"العراق والخليج الفارسي" [و‎‎١‎٣‎٠] (٨٦٢/٢٦٤)

CLIMATE
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variable in direction, but few winter dust-storms affect visibility for
any distance down the Persian Gulf. Rainfall during this period
much reduces the frequency of dust-storms, and rain together with
spring vegetation probably accounts for the low frequency of winter
dust-storms in the north.
Summer dust-storms appear to be caused by small variations in the
pressure gradient, particularly during the onset of the Shamal(y. 170),
and all summer dust-storms come from the north-west quarter.
The storms tend to increase in frequency and severity towards the
south-east, because of the increasing ‘fetch’ of the prevailing north
west wind in summer, and dust may be carried down the Persian
Gulf at this season, affecting visibility at sea. It may rise to over
10,000 feet, and from the air looks like a dense bank of cumulus,
visible 30 miles away.
In summer the first sign of a dust-storm is usually an abnormal
glare or dazzle round the sun, with dust rising in patches from the
ground, so that decrease in visibility is progressive. In winter,
particularly when associated with thunder-storms, the reduction of
visibility is much more sudden and the onset more violent; but the
severity of dust-storms is dependent on a number of factors. Thus in
central and southern Iraq it appears to be proportional to the wind
velocity at about 1,000 feet, and differs according to the time of day
at which the storm starts.
The severest storms occur in northern Iraq in October, in central
Iraq in March, and in southern Iraq in June and July. The months
of maximum frequency are May in northern Iraq and July in
central and southern Iraq (though they are also common here from
February to June). In the western desert (Rutba) they are sur
prisingly infrequent throughout the year; the reason can only be that
the desert surface is harder, more stony, and more difficult to break
up into fine particles of dust.
A curious and noteworthy feature is that dust-storms in Iraq
often occur about the same date in successive years; for instance,
a dust-storm or thick dust-haze was recorded at Hinaidi on 29 March
every year from 1929 to 1935, that in 1935 being one of the most
severe ever recorded. During this period the dates of maximum and
minimum dust-storm frequency between March and August were:
Maximum frequency . . 29, 30 March
11, 21 April
23 June
3 July (Shuaiba 3, 6, 13, 21, 25, 31)