As temperatures gradually decline, the UAE’s outdoor season is coming back to life, with cooler mornings and evenings encouraging residents to enjoy picnics, barbecues, desert trips and more time outdoors.

However, the seasonal shift has also brought back a troublesome morning weather phenomenon: dense fog, which can reduce horizontal visibility to near zero on some roads.

The UAE’s National Center of Meteorology (NCM) said the gradual decline in temperatures and increase in humidity create more favourable conditions for fog formation, particularly overnight and during the early morning hours.

The NCM told Gulf News that atmospheric stability, light winds, clear skies and high humidity near the ground are among the main factors behind fog formation.

Under favourable conditions, relative humidity can reach 90 to 100 per cent, bringing the air close to saturation. Fog forms when water vapour condenses in the lowest layers of the atmosphere.

Light winds allow moisture to remain near the ground. As temperatures fall, the air reaches saturation and tiny water droplets form, potentially causing a sharp reduction in horizontal visibility.

Moisture moves inland from the sea

Northwesterly winds can also play an important role in fog formation.

According to the NCM, winds that pass over the sea in the evening and move inland overnight can carry significant amounts of moisture into coastal and inland areas.

If these winds weaken and the atmosphere becomes stable, moisture accumulates near the ground, creating conditions for fog to spread across wider areas.

Several parts of the UAE have experienced fog in recent days.

High pressure in the upper atmosphere can intensify these conditions by preventing air in the lower layers from rising, trapping moisture near the ground.

Cooler nights increase the likelihood of fog

The return of fog is closely linked to the seasonal decline in temperatures.

September is considered the final month of the UAE’s climatological summer. Temperatures gradually fall during the month, particularly overnight and around sunrise, with the decline usually becoming more noticeable in the second half of September.

Astronomical autumn begins around September 23, when the Sun is directly above the equator before appearing to move south towards the Tropic of Capricorn.

Humidity levels also generally increase compared with August, particularly during the second half of September. The average maximum relative humidity during the month is about 77 per cent.

As the country moves further into the cooler season, the likelihood of fog also rises. The NCM identifies December, January and February as months when fog is more common because of lower overnight temperatures.

Why is radiation fog common in the UAE?

One of the most common types of fog in the UAE is radiation fog.

This type of fog forms when the ground loses heat overnight through longwave radiation. As a result, the air near the surface cools while the air above remains warmer.

This creates a surface temperature inversion, in which cooler air becomes trapped beneath warmer layers.

The stable layer prevents vertical mixing and allows water vapour to accumulate near the ground. As temperatures fall further, the moisture condenses and fog forms.

Clear skies can intensify this process by allowing the ground to lose heat more quickly overnight.

Stronger winds can prevent fog formation

High humidity alone does not necessarily lead to fog.

The NCM said stronger winds, including active northwesterly winds, can reduce the likelihood of fog by mixing the air near the ground and preventing moisture from accumulating.

Wind direction is also important because it helps meteorologists identify the source of an air mass and determine whether it is bringing moist or dry air into the region.

How does the NCM forecast fog?

The NCM uses satellites, weather stations and atmospheric data to monitor and forecast fog, paying particular attention to the difference between air temperature and the dew point.

The closer these two readings are, the greater the likelihood that the air will reach saturation and fog will form.

Conversely, a widening gap between air temperature and the dew point usually indicates a lower probability of fog.

Wind speed and direction, as well as conditions in the upper atmosphere, are also assessed. High pressure at higher levels can increase atmospheric stability, prevent moist air from rising and keep moisture trapped near the ground.

What is the difference between fog and mist?

The main difference between fog and mist is determined by horizontal visibility.

When horizontal visibility falls below one kilometre, the phenomenon is classified as fog. If visibility remains above one kilometre, it is classified as mist.

In cases of extremely dense fog, visibility can fall to near zero in some areas, making driving particularly hazardous during the early morning hours.