'Handbook of Mesopotamia. Vol. I. 1918' [‎30] (39/568)

CHAPTER II
CLIMATE
Introduction—Upper Mesopotamia—Lower Mesopotamia—Conditions affecting
aviat ion—Tab 1 es.
Introduction
The climatic conditions of Mesopotamia are those of a subtropical
area which lies at a distance from any ocean, and therefore are of
a semi-arid type, although an appreciable amount of rain falls in the
winter months. In the winter the atmospheric pressure is compara
tively high over northern Syria and Mesopotamia, since they lie on
the outskirts of the great high-pressure system of central Asia ; and,
while the air-currents at this season are somewhat variable in their
directions, winds from the north-west predominate in all parts of
the country.
Eain occurs during the passage of cyclonic depressions, some of
which come from the eastern Mediterranean, and others probably
pass across Asia Minor, but at other times the north-westerly winds
descending from the high plateau to the northward arrive at the low-
lying JezTreh as dry and comparatively warm winds. In summer
this effect is ipore strongly marked, and these north-westerly winds,
warmed by their descent from the plateau and in their passage to
lower latitudes, sweep over the valley of the Tigris and Euphrates as
hot, dry winds, which blow fairly continuously from May until
October. At this season the great low-pressure area of north-west
India, which is related to the monsoon of the Indian Ocean, extends
to the Persian Gulf The historical term used to describe the body of water between the Arabian Peninsula and Iran. , and the pressure gradient which exists from the
eastern Mediterranean to the Persian Gulf The historical term used to describe the body of water between the Arabian Peninsula and Iran. maintains this flow of air
over Mesopotamia throughout these months.
Consequently the land depends on the waters of its two great rivers
for its fertility, since the rainfall alone is insufficient to maintain
vegetation through the summer. Drawing their supply largely from
the snowfall in the mountains of Armenia, the levels of the Tigris
and Euphrates increase in the spring months, and begin to diminish