If you were trying to fly out of Doha over the past few weeks, you may have had your flights disrupted by fog. India, China and many parts of Europe have been shrouded in dense fog which has disrupted air travel. We’ve also had a couple of foggy mornings in the Gulf. Doha had some fog on 6th of this month, but it was worse in Dubai, where the fog was so thick that some flights bound for the Emirate had to divert to Doha.
Fog is a major challenge to air travel. Planes can land on thick snow or in torrential rain, but if it’s foggy, then, planes have to move slower and delays will therefore get longer and longer. Nowadays, aircraft have a vast array of technology on board, ensuring they can land and take off even with zero visibility, but most air traffic controllers prefer to space out the planes a little more on the approach, to allow for more room for manoeuvre should anything go wrong. This adds a short delay, but the real hold-ups occur once the plane has landed. Once on the ground, there is no special equipment and the pilot simply has to see where he’s going. If the fog is so bad that pilot can’t navigate safely, then the airport will grind to a halt and this is when planes are diverted to other airports.
As you can imagine, given the vast quantities of money involved in the airline industry, a large amount of money and effort has been spent on research into fog clearance. Attempts at clearing fog started as far back as the Second World War. The most successful attempts resulted from lining jet engines along the runway, in an attempt to raise the air temperature and clear the fog. Oddly enough, however, this proves quite expensive, and the higher the temperature of the fog the more expensive it becomes, as you need to heat the fog to a higher temperature to persuade it to clear. Hovering helicopters above the runway in order to mix the air and clear the fog also proved moderately successful, but this is something that only works in shallow fog.
If the fog has formed at a very low temperature, then it is slightly easier to clear. Liquid propane, which has a temperature of about -42C (-44F) can be injected into the air and this encourages all the water droplets in the fog to turn to ice. As the ice crystals grow, they become heavy and fall out of the air, leaving a hole in the fog. Hopefully, this clear patch will form over the runway, if not someone has made an expensive mistake!
Given the fact that no airport that I know clears fog as a matter of course, clearly all clearance methods are still too expensive to use on a regular basis. Therefore, diversions caused by fog are a hazard that we will have to endure every year as the temperature drops after the summer.
The reason that fog often forms at this time of year is because warm air can contain more moisture than cool air can. As autumn wears on and the winter weather starts to grip, the temperature across the northern hemisphere drops. As the air cools, it becomes saturated, and the excess moisture in the air condenses to form tiny droplets. If the winds are very light, then these droplets will hover near the ground, forming fog, but if there is a wind, this will lift the fog into low cloud.
Fortunately fog isn’t something we have to deal with very often in Doha and when it does appear, it usually clears pretty quickly, after the sun rises. Fog in colder climates, however, can stick around all day. This is mostly because the sun is often weak, so can’t supply enough heat to the fog in order to burn through it.
There is one type of fog that we get in Doha, though, which is quite spectacular. It appears like a wall of fog, which rolls in from the Corniche. This is more like the fog that rolls in across San Francisco or the south coast of Oman and is a result of the air being cooled by the sea.
For this kind of fog to form, the air has to be drawn in from the east, where the sea is generally warmer. The waters enter the Gulf around the Strait of Hormuz, then flow anti-clockwise. As they head northwards, they are joined by cooler water from the rivers in the north. The cooler water then flows down towards us here in Qatar and the water around the country is usually a few degrees lower than the waters to the east of us.
This temperature drop of just a couple of degrees is sometimes all is takes for an easterly wind to reach its saturation point. The humid, easterly air travels over the cooler seas around Doha, which causes the air to cool and allows the water droplets to condense. This type of fog is known as advection fog and it’s pretty impressive to see it roll in from the sea. Although it can disrupt flights and the traffic, for the ominous spectacle alone, it’s a shame it doesn’t happen more often.
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