With its magnificent deserts, amazing architecture and beautiful waterfronts, the UAE offers photographers scenes that few other nations can match. Perhaps the most striking of the many images of the Emirates are those that combine the country’s scenery with a natural phenomenon: early-morning fog. In particular, photographs showing the skyscrapers of Sheikh Zayed Road in Dubai emerging above the fog into clear air are truly iconic and have attracted attention around the globe. But what exactly creates this incredible sight, which tends to happen later in the year, as the weather cools? In the colder months, after a clear and calm night, the air is likely to be moist thanks to winds that blow in from the Gulf during the day. The ground, however, will have lost much of the heat absorbed the previous day, a process called radiative cooling, which in turns cools the air above it. In the early morning, the air temperature at ground level may be about 20°C, which may not be cold enough for fog to form (although on some occasions it will be cool enough and ground-level mist or fog will form). Further up, perhaps at 500 or 1,000 feet, the temperature may be 16°C or 17°C, which is cool enough for the water vap our in the air to condense into low cloud. Go higher still, however, and the air may actually become warmer — something known as a temperature inversion — because it has not been subject to radiative cooling in the same way that air lower down has been. So the cloud or fog does not stretch far up, making the upper sections of the skyscrapers visible in the clear air. While spectacular to witness, this effect is short-lived because, as the sun rises, it warms the air and the fog or cloud vanishes relatively quickly. A major reason is the UAE’s latitude, with Abu Dhabi and Dubai sitting about 25 degrees north of the equator. Here, high pressure weather systems predominate. To understand why, it is easiest first to consider conditions at the equator, which experiences the greatest amount of direct sunlight in the world. Here, the sun heats the air and causes it to rise. “In the tropics, there are large regions where the air is rising,” says Prof Sandrine Bony, director of research at the French National Centre for Scientific Research (CNRS). “The water vapour cools down when it’s ascending, causing it to condense and form clouds.” This heavy cloud cover frequently results in rain, so equatorial regions are often some of the wettest on earth. By contrast, at the subtropical latitudes, which lie between 23.5 and 40 degrees north and south of the equator, high pressure is more typical, in part because of wind patterns created by the rising air near the equator. “As the air rises in the tropics, it starts being transported away from the tropics at higher latitudes,” says Prof David Schultz, professor of synoptic meteorology at the University of Manchester in the UK. At subtropical latitudes, the air tends to descend, warming and meaning that, as Prof Schultz describes, it “is not favourable for the creation of deep, fluffy clouds”. So heavy cloud cover in the UAE is rare. “Many of the other deserts are in the subtropical latitudes, like Australia, the deserts of China, the deserts of North America. They’re devoid of lots of cloud because the air is descending,” says Prof Schultz. While global factors have a significant influence on the UAE’s climate, and are a key reason why cloud cover and rainfall in the Emirates is low, local elements also play a role. As described in a 2019 scientific paper, "Total Cloud Cover Climatology Over the United Arab Emirates", published in the journal <i>Atmospheric Science Letters</i>, Fujairah has the UAE’s highest average total cloud cover (TCC), the proportion of the sky covered by clouds. This is in part because of its proximity to the Hajar mountains, which cause air from the sea to rise, expand and cool, leading the water vapour to condense and form clouds and rain. “The land-sea temperature difference, along with the presence of Al Hajar mountain chain along the eastern coast of the UAE, significantly influences TCC in Fujairah, leading to orographic [mountain-related] cloud formations,” wrote the authors, who were from Khalifa University and the National Centre of Meteorology, both in Abu Dhabi. They said Fujairah’s vegetation also influenced upward air movements in a way that increased the emirate’s rainfall and cloudiness. “Fujairah also lies on the coast of the Gulf of Oman, making it more susceptible to the effects of the Indian Monsoon through proximity to the Arabian Sea,” the authors added. Al Ain had the next highest average TCC, while the lowest figures were recorded in Sharjah and Abu Dhabi. Regional geography explains why India, for example, experiences a monsoon from June to September, but the UAE — which sits roughly in the middle of India in terms of latitude — does not. In summer the Thar desert in north-west India heats up, forming a low pressure area that draws in moist air from the Indian Ocean, which lies south of the subcontinent. This moisture ultimately falls as the monsoon rains. South of the Arabian peninsula, however, the African continent occupies much of the space, so there is no vast ocean providing moist air of the kind that sits south of India. As a result, the same pattern, of moist air being drawn north onto land, does not produce the same results, although there are a few places on the Arabian peninsula, such as Salalah in Oman, that are famous for experiencing a monsoon season. “There’s not going to be a lot of moisture from the south,” says Prof Schultz. “There will be some, but not as dramatic as the air being flowed into India.”
