'IRAQ AND THE PERSIAN GULF' [‎130v] (265/862)

l8o CLIMATE, VEGETATION, AND FAUNA
Minimum frequency . . 13, 15, 28 March
23, 29 April
5, 10, 16 May
7 June
9, 10 July
The time of start of dust-storms is largely influenced by diurnal
changes of wind speed (p. 171). They normally start about 8 a.m. in
summer and about 10 a.m. in winter, that is, about three hours after
sunrise. They generally end in the afternoon, but occasionally persist
overnight, especially in March and July in central and south Iraq.
Between March and June they are liable to begin at night, and are
then almost always associated with thunder-storms.
Dust-devils are among the minor hot-weather manifestations. They
are very common and occur with light winds at any time of the
day. They are rapidly moving spirals of dust, originating in a
whirling dust cloud near the ground and curling upwards to thin out
to a diffuse dust cloud 600-700 feet above ground. They often
appear and vanish quite suddenly, and though locally violent do
little if any damage, and so are not recorded.
Visibility (Table VIII)
Mirage is the chief obstacle to visibility in Iraq. In all hot countries,
where the ground is raised to a high temperature, the air in contact
with it is much hotter than that a little distance above the ground, so
that the density of the atmosphere rapidly changes from the ground
upwards. This change in atmospheric density bends and distorts the
line of sight, and any object above the horizon of the eye appears
below its true position. A common form of mirage is the appearance
of water in a waterless desert, caused by an apparent lowering of the
sky below the horizon, the distortion and convection currents giving
to it the semblance of ripples. The actual horizon becomes very much
distorted, and the outline of hills, rocks, and other objects may be
completely altered or lost altogether. Objects may be distorted and
magnified through convection currents, some appearing nearer, and
all kinds of fantasies play tricks with the sight and with the imagina
tion. Iraq is particularly bad for mirage and refraction difficulties,
and in the War of 1914-1918 many strange incidents were reported.
At the battle of Shuaiba (p. 275), for instance, the Turks are said to
have altered their tactics to meet the threat of British reinforcements
seen through the mirage, though these were non-existent. Mirage is
absent in the early morning before the earth has heated up, but