What is Cirrus?
Written by : CHIU Chiu-yee
Cirrus is a kind of high-level cloud. Among all cloud genera, cirrus is the highest in the troposphere. The average height of cirrus is over 6,000 metres. It is so high in the upper air that when the sun has not risen above the horizon before dawn or has gone downhill after nightfall, sunlight can still shine on this detached, over-hanging and shadowless cloud. After scattering the sunlight, it produces an attractive red or orange hue, a very common sight on summer days.
Cirrus is composed mainly of ice crystals and is usually white in colour. Ice crystals make the cloud look even brighter and more translucent. Against a blue sky, the white delicate filaments of cirrus are spotlessly clean and display a silk-like gloss. Cirrus is usually thin, wispy and scattered. There are patches of narrow bands with fibrous texture which look like feathers, hair, tenuous trail or ponytails. It is fascinating to see cirrus in a variety of shapes and sizes.
"Bright white clouds floating on the azure sky. Like cotton fiber spreading on bluish green sea." - the most beautiful cloud is cirrus.
Since Cirrus is very high in the sky, even if it turns into water droplets, they tend to evaporate before reaching ground. This is the reason why rain is not detected on the ground and Cirrus is mainly associated with fine weather.
Do you know what kind of cloud will appear in the sky a couple of days before the arrival of bad weather or tropical cyclone? (Hint: This cloud is most probably present at a height more than 10 kilometres. It is among the highest clouds.) The answer is Cirrus. The rising air associated with tropical cyclones pushes moist air up to the height of 5 to 6 kilometres. Because of the low temperature there, water vapour condenses into small, clear crystals. A layer of glossy Cirrus forms which drifts out from the centre of the tropical cyclone because of divergence at that level. So, when the outer edge of a tropical cyclone reaches Hong Kong, we can see Cirrus high in the sky. People call it the “Mother of Typhoon”.
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