Bulletin 29 - July 1986: Understanding Meteorology in the UAE Part 1: Fog



Understanding Meteorology in the UAE Part 1: Fog

by Andy Best

It is 6.30 on an unusually cold December morning. I head for the peel-back curtains and do just that, only to find that there is nothing out there. Nothing at all. Well, nothing but a thick, grey shroud of wet, clammy, cold stuff. For a change, I think, I should have listened to the forecast last night. Now, am I going to wait at the boat for the others as if nothing were wrong and confidently muddle my way over to Sadiyat Island, or shall I phone the forecaster and ask him when it will clear? This is a typical example of how the weather in Abu Dhabi can affect man, particularly, it seems, on Fridays. Of course, the other time that fog really affects us is when we emerge from our block one morning to drive to work and suddenly realise we should have replaced those frayed wipers, as two minutes after setting off there is that tell-tale band of mud smeared across the windscreen.

Fog is indeed a problem. It affects all forms of transport except submarines and radar-equipped police patrol cars. But what really is fog, and what causes it here in the Emirates? Actually, it is easy to answer the first part of this question, but very difficult to come up with a solution for the second. The 'Observer's Handbook' (the Met. observer's most valuable aid next to his eyes) gives the official definition as:

'A suspension of very small, usually microscopic, water droplets in the air, reducing visibility at the earth's surface to less than 1000 m. When sufficiently illuminated, individual fog droplets are frequently visible to the naked eye; they are often to be seen moving in a turbulent manner. In general, the relative humidity is, or close to, 100%.'

There are two distinct types of fog formation known to meteorologists: radiation fog and advection fog. To understand the mechanisms of fog formation it is necessary to understand the concept of Dew Point. If air is cooled until with respect to its water content it becomes saturated, then the temperature at which saturation takes place is called the dew point temperature. So, if some air cools until its temperature falls below the dew point then condensation will occur as microscopic droplets on the condensation nuclei in the air (these are less than 0.1 microns in diameter and are found in smokes and sea salts). Radiation fog is formed when the air, or the ground in contact with the air, radiates its internal infrared radiation on clear, calm nights.

As droplets form during the radiative cooling process the radiation emitted by this shallow layer of air increases and more heat is lost, causing fog. Advection fog can also cause condensation when an air mass moves horizontally over the ground towards a colder region and the air mass itself is cooled by turbulent heat conduction to the ground.

The Emirates are interesting in their topological aspect for the favourable formation of fog, and both radiative and advective forms are quite common. Basically, the country can be considered a large triangle mostly covered with sand, surrounded on two sides with water. On the one hand the water is relatively warm in summer compared to the land, and on the other it is relatively cool in winter. It is well known that arid deserts are hot by day and cool at night. The radiative cooling effect in the area south of Abu Dhabi known as Ad-Dhafrah causes moist air in that region brought in by the afternoon sea breeze to condense and form fog; indeed, in areas where there are many small undulating hills (dunes) and valleys, fog formation tends to be higher, the valleys acting as cold air traps enhancing the surface condensation effect. Once the fog has formed in the desert there is a definite change in the conditions for the outward transfer of infrared radiation, since the top of the fog then starts to radiate at a rate of about 2 - 5 degrees Celsius per hour, while the cooling at the ground ceases. This can lead to the very thick fogs that are encountered in Abu Dhabi Emirate - as the fog bank grows in desert areas it gradually envelops the airport and penetrates to the coast. As solar radiation is weak in winter it can take quite some time to 'burn off' this fog.

Although fog is most common in winter months it is not unknown during the 'spring' or early 'autumn'. This type is advection fog, when air of high dew point over the sea is carried over the cooler land. As the moist air moves landwards, it is forced to rise over the denser desert air. Such blankets of fog may penetrate inland for many miles.

In Abu Dhabi advection fog off the sea can be caused by local differences in sea temperatures in the Arabian Gulf. Fog forms over the sea due to these differences and drifting coastwards can at times be a major influence on traffic. later, as the land/sea temperature contrast increases, the fog may drift back out to sea and wait there until later in the evening when it can drift back onshore.

Airports are greatly influenced by fog formation in the Gulf, and every Gulf airport experiences closure at some time or another due to low visibility; however, because fog is localised, it is highly unlikely that all airports would be affected at the same time. Fog clearance systems are available, with huge heaters attached to fans, but none of the Gulf airports are thus equipped.

Some of the areas most prone to fog in the Emirates are in the south west of the region around Buhasa and Asab oilfields, as well as along the major roads leading east and north of Abu Dhabi. Forecasting fog here is a very difficult process and meteorologists at the New International Airport rely on hard data from surface observations, rig observations, aircraft sightings, satellite photographs and a past history of the general meteorological situation. Hydrogen balloons are sent aloft four times a day to study vertical profiles of wind, temperature and relative humidity.

 


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