The research also found, however, that the fog that forms tends to be less dense, possibly because urbanisation has — against expectations — reduced the amount of particulate matter in the air.
Fog causes major disruption in the UAE, particularly during the winter months, affecting airline schedules and causing significant hazards on the roads.
In highlighting how large-scale weather systems outside the region influence fog formation in the Emirates, their research could help forecasters predict when long-lasting fog events will occur.
They analysed data for the Emirates over several decades and found that periods when fog is seen on multiple days have become more common and tend to last longer. However, these events are now on average less intense, with visibility not hampered quite as much.
“We think that the increase [in fog events] can be due to the increase of the water vapour content in the atmosphere due to global warming,” said Dr Diana Francis, head of the ENGEOS lab.
“We know that a warmer atmosphere can hold a larger amount of water vapour than a colder one.”
Meteorological records showed that between January 1983 and August 2021 there were 102 fog events in the UAE lasting two or more days.
During the day, the sea breeze from the Arabian Gulf brings moist air inland and fog forms because of a steep temperature gradient between the air near the ground, which is cooler, and the atmosphere above it, which is warmer. Cooler temperatures cause the water vapour in the air close to the ground to condense and form fog.
“The steepest difference in temperature is seen in the UAE during winter, when the surface cools down much faster than the atmosphere above it after sunset,” she said.
This cooling of the land surface is typical for desert regions, where the sand loses heat much faster than air masses do.
As well as increased water vapour in the atmosphere, changes in weather circulation in the region and the northern hemisphere more widely explain the increase in the number of fog events, the researchers found.
These changes relate to complex patterns of alternating high pressure and low pressure systems from outside the Middle East. The new study is the first to identify how these influence fog formation over the UAE.
“The findings of this work will aid in the forecast of fog in the region, as the link to larger-scale [weather systems] had not been made,” the researchers wrote.
The “positive phase” of three major weather systems was found to make fog more likely. As an example, one of these systems is the East Atlantic/Western Russia teleconnection pattern, the positive phase of which means there is high pressure over western Europe and low pressure over western Russia.
Also affecting fog formation are tiny particles or aerosols in the air, as they act as “condensation nuclei”.
Over the four decades covered by the research, the fog has become less optically thick, meaning that more sunlight can pass through it than before.
This reduction in thickness could be due to a decrease in the amount of aerosols in the atmosphere in the UAE in recent years, Dr Francis said.
“There is a decline in the load of aerosols since 2009, mainly due to urbanisation and changes in land cover and land used,” she said.
As reported in The National, in previously published research Dr Francis and colleagues found that increased urbanisation in the UAE has reduced the quantity of aerosols in the country’s air by cutting the amount of dust whipped up by the wind.
Mineral dust accounts for a significant fraction of the aerosols or suspended particles, and development, including the creation of green spaces, may reduce quantities released into the air.
As well as urbanisation, an increase in rainfall may also have contributed to the reduction in the quantity of aerosols.