Page 158 - Arabian Studies (II)

150 Arabian Studies II

greater than those occurring over the Jiri Plain (less than 3°). The
surface of the Jiri Plain has become smoothed by aeolian activity
that has prevailed since the sub-pluvial period, accompanied by
limited sheet-flow. The sand dunes have further encroached, with the
result that the finest outwash deposits are now entirely covered by
sand dunes. The Jiri Plain thus consists almost entirely of the coarse-
and medium-textured outwash deposits, and the restricted areas of
heavy textured material found along the western edge of the plain
reflects contemporary deposition by limited sheet flow across the
plain.
North of Digdaga and Fahlain, the area between the outwash fans,
formed during the sub-pluvial period, and the sand dunes is
ill-drained and made up of heavy textured sediments, silts and clays.
These deposits are both saline and alkaline, and reflect a former
inland extension of the Ras al-Khaimah lagoon which is now only a
fraction of its former size.
This part of the Arabian Peninsula has a hot arid desert climate
with high summer temperatures and warm winters with some rainfall.
There are, however, no long term meteorological records for Ras
al-Khaimah — a rain gauge was only installed at Digdaga Agricultural
Station in 1959 with additional equipment being added in the
mid-1960s. However, at Sharjah, some 75 km. from Ras al-Khaimah
but also on the Arabian Gulf coast, a full range of meteorological
observations have been kept since 1950, while rainfall records date
back to 1934.
The coldest months at Sharjah are December and January with
mean minimum temperatures of about 7°C., while July and August
are the hottest months with mean maximum temperatures of about
43°C. Temperatures below 0°C. have not been recorded at Sharjah
but the station at Digdaga, about 12 km. inland from Ras al-
Khaimah, first recorded a temperature of —0.6°C. in February 1967.
With increasing distance from the sea, the frequency of occurrence of
such temperatures will undoubtedly increase. A consequence of the
high summer temperatures is that the Arabian Gulf is noted for its
high humidities, though it has been suggested (Sir W. Halcrow and
Partners, 1969) that the highest relative humidities occur during the
winter months. Nonetheless, high summer humidities can make
summer working conditions very difficult especially during the
months May to October inclusive. There is also a high rate of
evaporation during the summer months and a U.S. Weather Bureau
Class A pan was installed at Digdaga in 1966. The average daily
evaporation rates (in mm.) over the period October 1966 -
September 1969 have been as follows: