CLIMATE

middle eastern climatic conditions vary greatly, depending on the season and the geography.

The Middle East and North Africa are perceived as both homogeneous and intensely arid, but the region is best characterized by its climatic variation. Although the hot arid, or desert, climate predominates in the region, the well-watered highlands of Turkey and the mountains of Iran and Ethiopia are important as sources of the region's major rivers. Climatic variation finds further expression in the temperature regimes of the northern and southern parts of the area. Average July maxima for inland


locations near 30° north latitude are as high as 108°F (42°C), while summer maximum temperatures in northern locations such as Ankara, Turkey, do not exceed 86°F (30°C). Black Sea coastal stations' (e.g., Trabzon, Turkey) average summer maxima may be as low as 79°F (26°C). January average minimum temperatures fall to 50°F (10°C) in Aswan, but reach 10°F (—12.5°C) in Erzurum on the Anatolian plateau.

Desert conditions are primarily the result of the subtropical zone of high pressure that coincides with 30° north latitude. In this area, cold, subsiding air warms as it approaches the earth, thus increasing its ability to hold moisture. This results in extreme evaporation from all surfaces, and under such conditions, very little rain falls. During the summer solstice, the sun is directly overhead at 23° 30′ at north latitude (e.g., at Aswan, Egypt). Annual periods of high sun in combination with clear skies through much of the year allow intense solar radiation with subsequent extreme evapotranspiration demands.

Evapotranspiration refers to the water needed by vegetation to withstand the energy of incoming solar radiation. This is accomplished through the mechanism of heat transfer by means of evaporation from inert surfaces and transpiration from stomata (pores) on leaf surfaces. Total demands made upon an individual plant are termed potential evapotranspiration (PE). Actual evapotranspi-ration (AE) is the amount of water actually available and used by the plant and reflects climatic conditions rather than optimal plant requirements. The difference between PE and AE defines the degree of aridity or drought and also the amount of irrigation water that would have to be applied for such vegetation to survive.

In the deserts of North Africa and Southwest Africa, total annual precipitation is between 2 inches (50 mm) and 14 inches (350 mm). The area from Aden to Baghdad receives from less than 2 inches (50 mm) annually to about 6 inches (150 mm). More than 39 inches (1 m) of water would be required in those places to sustain rain-fed agriculture. Under such conditions, sparse natural vegetation allows animals some seasonal grazing at best. Hyperarid areas, which seldom if ever receive rain, have no vegetation at all. Rainfall variability within the area of desert climate exceeds 40 percent, reducing to 20 percent on the moist margins of the semiarid zone, which forms a transition between the true desert to the south and the more humid areas farther north.

Precipitation on the semiarid margins of Middle Eastern deserts ranges from 14 inches (350 mm) to 30 inches (750 mm) annually. Dry farming of grains employing alternate years of fallow can be carried out with 16 inches (400 mm) or more of rain. It should be remembered that, while rainfall variability is greatest in the desert, this also means that aridity there has high predictability. Thus, the semiarid transition between regions of predictable aridity and predictable rainfall is one where rain-fed agriculture is possible but has a high chance of failure. This is biblical country—years of plenty followed by years of famine—and one to which pastoral nomadism was a practical adaptation.

The Black Sea coast of Turkey receives from 78 inches (2,000 mm) to 101 inches (2,600 mm) per year, although the transition from the windward, watered side of the Pontic range to the leeward, dry side can be very abrupt due to the topography. The Mediterranean climate, which is limited to a narrow coastal strip reaching from Gaza to Istanbul and from Tunis in the west to the Atlantic, is marked by mild winters with ample rain and long, hot summers when Sahara-like conditions prevail.

Precipitation results from three different processes. Orographic precipitation occurs on the Pontic and Taurus mountains of Turkey; the Elburz and Zagros mountains of Iran; the peaks of Lebanon and the hills of Israel, the West Bank, and Jordan; the highlands of Ethiopia; and the Atlas and Anti-Atlas mountains of northwest Africa. Such precipitation occurs as warm, moisture-bearing winds are forced to higher elevations over the mountains. When the air cools, it loses its ability to hold moisture, and rain or snow falls on the wind-ward sides of those ranges.

The Anatolian plateau and the steppes of northern Syria experience small quantities of rain in the form of convectional summer showers from thunderstorms. Equatorial convectional rains provide the waters of the White Nile.

A third cause of precipitation, particularly in the wintertime, is the passage of frontal systems from west to east across the region bringing alternating high and low pressure cells with associated cold, clear, or moist warm air masses. Frontal systems are propelled eastward by the subtropical jet stream, the position of which varies latitudinally by as much as 15° from a winter position in the north to its summer position in the south. Summer months find the path of the jet stream located from central Turkey northeastward to central Asia. Six months later the jet stream is at its maximum along a path traced across the Gulf of Suez to the head of the Gulf of Aqaba and beyond. This shift accounts for the changes in temperature and precipitation noted above.

Surface winds in the Middle East have distinctive qualities and have received local names famous throughout the region. The cold northern wind blowing from the Anatolian plateau to the southern Turkish shore in the winter is the Poyraz (derived from the Greek: bora, i.e., north); the warm on-shore wind in the same location is known as the meltem. Searing desert winds are infamous: The Egyptian khamsin, which blows in from the desert, is matched by the ghibli in Libya and the simoon in Iran.

Bibliography

Beaumont, Peter; Blake, Gerald H.; and Wagstaff, J. Malcolm. The Middle East: A Geographical Study, 2d edition. New York: Halsted Press, 1988.

Blake, Gerald; Dewdney, John; and Mitchell, Jonathan. The Cambridge Atlas of the Middle East and North Africa. Cambridge, U.K., and New York: Cambridge University Press, 1987.

Goudie, Andrew, and Wilkinson, Jon. The Warm Desert Environment. Cambridge, U.K., and New York: Cambridge University Press, 1977.

Grigg, David. The Harsh Lands: A Study in Agricultural Development. London: Macmillan; New York: St. Martin's, 1970.

John F. Kolars