There are thousands of named clouds however the ones in the above picture are the ones we see here in the UK most frequently and the ones which tell us something useful about the weather. Are we dominated by high pressure or are we about to experience a low pressure system with the usual accompanying weather front ? This short blog will go through the clouds associated with a frontal system and the key elements of weather associated.
Cirrus – high wispy cloud formed of ice crystals, these will come from the west and are usually the indication that a depression is heading our way. At this stage the low pressure could be up to 600 miles away, the speed at which the clouds appear will give an indication of the speed at which the low is moving. Often these cirrus clouds will have mares’ tails which look like little hooks at the end of the cloud, this shows us that these clouds are close to the jet stream. Just a little aside at this point Buys Ballot’s Law states that is you stand with you back to the wind in the Northern Hemisphere the area of low pressure will be on your left.
Cirrostratus – high level layer cloud, this is a thicker type of cloud than cirrus still at a very high level and still formed primarily of ice crystals. The appearance of this type of cloud tells us that the front is getting closer. The warm air which is approaching behind the warm front is climbing up over the cool air in front of it and condensing to form ice crystals.
Cirrostratus with halo effect – this is the same type of cloud as above but with the light from the sun being refracted through ice crystals. This halo effect can also be caused in the same way by the light from the moon. At this point in the movement of the system the barometer should have started to fall, the speed of the fall of the barometer will give an indication of the speed the system is moving.
Altostratus – mid-level layer cloud. As the warm front approaches the cloud will get lower as more warm air is climbing aloft, the cloud gets thicker. At this point in the system we have not yet experienced any precipitation and our barometer will still be falling.
Altocumulus – mid-level lumpy cloud, resembling cotton wool, warm air is still climbing upwards over the top of the cold air ahead, again we are still dry but the barometer continues to fall.
Nimbostratus – low level rain clouds, these clouds will be seen just ahead of the warm front and as the front passes overhead. Heavy rain will be experienced and the wind will veer to the south west. Once the warm front has passed the rain will diminish to drizzle and we will be in the warm sector. The barometer will fall less quickly after the warm front has passed. Visibility will be poor at this point due to the rain and the possible formation of fog.
Stratus – low level layer cloud, this type of cloud is typical of the warm sector and brings with it drizzle. There should be an increase in temperature as we are in a warmer air mass (tropical maritime) however sometimes the temperature increase is not hugely apparent. Visibility is often poor due to the drizzle or fog. At this point the barometer should have leveled off, if it is still falling the depression will be deepening.
Cumulonimbus – towering lumpy rain clouds, these clouds are typical of the cold front approaching and passing overhead. The cold air behind the cold front is cold and dry and therefore will not rise up above the warm air in front of it, instead it will push the warm air up on a much more vertical plane than the warm front climbing over the cold air ahead of it.This movement of air will create towering cumulonimbus clouds which will result in heavy rain with big droplets possible hail and electrical storms. Visibility under the cold front will be poor because of the rain. After the cold front has passed the wind will veer to the west or north west and the barometer will start to rise.
I hope the above was helpful and you recognised some the the clouds as being typical of UK weather. There are some far more spectacular and interesting clouds to be seen in other areas of the world, I have picked what I consider to be the two most spectacular.
The Roll Cloud or Arcus Cloud
The roll cloud is a long tube-shaped cloud and can stretch over 500 miles, these clouds are typically seen in the month of October in Queensland Australia but can and do appear in many other parts of the world, even the English Channel, I have yet to see one but I will be keeping my eyes open !!!
The Mammatus Cloud
The Mammatus cloud is a sack like cloud and is unusual because it is formed in sinking air, most clouds are formed when air is rising. Mammatus clouds are most frequently formed on the underside of cumulonimbus clouds but can develop under cirrocumulus, altocumulus and startocumulus.
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