Climate

Low rainfall and high temperatures characterize the climate of the UAE. The northern Emirates traditionally receive more rainfall. The long-term average at Sharjah (where records go back to the 1930) is 95.8 mm annually, and for parts of Fujairah is over 150 mm. Average annual rainfall for Al Ain during 1966-1979 was 77.6 mm. Enormous damage may be caused by heavy winter and spring storms, not only to property but also to vegetation. On March 29th 1987 Shwayb dam in the Madam valley burst and floodwater spilled into the desert to the north and east over several square kilometres, destroying all Acacia tortilis and Calotropis procera trees in its path and leveling the dunes. A vast tract of Hammada elegans (= Haloxylon salicornicum) and Calligonum comosum shrubs disappeared entirely (Western 1989). Absolute maximum temperatures rise to 49°C on the Arabian Gulf coast in July (Table 1). In January temperatures can be as low as 5°C, though this is rare on the coast because of the moderating influence of the sea. The mean maximum for Abu Dhabi in July is 40.1°C, and the mean minimum in August is 32.2°C. As with rainfall, there can be break temperatures too. Between 8th and 10th July 1987 temperatures reached 52°C on the Fujairah coast, causing extensive damage to fruit plantations there. The mangroves at Khor Kalba were badly scorched for the first time in living memory. Most precipitation occurs between December and April, though inland the odd summer shower along the mountainous border with Oman is not uncommon (Table 2). The temporary freshening effect of summer showers is soon dissipated by heat and evaporation, but such rainfall is important for the germination of some species, such as Tribulus. Winter and spring rains are of the cold front type, although essential for aquifer recharge and to dampen the soil surface, the main catalyst for germination of ephemerals would appear to be a rise in mean temperature, which may not occur until several weeks after the most recent rainfall. Mean annual relative humidity is over 60% for Abu Dhabi, with winter months generally over 70%. Diurnal means, display great variability. Foggy days, i.e. with rising sand, are recorded in all months. In March 1983 five such days were recorded at the new Abu Dhabi Airport (visibility less than 1000 m), but in April that year only one. In summer there is a high incidence of suspended dust throughout the country brought by the prevailing wind from the head of the Arabian Gulf. The dust haze remains in circulation because of the barrier effect of the Hajjar Mountains. Visibility is less than 8000 m for half the summer; August 1984 witnessed 26 days of restricted visibility at the new Abu Dhabi Airport. Local fogs are frequent in the early hours of the morning inland of the sabkhas. The radiative cooling effect around Al-Dafrah, south of Abu Dhabi, causes moist air in that region brought in by afternoon sea breezes to condense and form fog. Cold air valleys between the dunes enhance this surface condensation effect and dense fogs may result which can penetrate to the coast. As solar radiation is weak in winter, it takes some time to 'burn off this fog. Advective fogs also occur, when air of high dew point over the sea moves landwards where it is forced to rise over the cooler and denser desert air. Such fog blankets may extend inland for up to 100 km. Dewfall is the only fairly regular source of moisture for plants, since fogs and rains are mostly restricted to the winter and spring months. Dew does not occur every night, but it is a feature of a large number of nights each month, and in the summer constitutes the only source of surface moisture over much of the desert. Little data has been collected within the UAE, but the results of experiments conducted by a Royal Geographical Society team in the Wahiba Sands south of Muscat in Oman in early 1986 indicate that dewfall is a most important contributor of surface moisture. The persistent summer wind is the northwest shamal, which occurs as a result of two circulating pressure centres, a low one over Iran and Afghanistan and a high one over Saudi Arabia. The Gulf in between acts as a conduit for enhanced winds from surface level up to 5000 feet, and it is this force, which brings so much dust haze into the country. The winds generally drop during the night. In the winter winds are more variable but velocities are raised during storms caused by low pressure and passing jet streams. Gusts of up to 120 kph were recorded in Abu Dhabi on 27th March 1987. Mean daily sunshine for the year is 10.3 hours at the new Abu Dhabi Airport (1985 figures) with a maximum daily mean of 11.4 hours in June and a daily mean of 8.4 hours in January. The world map of desertification (UNESCO 1977) classified the UAE into three main bioclimatic zones based on the ratio between precipitation and evapotranspiration (P/E). These zones are the hyperarid zone (P/E <0.03), arid zone (P/E = 0.03 - 0.20) and semi-arid zone (P/E = 0.2 - 0.5). Within the semi-arid zone there is a sub humid mountainous area that receives a higher amount of rainfall (Fig. 3) and its temperature is usually lower than that the surrounding semi-arid zone (Satchell 1978).