By Steff Gaulter
The unsettled weather has disappeared and probably won’t return until after the long hot summer. I’ll certainly miss it, particularly the thunderstorms. Whilst they were with us, they brought a whole variety of weather, not only rain and lightning, but flooding, hail and even some dust storms.
Thunderstorms are an impressive show of nature’s strength and understandably some people are petrified of them. In England, one lady used to ring up the Met Office every time she heard there may be thunder in the forecast. She would hide in the cupboard under the stairs until the forecaster assured her that the risk had passed.
Air within a thunderstorm cloud is continually moving, which gives the clouds the shapes like cauliflowers. It’s this constant movement which causes lightning, because as the particles within the storm rub against each other, they become charged. It’s the same charge that builds up when you shuffle across a carpet then touch another person; the electric shock you feel is like a tiny lightning strike. If you turn the light off, sometime you can even see the spark.
In a towering thunderstorm cloud, the negative charges build up at the bottom of the cloud and the positive ones gather at the top of the cloud. As opposites attract, positive charges will also build up in the ground below the storm. The charges will concentrate around anything that sticks up: buildings, poles, trees or even people.
Eventually the charges become so great that the electricity will discharge, and this is what a lightning strike is — electricity levelling itself out. This can either happen within the cloud or from the cloud to the ground.
Thunderstorm clouds, officially called cumulonimbus clouds, also can produce hail, and they’re the only clouds that can. In order to produce a hail stone, a droplet of water has to move high up into a cloud and freeze, then drop through the cloud, melting and bumping into as much water as possible.
After melting and re-freezing many times, eventually the hail stone becomes so large that it falls to the ground. Strangely the dry climate that we have in Doha ensures that hail isn’t so uncommon. Most of our rain comes from a thunderstorm cloud and if the rain doesn’t evaporate before it hits the ground, then there’s a good chance that the hail won’t have melted either.
Thunderstorms can also produce some rather impressive dust storms; you may have seen the images from Riyadh on May 7 as a huge dust storm rolled across the city.
It was a daunting sight; the wall of dust swallowed everything in its path. Once inside the storm, the dust was so thick that barely anything was visible. The same storm also engulfed Qatar as well, but it was nearly 11pm by the time it did, so it didn’t have the same imposing effect.
A great rolling wall of dust like the one which struck on May 7 is known as a ‘haboob’. The name comes from the Arabic word ‘habb’ meaning ‘wind’ or ‘to blow’. As they sweep over you, the temperature drops, the winds pick up and the visibility drops to nearly nothing. They pose a daunting sight, towering up to a thousand metres (93,000 ft) high and over a kilometre long. They can occur wherever the ground is dry enough, including in the Sahara desert, the US states of Arizona and Texas, and also parts of China.
A haboob will normally form in association with a thunderstorm. Rain from a cloud is obviously falling towards the earth, and as it does so, it also drags down some of the air around it. However, if the air underneath the thunderstorm is very dry, there is also a far more powerful process which causes the air to fall to the earth.
Dry air underneath a thunderstorm will cause some of the rain to evaporate, but evaporation needs a lot of energy. When you get out of a swimming pool, the water left on your skin uses your body’s heat as energy to evaporate, and that’s why you feel cold. The rain falling from a thunderstorm also uses heat as energy to evaporate, but it uses the heat from the surrounding air. This causes its temperature to drop. Whereas hot air rises, cold air sinks. Therefore the evaporation causes a downwards rush of air, which hits the ground and then spreads out.
As the air slams into the earth, it kicks up all the tiny loose particles and pushes them ahead of the thunderstorm. In Riyadh, as the haboob swallowed the city, the temperatures dropped from 33oC (91oF) to 24oC (75oF) in just an hour.
That’s a fall of 9oC (16oF)! As the temperatures plunged, the winds were gusting over to 90 kph (55 mph) and the visibility was just 100m (100 yards). Then, just when you think the weather couldn’t get any worse, it started to rain, sticking the dust to everything.
At this time of year, it’s often hot and settled for months on end, which means that thunderstorms are unlikely. However, dust storms can still be expected, which is a great shame; whereas a torrential thunderstorm can do wonders to clear the air, a dust storm does the opposite, in an instant.