Cooler UAE weather returns but so does the season of morning fog

Abu Dhabi: The UAE’s outdoor season is beginning to stir back to life, with cooler mornings and evenings drawing residents towards barbecues, desert trips and time outside.

But the same seasonal shift is also bringing back one of the country’s most disruptive early-morning weather hazards: dense fog that can reduce visibility to near zero on some roads.

As temperatures gradually fall and humidity rises, conditions are becoming increasingly favourable for fog formation, particularly overnight and during the early morning, according to the National Centre of Meteorology (NCM).

In comments to Gulf News, the centre explained that stable weather, light winds, clear skies and high levels of moisture near the surface are among the main ingredients needed for fog to develop.

Under the right conditions, relative humidity can climb to between 90 and 100 per cent, bringing the air close to saturation.

Fog develops when water vapour condenses in the lowest layers of the atmosphere.

Calm winds allow moisture to remain concentrated near the ground, while falling temperatures cool the air until it reaches saturation, producing tiny suspended water droplets that can sharply restrict horizontal visibility.

North-westerly winds can also play an important role.

The NCM noted that winds moving across the Arabian Gulf during the evening and continuing inland overnight can transport significant amounts of moisture towards coastal and interior areas.

If those winds weaken and the atmosphere becomes stable, moisture can accumulate near the surface, creating favourable conditions for fog to spread across wider areas.

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

High pressure in the upper atmosphere can reinforce these conditions by limiting the upward movement of air from the lower layers, helping trap moisture close to the ground.

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

September marked the final month of climatological summer in the UAE, with temperatures beginning to ease gradually, particularly at night and around dawn. The decline typically becomes more noticeable during the second half of the month.

Astronomical autumn began around September 23, when the sun was directly above the Equator before appearing to move southwards towards the Tropic of Capricorn.

Humidity also generally increased compared with August, particularly later in September. The average maximum relative humidity during the month is about 77 per cent.

As the country moves deeper into the cooler season, the likelihood of fog rises further. The NCM highlighted December, January and February as months when fog becomes more frequent because of lower overnight temperatures.

One of the most common forms in the Emirates is radiation fog.

It forms when the ground loses heat during the night through long-wave radiation. The surface cools the air directly above it, while air at higher levels remains warmer.

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

The stable layer restricts vertical mixing and allows water vapour to remain concentrated near the ground. Once temperatures fall sufficiently, that moisture condenses and fog begins to form.

Clear skies can make the process more effective because they allow the ground to lose heat more quickly overnight.

High humidity alone does not guarantee a foggy morning.

The centre pointed out that stronger winds, including active north-westerlies, can reduce the likelihood of fog by mixing the air near the surface and preventing moisture from accumulating.

Wind direction is also important because it helps meteorologists identify where an air mass originated and whether it is carrying moist or dry air.

NCM monitors fog using satellites, weather stations and atmospheric observations, with particular attention paid to the difference between the air temperature and dew point.

The closer those two readings become, the greater the likelihood that the air will reach saturation and fog will develop.

A larger difference between temperature and dew point generally means a lower probability of fog.

The NCM also tracks wind speed and direction and conditions higher in the atmosphere. Upper-level high pressure can increase atmospheric stability, suppress rising moist air and allow humidity to remain trapped close to the surface.

The distinction is largely determined by visibility.

Fog is recorded when horizontal visibility falls below 1,000 metres, while similar conditions are classified as mist when visibility remains above one kilometre.

In particularly dense episodes, visibility can fall much further, in some locations reaching near-zero levels and making early-morning driving especially hazardous.