x Abu Dhabi, UAESaturday 22 July 2017

The science behind the clouds - and rain and hail

ABU DHABI // The weather system that ripped through the UAE over the past week eased yesterday, but meteorologists say there may be more storms on Friday. The gales and downpours that contributed to at least 18 deaths and caused millions of dirhams in damage were severe, but not unusual for this time of year. The thunderstorms erupted when warm, moist air from the Arabian Sea travelling at surface level collided with a cold upper air mass from the eastern Mediterranean. Low pressure weather systems are usually associated with wind, cloud and rain while high pressure systems mainly result in hot, dry conditions.

The recent storms were prolonged because the low pressure trough became trapped by a ridge of high pressure around Egypt and the Arabian peninsula. "The trough we have been experiencing has now moved east and is about to pass over Iran," said a meteorologist at the UAE's National Centre of Meteorology and Seismology (NCMS). "Unfortunately a second low pressure trough is due to arrive here from the north-west on Friday. We expect it will stay here for around two days. It will bring more rain and winds."

Jebel Jais, around 30km north of Ras al Khaimah, was the wettest region, with 162mm of rain last week. Fujairah had 127mm, Ghantoot 98mm and Abu Dhabi 27mm. Figures were not available for Dubai. Rain was concentrated in the north of the Emirates as mountainous regions receive more rainfall. Wind speeds peaked at about 90 kph. "There was a lot of humidity which arrived from the Arabian Sea and Indian Ocean and it caused the instability in the weather that we have been seeing for the past few days," said the meteorologist. "This caused the formation of cumulonimbus clouds and led to the wet weather.

Another meteorologist at Dubai International Airport added: "There was a great deal of convection clouds formed and these rose higher than normal; 40,000ft rather than the usual 30,000ft. "The weather was quite severe but it was not unusual. Last year was drier and not like this but in 2007 there was a very similar weather pattern at this time of the year. In 2005 it was the same. "It is cyclical. These weather patterns come and go. The 1980s were wetter. The 1990s were drier."

Storms form when warm air and cool air collide. Warm air has a lower density than cool air and rises within the cooler air mass. As it rises it cools and the water vapour condenses before freezing inside the cloud. This condensation releases heat energy that warms the outer layer of air, triggering a convection effect that sucks this cold air core up to the tropopause. This leads to the formation of huge cumulonimbus clouds.

The cold air then falls and leads to rain or hail and also causes downdrafts below the storm. As the warm air updrafts mix with the freezing downdrafts, the water particles collide, causing friction that electrifies the cloud particles and these charged particles separate. The positive charge fixes near the top of the cloud while the negative rests at the bottom. Eventually, the charges become so great they form an electrical discharge in the form of lightning.