UAE humidity: why is 'Sweat-tember' so muggy?

Changes to regional weather systems affect the balance between sea and land breezes

A muggy Downtown Dubai on a humid late summer day. Changing weather patterns at the start and end of the summer lead to high humidity. Pawan Singh / The National

Humidity can reach 100 per cent in September, even though temperatures are cooler than the high summer months. Pawan Singh / The National

Dry heat can make the hottest summer months, July and August, more comfortable than June and September, which tend to be very humid. Pawan Singh / The National

People wearing protective face mask and buying fruits and vegetables at the Al Awir fruit & vegetable market during the hot and humid weather in Dubai. Pawan Singh / The National

People wearing protective face mask and buying fruits and vegetables at the Al Awir fruit & vegetable market during the hot and humid weather in Dubai. Pawan Singh / The National

People using umbrella during the hot and humid weather at International City in Dubai. Pawan Singh / The National

It is temping to think that the UAE only has two kinds of weather — hot and very hot.

But while the mercury is usually high in the Emirates, there can be a big difference between days when the heat is dry and days when it is humid.

As many residents have observed, humidity often tends to increase later in the summer, typically from September onwards, so while temperatures may be starting to fall from their summer highs, going outside is not necessarily a more comfortable experience.

But why does the air feel more humid after summer temperatures have peaked?

Autumn humidity

In September, temperatures fall slightly compared to their highs during August, but Dr Ahmed Habib, from the National Centre of Meteorology, says that residents typically do not find that being outside becomes more comfortable.

In August, daily maximum temperatures may be 49°C or 50°C, while in September, during the hottest part of the day, they may be several degrees cooler. Daily minimum temperatures may be at least 5°C lower in September, compared with August.

“If the temperature decreases, the relative humidity increases,” Dr Habib says. “In September, especially at night and early morning, the relative humidity increases.

“When the relative humidity increases, the human feel of the temperature increases also. The temperature is coming down, but the people talk with our centre here, they say, ‘The temperature is coming down, but we feel that it is not coming down.’”

So, the temperatures fall slightly from September but this slight cooling is more than balanced out by greater levels of humidity.

Sea breezes versus land breezes

The balance between sea breezes and land breezes helps to explain why the weather becomes more humid in much of the UAE from September and sometimes into October.

During the day there is often a sea breeze, which is a wind, typically humid, that comes off the sea on to land. At night-time, a land breeze, which tends to be drier, is often felt.

As the sun rises, the land heats up faster than the sea, so the warmer air above it rises, and draws in air from the sea — a sea breeze — during the day.

After dark, the land loses heat faster than the sea does, so the air above the sea rises and draws in cooler air from the land, creating a land breeze.

From September onwards, the sea breeze tends to become more important, blowing for a longer period of time and so making the weather more humid.

“Late in the day, all the night and early morning, we are affected by the south-east or south-west wind as a land breeze,” Dr Habib says.

“But if we see the way we are affected by the north-westerly, it is very little — a short period, a few hours, maybe three or four hours maximum — in August.

“In September, when we are affected by the pressure system … the hours of the sea breeze will be increased. So, more water vapour is coming from the Arabian Sea to our area.”

Changing pressure systems

In July and August, a low-pressure system associated with easterly and south-easterly winds (winds that blow from the east and the south-east) tends to predominate.

These winds tend to be drier, so while the temperatures are extremely high, the air is less humid than it becomes from September onwards.

“In September, the pressure system changes,” Dr Habib says. “The low pressure returns back to the east and there is the start of high pressure coming from the north-west and affecting the UAE. The high pressure brings north-west winds over the Arabian Gulf.

“The humid air masses are coming over the sea, the Arabian Gulf, towards our area. The humidity is increasing gradually in September and October and November, associated with the north-west wind coming from the Arabian Gulf.

“When the UAE is affected by the north-west wind, it is a source of humidity for Abu Dhabi and Dubai and the northern part [of the country].”

So much of the UAE becomes more humid as this high pressure system takes control and causes an increase in north-westerly winds bringing moisture from the Arabian Gulf on to land.

Before September, a low-pressure system often causes easterly and south-easterly winds, which bring drier air.

“The temperature is coming down and the humidity is starting to rise up in September,” said Dr Habib.