Straddling the Tropic of Cancer, the UAE is warm and sunny in winter and hot and humid during the summer months. Winter daytime temperatures average a very pleasant 26°C, although nights can be relatively cool, between 12–15°C on the coast, and less than 5°C in the depths of the desert or high in the mountains. Local north-westerly winds (shamal) frequently develop during the winter, bringing cooler windy conditions. Summer temperatures are in the mid-40s, but can be higher inland. Humidity in coastal areas averages between 50 and 60 per cent, touching over 90 per cent in summer and autumn. Inland it is far less humid.
Rainfall is sparse and intermittent. In most years it rains during the winter months, usually in February or March, but occasionally earlier. Winter rains take the form of short sharp bursts, which, if occurring in the Hajar Mountains, run off rapidly into wadis and onto the downwashed gravel plains. Localised thunderstorms occasionally occur during the summer. Generally appearing over the mountains of the south and east of the country, these rumbling cloudbursts can give rise to severe flash floods.
Some years are totally dry and it is only through the regular formation of dew that vegetation and wildlife can survive. This applies even to those places that experience a relatively high annual rainfall: at the Hajar Mountain town of Masafi, for example, 350 mm may fall in a ‘wet’ year, whereas as little as 30 mm may be recorded in a ‘dry’ year.