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.