Always present yet constantly changing, clouds are a kind of global language of the skies, offering cues to what is happening in the atmosphere. Regardless of where in the world we live, it is easy to recognize fluffy white clouds floating through a blue sky on a sunny day or dark, ominous thunderheads building as a warning of stormy weather. Clouds play an integral role in balancing Earth's energy, shaping the global distribution of water resources and influencing local and global climate and weather. They are central to forecasting weather conditions, modelling the impacts of future climate change and predicting the availability of water resources.  
 
The theme of World Meteorological Day on 23 March 2017 was Understanding Clouds. Along with acknowledging the importance of the science behind understanding clouds and their impact on our Earth systems, World Meteorological Day provided an opportunity to celebrate the inherent beauty and aesthetic appeal of clouds that has inspired artists, poets, musicians, photographers and countless other enthusiasts throughout history.  
 
The day also marked the release of the revised edition of the International Cloud Atlas - the single most authoritative and comprehensive reference for identifying clouds. First published in the late 19th century and last revised 30 years ago, the Atlas - available digitally for the first time - includes images, classification details and information about clouds and other weather phenomena such as rainbows and hailstones.  
 
Clouds are warmer than the surrounding air, meaning that they float because warm air is lighter than cool air.  
 
This is because when water vapour cools into water droplets a small amount of heat is released.  
 
All the main cloud types live in the troposphere; this is the lowest part of the atmosphere, closest to the ground.  
 
Above the troposphere is the stratosphere and above that is the mesosphere, thermosphere and exosphere.  
 
There are 10 different cloud types all given Latin names based on their shape and position, these are:  
 
Cumulus cloud  
These look like fluffy balls of cotton wool. Usually cumulus are found on calm clear days and indicate fair weather, but they can grow into towering thunderstorms given the right conditions.  
 
Stratus cloud  
 
These are flat, grey, featureless layers which often sit low to the ground, hiding the clouds above. Sometimes these can produce a light drizzle. Fog is simply a stratus cloud which has come down to surface level, so when you walk through fog, you are actually walking through a cloud.  
 
Stratocumulus cloud  
 
A layer of stratus clouds can break up to form cumulus or an area of cumulus can squeeze together to form stratus. The period in between is its own type of cloud, known as stratocumulus.  
 
Altostratus cloud  
 
The 'Alto' clouds are found in the middle of the troposphere, between the low level water-based clouds and the high level ice-based clouds. Altostratus is a mid level stratus cloud which is usually thinner than stratus and usually lighter. If you look carefully you may be able to see the sun shining through.  
 
Altocumulus cloud  
 
Like altostratus, altocumulus are just a mid level cumulus cloud. There is a difference however, as altocumulus are visibly smaller than cumulus and are made of both ice and water.  
 
Cirrus cloud  
 
Cirrus are the highest level clouds, made entirely of ice, this are wispy looking clouds which look like horses tails.  
 
Cirrocumulus cloud  
 
These are cumulus clouds at cirrus height. Cirrocumulus is composed entirely of ice and look like little fish scales in the sky.  
 
Cirrostratus cloud  
 
Thin wispy layers high in the sky, cirrostratus are unlike their mid and low level cousins as they can give rise to beautiful optical effects such as Haloes. The sun still shines brightly through these layers so even though the sky may be covered you still can see your shadow faintly on the ground.  
 
Nimbostratus cloud  
 
'Nimbus' is the Latin for rain cloud. Nimbostratus produces long lasting rain or snow that is light to moderate intensity. These are tall stratus clouds existing at low and mid levels.  
 
Cumulonimbus cloud  
 
Also known as 'the king of clouds' cumulonimbus are responsible for bringing very heavy rain and hail to an area over a short space of time.  
 
These are also the only clouds that can produce thunder and lightning. Cumulonimbus are very tall and grow through all layers of the sky often with a giant 'anvil' shaped top.