Download What is Cirrus? - Hong Kong Observatory

 What is Cirrus? Written by : CHIU Chiu­yee Cirrus is a kind of high­level 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,
over­hanging 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 silk­like 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”. Reference:
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天氣雲圖，上海科學技術出版社 (in Chinese only) 地面氣象觀測，氣象出版社 (in Chinese only) 天氣，三聯書店(香港)有限公司 (in Chinese only) International Cloud Atlas, World Meteorological Organization Observer's Handbook，Meteorological Office, HMSO The Wonders of the Weather, Bureau of Meteorology