The Cloudspotter's Guide by Gavin Pretor-pinney

Author:Gavin Pretor-pinney
Language: eng
Format: epub
Publisher: Penguin Group USA, Inc.
Published: 2006-06-17T16:00:00+00:00

So that’s how Cirrus are formed.

You wouldn’t think it, but Cirrus are in fact precipitating clouds. The reason they are not defined as such is that their ‘precipitation’ evaporates away in the warmer air below the cloud. Nevertheless, to look up at Cirrus clouds is to see snow–well, ice crystals, to be precise–falling too high to reach the ground. Cloudspotters may live in regions too warm for snow but, in the Cirrus clouds, they can still see how it looks from a few miles off.

The movement of the falling crystals, as they are whipped by the winds high in the troposphere, is what gives Cirrus their distinctive, flossy formations, known as ‘fallstreaks’. Up at the top of the troposphere, winds will often be as strong as 100–150mph. Forming in gales like this, Cirrus aren’t clouds that stick in one place for long.

Compared to low Cumulus gliding along in the breeze, these clouds can look almost stationary. Cloudspotters should remember, however, that the further away something is, the more difficult it is to notice its motion. In fact, Cirrus move a lot faster than Cumulus clouds. Not only are they the highest of the common clouds, they are also the fastest.

On average, wind speeds decrease with descent through the troposphere. So, as the ice crystals of a Cirrus cloud fall, they tend to pass into slower air currents below, where they lag behind. The varying temperatures and humidities on the way down can cause the crystals to grow and multiply in some regions and diminish in others. It is this variation in the wind speed, temperature and moisture content of the air through the crystals’ descent that determines the wavy forms of the Cirrus fallstreaks.

Download

Copyright Disclaimer:
This site does not store any files on its server. We only index and link to content provided by other sites. Please contact the content providers to delete copyright contents if any and email us, we'll remove relevant links or contents immediately.