ABSTRACT
Aridity is basically defi ned as a lack of moisture and is therefore based upon a number of climatic variables. The exact boundary of the arid zone has long been a source of contention but in 1977 the United Nations Educational, Scientifi c and Cultural Organisation (UNESCO) published a map which is now accepted as standard (Map 21). The boundaries illustrated enclose both the arid and the hyperarid regions and include all the areas within which the mean annual precipitation divided by the mean annual potential evapotranspiration (both in mm) is less than 0.2. The main areas of hyperaridity are in North Africa and the Arabian Peninsula and the map confi rms the fact that the Middle East is the major area of global aridity. As can be seen, from the map, it is not the only area of aridity but it is the most extreme for any area with such a relatively high population and such a crucial strategic resource as oil. The climate of the entire region, with the exception of the mountainous areas of the Maghrib, Turkey, Iran and Afghanistan together with southern Sudan, falls within the UNESCO classifi cation of arid.
