Cirrus
Topic: Water in the Atmosphere: Humidity, Condensation, Clouds and Precipitation · NCERT: Class 11, Ch 10 "Water in the Atmosphere"
Meaning
Cirrus is the highest of the four basic cloud types. It forms at about 8,000–12,000 m, where the air is so cold that the cloud is made of ice crystals, not water drops. It looks thin, feathery and detached, and it is always white. Cirrus gives no rain itself. It matters because it often comes before a warm front, the line where warm air slides up over cold air and brings long spells of rain.
Example
Over north-western Europe, for example over Britain, feathery cirrus often spreads across a clear sky well before a warm front arrives from the Atlantic. Lower and thicker clouds then follow, and steady rain comes after them.
Don't confuse with
- Cirrostratus: also a high ice cloud, but it is a thin, even sheet over the sky and makes a halo round the Sun or Moon. Cirrus is made of separate wisps.
- Cirrocumulus: also high and icy, but it is made of small puffs in rows or ripples. Cirrus is made of streaks.