Clouds have two competing influences in the radiation budget:

1) They reflect solar radiation and prevent it from reaching the surface of earth, similar to an umbrella, and preventing the warming of the atmosphere through absorption of the radiation.

2) They absorb the infrared radiation escaping from the surface of the earth and thus trap the warmth in atmosphere, similar to a greenhouse.

To determine which of these processes is dominant, we first need to emphasize that not all the clouds behave the same. In fact, their microphysical properties determine their radiative properties.  Microphysical properties means the nature of the condensed phase of the cloud particles (ice crystals and their shape and/or droplets)--the number of the particles and their size.

For example, upper level cirrus clouds, which contain only ice, are believed to have a weak albedo (albedo = capability to reflect incoming solar radiation) and a strong absorption of the outgoing infrared radiation.  As a net effect, then, they will warm the surface.  In contrast, low-level clouds have a strong albedo and a weaker absorption of the infrared, and thus tend to cool the surface of the earth. As you can imagine, it's impossible to give a single answer on the effect of clouds on climate.


source: ESPERE - E.U.

Animation: The different effects of clouds:
Cirrus clouds reflect only a small percentage of the incoming radiation from the sun (yellow).  Infrared radiation from the earth (red), however, is efficiently absorbed.  Both effects lead to a net warming.  On the other hand, low stratiform clouds reflect most of the radiation coming from the sun, so that only a small fraction reaches the earth.  Most of the infrared radiation, though, can pass through the clouds.  This causes a net cooling.

In total, the cooling effect of the more abundant low clouds is stronger than the heating effect of high clouds. Therefore clouds, if regarded in sum and without differentiation, can be regarded as opponents of global warming.

Why do we need to understand clouds today ?

At the moment, within the international community on atmospheric sciences, cloud physics commands a high priority.  Understanding the formation and the development of clouds, how pollutants are absorbed within the clouds, and the link between cloud microphysics and radiative properties is an extremely important undertaking.  Consider the following two points:

The sulfate aerosol:

Man puts a lot of aerosol particles into the atmosphere.  First, if these particles are chemically toxic and are absorbed by the droplets, they will reach the soil through precipitation and contaminate the water (thus the problem of acid rain in some polluted areas).

Secondly, aerosols drastically affect the radiative budget, through what we call the indirect effect of aerosols.  With more aerosol particles in the air, there are more available cloud condensation nuclei, and thus more droplets. As these droplets are more numerous, and yet share the same amount of water within the cloud, they tend to be much smaller than clouds without enhanced aerosol levels.  Since the droplets are smaller, the cloud will form less precipitation and have a longer life expectancy.  Smaller droplets are more efficient at reflecting solar radiation and thus, when combined with their larger population and the longer cloud lifetime they induce, tend to greatly increase the reflectivity of the cloud (the cloud albedo).  Thus clouds forming in air with enhanced aerosol levels ("polluted air") can have a significantly altered affect on the radiation budget of the earth and atmosphere.

[Put the picture of the ship tracks near the US coast and explain it's not smog but cloud ]

The Contrails:

Airplanes emit large numbers of aerosol particles in their exhaust, and are frequently flown at an altitude where development of cirrus clouds is predominant.  If the contrails persist we can see enhanced coverage of cirrus clouds.  As these clouds are believed to increase the greenhouse effect, as previously discussed, airplanes can have the effect of  warming the surface in the region of these contrails.

[Put a picture of old contrails.]

Thus, if we want to quantify the human impact on climate, and understand how to behave to prevent possible irreversible changes in our climate, we should include in this study the cloud.  And, as you've read, we're beginning to understand them to some degree, but there are quite a few things about them that we still need to understand to fully describe the impact both of the clouds and of human influence on the clouds.

Text: Marie Monier - Université Blaise Pascal de Clermont Ferrand / France
Reviewing and corrections: Prof. Andrea Flossmann - Clermont Ferrand; Stephen Gawtry - University of Virginia

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