Roughly 90 percent of the annual rainfall occurs between November
and April, most of it in the winter months from December through
March. The remaining six months, particularly the hottest ones
of June, July, and August, are dry.
Except in the north and northeast, mean annual rainfall ranges
between ten and seventeen centimeters. Data available from stations
in the foothills and steppes south and southwest of the mountains
suggest mean annual rainfall between thirty-two and fifty-seven
centimeters for that area. Rainfall in the mountains is more abundant
and may reach 100 centimeters a year in some places, but the terrain
precludes extensive cultivation. Cultivation on nonirrigated land
is limited essentially to the mountain valleys, foothills, and
steppes, which have thirty or more centimeters of rainfall annually.
Even in this zone, however, only one crop a year can be grown,
and shortages of rain have often led to crop failures.
Mean minimum temperatures in the winter range from near freezing
(just before dawn) in the northern and northeastern foothills
and the western desert to 2o-3° C and 4o-5° C in the alluvial
plains of southern Iraq. They rise to a mean maximum of about
15.5° C in the western desert and the northeast, and 16.6°
C in the south. In the summer mean minimum temperatures range
from about 22.2° C to about 29° C and rise to maximums
between roughly 37.7o and 43.3° C. Temperatures sometimes
fall below freezing and have fallen as low as -14.4° C at
Ar Rutbah in the western desert. They are more likely, however,
to go over 46° C in the summer months, and several stations
have records of over 48° C.
The summer months are marked by two kinds of wind phenomena.
The southern and southeasterly sharqi, a dry, dusty wind
with occasional gusts of eighty kilometers an hour, occurs from
April to early June and again from late September through November.
It may last for a day at the beginning and end of the season but
for several days at other times. This wind is often accompanied
by violent duststorms that may rise to heights of several thousand
meters and close airports for brief periods. From mid-June to
mid-September the prevailing wind, called the shamal,
is from the north and northwest. It is a steady wind, absent only
occasionally during this period. The very dry air brought by this
shamal permits intensive sun heating of the land surface,
but the breeze has some cooling effect.
The combination of rain shortage and extreme heat makes much
of Iraq a desert. Because of very high rates of evaporation, soil
and plants rapidly lose the little moisture obtained from the
rain, and vegetation could not survive without extensive irrigation.
Some areas, however, although arid do have natural vegetation
in contrast to the desert. For example, in the Zagros Mountains
in northeastern Iraq there is permanent vegetation, such as oak
trees, and date palms are found in the south.
Data as of May 1988