'IRAQ AND THE PERSIAN GULF' [‎127v] (259/862)

174 CLIMATE, VEGETATION, AND FAUNA
The Persian Gulf The historical term used to describe the body of water between the Arabian Peninsula and Iran. and its coasts are notorious for their ‘damp’ heat
in summer. At nearly all its ports the wet-bulb temperature exceeds
78° from June to September, both morning and afternoon. At
Bahrein on August afternoons it averages as much as 87°, and when
the dry-bulb (air) temperature is between 95 0 and ioo° the wet-bulb
thermometer may read only two degrees lower, especially if the wind
drops to a calm. The oppressive character of this summer climate is
shown also by the relative humidity, which averages 70 per cent, with
little diurnal variation. As an example of the effects of this oppressive
heat may be mentioned the experience of a troopship bound for Basra
at the end of August 1917, when the sea-temperature was near blood-
heat and the atmosphere nearly saturated. In consequence, there were
many cases of heat-stroke, the number of deaths being increased by
the failure in the supply of ice.
Conditions inland begin to change almost at once. At Shuaiba,
south of Basra, the mean relative humidity for the months from
June to September is as low as 27 per cent., but there is a fairly wide
day-to-day variation from the monthly means. On some days the air
is almost completely dry, a condition brought about by a strong
wind from the desert quarter; on others the air may feel ‘damp’ for
some days on end, when it is calm and the air has picked up moisture
from the marshes. The river front at Basra and the airport may be as
much as 6 degrees cooler than Shuaiba, but much ‘damper’ (Table
IV). 1 It is at such periods that there is danger of heat-stroke because,
though the relative humidity may not rise above 50 per cent., very
high air temperatures are recorded (p. 172) and the wet-bulb tempera
ture approaches blood-heat. In such a climate a comparatively low
relative humidity may have a ‘damper’ significance on a calm day
than the higher monsoon figures of Bombay and Calcutta (85 to 95%). 2
The important point is the difference between the amount of vapour
actually present in the air (that is, the absolute humidity) and the
amount contained by saturated air at body-temperature. This
difference is the physiological saturation deficit. The smaller this
deficit, the less vigorous the evaporation from skin and lungs, and
the ‘damper’ and the more ‘oppressive’ the atmosphere feels. If the
air is stagnant, the local deficit in contact with the body becomes
1 The figures in Table IV are not strictly comparable for this argument, but they
are significant of the general conditions.
2 Vapour pressure at no 0 F. with relative humidity at 50% is about 46 mb. and
is representative of the worst conditions at Basra. Vapour pressure at 90° F. with
relative humidity at 90% is about 43 mb. and is representative of the steamy heat of
Bombay in monsoon breaks.