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Clouds & Particles

Basics

Formation of clouds

A cloud is composed of millions of little droplets of water (or ice crytals when temperature is very low) suspended in the air. Hence a cloud can form when water vapor becomes liquid, i.e. when the humid air is cooled and condenses on tiny particles. There are several major processes for cloud formation.

 

 

1. Formation of clouds by convection process. Author: J. Gourdeau.

 

Convection :

On Earth, because of air density depending on temperature, the warm air rises and the cold air sinks as warm air is less dense than cold air. That is what scientists call convection.
Convection is one of the processes that allows cloud formation. When the sun shines, the air at the ground level that contain water vapor is heated and it begins to rise, and as the air rises, it begins to cool. Clouds are formed when the humid air is cooled below a critical temperature : the water then condenses on tiny suspended particles and forms droplets in the atmosphere.  

 

 

 

Topography (mountains) 

Clouds can also form because of the Earth's surface topography, mountains or hills for example. They are then called "orographic clouds".The air is forced to move over the mountain, and as it ascends, the air cools. If it cools to it's saturation point, condensation occurs and the water contained within the air becomes visible as a cloud.

 
2. Source: NOAA
3. Source: NASA

The Foehn effect

When air rises over the mountains, it cools and becomes saturated with water vapour. Condensation occurs and the water vapour becomes liquid. It stays under the form of a cloud or rains, while the air continues to rise. When the air mass descends on the other side of the mountain, it is more dry and warm. This difference in temperature is known as the Foehn effect.

Meeting of air masses 

Not only mountains can force air to rise : when warm air meets a mass of heavier cold air, it is also forced to rise. The boundary between warm and cool air is called a « front ». As the warm air ascends it cools, and then clouds may be formed.

 
4. Cold front.
A: cold air; B: warm air.
Author: J. Gourdeau
5. Warm front.
A: cold air; B: Warm air.
Author: J. Gourdeau

 

Horizontal motion

Sometimes winds bring warm and moist air into a region. If the warm moist air flows over a much colder surface, it is cooled and moisture will condense and form fog. This mechanism is frequent on the coast of continents.

6. Fog over a lake. Source: Ph. Osset
 
About this page...
Author: Justine Gourdeau LaMP Clermont ferrand/France
date of generation: 2003-03-12
1st reviewer: Pr. A. Flossman, LAMP.
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