"كتيّب بلاد الرافدين. المجلد I. ١٩١٨" [‎٣‎٩] (٥٦٨/٤٨)

CLIMATE
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during the hot months. From light airs at sunrise the wind
increases to a moderate breeze by about 10 a.m., and by 2 p.m. to
4 p.m. has become a fresh or even a strong breeze. It is strong
enough to raise dust and even sand, so that in the afternoon the
horizon is usually obscured. About sunset the wind drops, to rise
again an hour or two later as a light breeze, which may continue
during the night, falling to a calm before sunrise.
The fresh northerly wind {shamal) which blows in summer mitigates
to some extent the great heat, and contributes to the decrease of the
annual floods in lower Mesopotamia. The main period of the
summer shamal is about 20-30 days in length, but it is usually
interrupted by a few short breaks. This main period begins as
a rule in the first half or middle of June, and is then followed by
a few short periods in July and August. But in 1916 the main
period began extraordinarily late (July 21).
Gales are said to be rare, but probably high winds occur when
depressions pass over the country in the rainy season. In the
summer the afternoon wind is said occasionally to reach gale force,
but this seems to be exceptional.
Sandstorms
Sandstorms are most common in the spring months. They are
strong winds carrying dust and sand like the of Egypt.
They are preceded by dull weather of great and oppressive heat and
usually by a light wind which grows in force until the sand-cloud
which it has raised approaches with great rapidity. During the
passage of the sand-cloud, which may last for some hours, the wind
often blows with extreme violence.
Conditions affecting Aviation
Density. —The density of the air in Mesopotamia has been computed
for four months of the year, including those of mid-winter and mid
summer, and the results are given in the following table:
Table op Density in ( t Rammes per Cubic Metre
Place,
January.
April,
July.
October.
Mosul ....
1,233
1,170
1,089
1,149
Baghdad
1,254
1,194
1,127
1,172
Babylon
1,260
1,166
1,121
1,177
Basra ....
1,250
1,185
1,130
1,176
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