainted and wished in himself to degree, so that daily ranges of tem- die". Also in this respect no varia- perature, averaging 70 F ° and more, tion of climate has taken place from occur.6 These enormous differences ancient times till now in the eastern between the daily extremes make this Mediterranean regions. climate more bearable because the The winds of high temperature, low nights appear relatively cool in com- humidity, often laden with huge 7 parison with the very hot days. masses of dust are characteristic of 5Gautier-Mayhew: "Sahara." New York, North Africa from Algeria to Pales- 1935. tine. These winds are known under 6Dakhla Oasis, Egypt, 25° 29' N 64 F° in June. Babylon: August and September, 68 manifold names as: Khamsin,8 Si- F°: Mossul (36.3°N, 43.1°E) 76.5 F° (Babylon and Mossul, only one year's observations ; after moom, Samyel (Sam = Poison, Yel = Zistler). Wind) Ghibli, Chili, Dshavi, Chichili, 7Schlafli (Neue Denkschr. Schweiz. Ges. Naturw., vol. 20, 1864) reports in his excellent etc. description of the life in Baghdad that the last two hours before sunrise have a delightful In spite of the different names, coolness. "However this is very relative and these winds have the same character is felt only by people accustomed to the blazing heat of Baghdad because the average minima and origin in the south and east Medi- at sunrise do not fall below 81° to 79°F. . . . In the later afternoon a well marked change terranean regions: i.e., the desert, of temperature occurs which is longed for as especially the sand desert. There is a relief from the heat. . . . The open, well watered yard is now pleasantly cooled although the source region of the dry air the thermometer still shows 104° to 109°F." C. H. E. Ridpath, Symon's Met. Mag. 1919, p. 91, writes; ". ... by 8 a.m., for instance, 8An Arabic word meaning "Fifty" referring flying is no longer possible, as engines would to the period of 50 days after Shemel Nessim, seize up. Traveling in trains is forbidden (to the Coptic Easter Monday during which time military persons) after noon." these unpleasant winds are most frequent.

Unauthenticated | Downloaded 10/08/21 10:28 PM UTC FIG. 18. Cyclonic Winds in North Africa. masses of high temperature. illustrative of the conditions from These winds are caused by the de- Algeria to Palestine. pressions which move either along the The different names mentioned northern coast of Africa or cross the above, "Khamsin", "Simoom", etc., deserts from southwesterly to north- mean nothing more than the hot easterly directions. Hot dry air winds of the warm sector and may be masses from the interior of the des- summarized under the name of "Si- erts are transported northward and rocco" for the entire Mediterranean westward at the front sides of the region. depressions. The mechanism of the fronts in FIG. 18 shows a front analysis9 of these sirocco-depressions is not so a cyclone passing the theatre of war clearly developed as it might be ex- in Algeria. pected10 because of the very low dew The front between the invading points in consequence of which gen- cool, moist Mediterranean air masses erally no condensation takes place in and the warm, dry Saharan air the ascending warm air. (see p. 139) masses originating from the south, is This genuine sirocco should not be

9L. Petit jean, La frontologie en Afrique du 10See: J. Namias, "Air Mass and Isentropic Nord. Beitr. Physik d. freien Atmosphdre, vol. Analysis." Chapter V. Elements of Frontal XIX, 1932, p. 163. Structure—The Warm Front. 5th edit., 1940.

Unauthenticated | Downloaded 10/08/21 10:28 PM UTC confounded with a very moist and quent occurrence of dust devils.11,12 warm wind of the warm sectors A former commander of a British air of the Adriatic secondary depres- base in Mesopotamia wrote11: sions which is also called "sirocco", "During the summer, dust devils especially in the northern portion of may be seen every day. They travel the Adriatic Sea. with the wind and possess plenty of The former idea that the sirocco has energy and lifting power. They ex- always a descending component which tend many hundred feet into the air. causes extreme high temperatures A pilot in our squadron happened and low humidities by means of a once to fly over the top of a dust devil. foehn-effect, is not verified. At He was not lifted but fell 1000 feet Azizia, however, it was shown that so quickly that the machine barely the properties of the sirocco can be stood the strain when it suddenly intensified by a local foehn. encountered denser air again". The steep lapse rates of the Sahar- These dust devils as well as especi- an air masses strengthen the turbu- ally the great sand storms show in- lence by which great masses of dust tense atmospheric-electrical phenom- are lifted to considerable heights. ena. Thus it can be understood why The dust content varies directly with wireless communications are stopped the instability and the steepness of in such storms, a fact of great impor- the pressure gradient. The genuine tance for warfare. sirocco often crosses the Mediterra- 12. Bioclimatological remarks.—For nean Sea and is experienced on the the standpoint of bioclimatology the islands and in southern Spain, Sicily, Mediterranean region offers the great- and Greece. The enormous masses of est contrasts. On the one hand, the dust transported by these winds, are south coast of Europe from Gibraltar carried on toward higher latitudes by perhaps to Corfu, the islands, especi- other winds with southerly component. ally in the western basin, and Egypt, Saharan dust deposited by rain and the Nile up to Aswan, are longed for snow was observed even at the coast by invalids and infirm of Europe be- of the Baltic Sea. cause of their outstanding winter According to records of pilot- and health resorts. Every winter a mi- sounding-balloons, the thickness of the gration takes place of ten thousands sirocco layer itself is about 5500 feet of wealthy people to the Mediterra- to 7000 only. The upper air current nean coasts and islands. In summer, from the sea sets in perhaps at a on the other hand, the long duration height from 8000 to 8500 feet. of high temperatures (see the maps The frequencies (%) of sirocco FIGS. 2, 3, 4,) and the scarcity of winds and depressions in Egypt in the UC. H. E. Ridpath, Notes on the Climate of average course of the year are: Mesopotamia. Symons's Met. Mag.. 1919, p. 107. ^Dust devils are small whirls with a diame- ter of perhaps 30 ft. or more. They are Winter Spring Summer Fall generally smaller than waterspouts and "wind- spouts". The water spout of Cottage City Lows 42 39 3 15 (W. Va.) was photographed by L. N. Cham- berlain ; F. H. Bigelow found out the exact Siroccos 33 40 6 20 dimensions of the spout from this photo. The diameter of the cloud column was 236 feet, The frequencies of the two phenom- that of the water spray cloud of the foot 722 feet. The height of the column was 3610 ena are strongly correlated. feet. (M. W. R., 1906, p. 37). A trigonometrical measurement of the dia- 11. Dust devils. — Superadiabatic meter of a wind spout in Bavaria, Europe, re- gradients and very unstable surface sulted in 30 feet. The estimate of different diameters varies between 330 feet and 10. air layers explain some properties of The average of twelve wind and water spouts was 60 feet. (Alfred Wegener, Wind- und the sirocco. They also favor the fre- Wasserhosen in Europa. Braunschweig 1917)

Unauthenticated | Downloaded 10/08/21 10:28 PM UTC water are injurious to health of IV. CONCLUSION people from temperate climates. Long The scene of the Old and New Tes- lasting spells of sirocco and parti- tament is the Mediterranean region. cularly the end of it with rapid varia- Thorny bush and olive tree, palm and tions of temperature and humidity, wine played a great role there a few can favor diseases of heart and stom- thousand years ago. These plants ach. Malaria and some other diseases are still the leading flora of the Medi- of hot climates are endemic in some terranean region. This fact is not the regions. According to personal ex- only reason against the opinion, fre- perience of the author and particu- quently expressed, that great climatic larly that of medical authorities, changes have taken place in this part the greater part of these diseases is of the surface of the earth within the not of real climatic nature. They are last 2000 to 4000 years. perhaps promoted by the climate but The beauty of the landscape, the are, above all, a consequence of back- history and the climate of the Medi- ward civilization of the poor and terranean basin offer a mysterious uneducated population. A modern and mixture of joy and grief, of paradise strict hygienic discipline will prob- and hell, whence religions and cul- ably protect the army of strong young tures have developed,— cultures and men from earnest prejudices to civilization of the modern nations health. of Europe and America.

An optical hygrometer, Current been found by various trials and ob- Science, v. 10, no. 2, Feb., 1941, p. servations that the mixture of these 100.—Dr. L. D. Mahajan, Physics two powders has high power of ab- Research Laboratory, Mahendra Col- sorption and desorption of moisture. lege, Patiala writes:—"An optical Suitable weights are added into the hygrometer has been devised in this other pan till the beam is almost laboratory in order to study the vari- horizontal. ation in the humidity of the air. "A beam of light is thrown from a "To a zinc rod (balance beam), lamp and scale arrangement on the about 1 mm. thick and 10 cms. long, spherical mirror and the reflected a cup of zinc of 1 cm. square and 2 beam is allowed to fall somewhere mm. deep is fixed rigidly to each end. in the middle of a vertical scale at a In the middle of the beam, a revolv- distance of about 200 cms. from the ing rod of copper, about 1 mm. thick revolving instrument. The whole in- and 7 mms. long, is rigidly attached strument is placed inside a rectangu- at right angles to it. A small spher- lar glass vessel perforated with holes ical mirror of about 2 metres focal at the base to permit free circulation length is fixed to the revolving rod, of air inside it and to avoid any dis- just on one side of the junction of turbance to the revolving arrange- the revolving rod and the balance ment due to any direct and strong beam. currents of air. Then the instru- "The two ends of the revolving rod ment is ready for measurements. are made to rest on two fine, well pol- "With a small change in the hu- ished and equally levelled glass plates midity of the air, the spot of light fitted on a stand. One of the cups is moves through a great distance on the filled with a powder containing about vertical scale. The shift of the spot of 97 per cent, plaster of Paris and 3 light is proportional to the change in per cent, calcium chloride. It has the relative humidity of the air.

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