World Meteorological Day 2017

This year the theme of World Meteorological Day (23 March) is "Understanding Clouds". 

Clouds play a key role in weather forecasts and warnings and in honour of this year's theme we explain how clouds are formed and the most common kinds.

Want to learn how to spot the different types of clouds yourself? View our useful guide here

Formation of clouds
Everywhere in the world, the air in the atmosphere contains water vapour, most of which has evaporated from the oceans. Clouds are composed of water droplets and/or ice crystals, which form when water vapour condenses. Condensation results from air being cooled by one or more of the following processes:

•    Convection (when the sun heats the ground)
•    Frontal uplift (when two air masses meet)
•    Orographic uplift (when air is forced over hills or mountains)

You may have wondered how the various cloud types got their names. The names are based on the original cloud classification scheme devised by Luke Howard, an amateur meteorologist, in 1803. He wrote a paper (and later a book) entitled “On the modification of clouds” which formed the basis for the international Cloud Atlas which was subsequently published in 1896. Luke Howard classified the clouds according to their altitude and visual appearance.

•    Low clouds are defined as those below 2000m (e.g. cumulus, stratus)
•    Medium clouds between 2000 and 6000m (usually prefixed by alto)
•    High clouds are above 6000m (e.g. cirrus or prefixed by cirro) 

•    Cirrus - fibrous, hair like 
•    Cumulus - a heap or pile 
•    Stratus - a horizontal sheet/layer 
•    Nimbus - rain-bearing 

These attributes can be combined to form the 10 best-known cloud types:
1. Cumulonimbus
2. Nimbostratus
3. Cumulus
4. Stratocumulus
5. Stratus
6. Altocumulus
7. Altostratus
8. Cirrus
9. Cirrocumulus
10. Cirrostratus

However, there are now many more cloud types than this, including castellanus, lenticularis, fractostratus, undulatus, mammatus, aircraft contrails etc.

Want to know more about clouds? Our useful guide provides more information and helps you to identify clouds yourself! Download here.

For more information about World Meteorological Day visit the website here

There are 10 best-known cloud types, but there are now many more cloud types than this.

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