Clouds: an example of cirrus

In addition to my experiences I would also like to post features and interesting articles on this blog. Also this gives me the opportunity to post some nice pictures.

Most of the weather happens in the lowest layer of the atmosphere, called the troposphere. It extends to around 10 km, so around the cruising altitude of a long haul flight. In general, clouds are divided into three groups depending on their height, which also affects how they are formed. The lowest level is from the ground up to around 5000 ft (yes, for some reason meteorology uses feet for any vertical distance), which in most places are dominated by convection from heating of the surface. In Antarctica this is almost never the case, so we mostly see layer type clouds called stratus at low levels. For clouds in the layer above this, the prefix alto is used, again dividing the clouds into two main types of stratus and fluffy cumulus clouds. These extend up to 15 000 ft, above which the top level clouds reside. Because of the cold and the air circulation around Antarctica, these levels are a bit lower than elsewhere, so the numbers I give here are a rough guide that we use at Halley, but will not be valid in other places. Above the midlevel the prefix cirrus is used resulting in cirrostratus (sheet), cirrocumulus (cells) or just cirrus (whispy) clouds. These top level clouds are composed of ice crystals.

Halley provides some beautiful examples, especially of cirrus clouds. The cold temperatures allow the formation of ice crystals lower in the troposphere, but most of the cirrus clouds are aroud 20 000 ft. They also produce amazing optical phenomena like halos around the sun and moon, as well as parhelia (sundogs), circumzenithal and circumhorizontal arcs depending on the shapes of the ice crystals, which in Antarctica are mostly long hexagonal cylinders.

DSCN1795-cirrus-copy (1280x960)

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