The Dual Role of Clouds
Clouds play a prominent role in the climate machine. Apart from their role in generating precipitation, they have a considerable influence on radiation. Their influence on our perception of the weather is not a mere coincidence.
With regard to solar radiation, clouds act mainly as parasols, reflecting back into Space a large proportion of the Sun's rays. The reflecting ability, or albedo, of thick clouds is very high—approximately 80%.
The effect of clouds on infra-red radiation emitted by the Earth is less obvious, but just as perceptible. For example, we know that the coldest morning frosts follow nights with clear skies. In fact, during the night, the Earth, which is no longer being heated by the Sun, cools down by emitting infra-red radiation. Clouds block this radiation and act like an insulating layer above the Earth's surface. This is the greenhouse effect at work.
The clouds thus play a "dual role": they cool the planet with their parasol effect and warm it with their greenhouse effect.
We are able to measure these two effects from Space. Each one is considerable in terms of absolute value and can represent a few tens of W/m². Their modification by the processes of climatic interaction could therefore correspond to very large variations, compared with the effect of man-made greenhouse gases which only account for a few W/m² (about 1.5 W/m² for carbon dioxide alone). These two effects also vary greatly according to the regions concerned. Though we know that overall the net cloud radiative forcing has a cooling effect on the planet (cf. fig. below), we are still unsure about the consequences of global warming on cloud cover, type, and distribution. Similarly, we do not know how any such modifications would affect the Earth's radiative budget.
 | Radiative forcing as a zonal mean at the top of the atmosphere obtained from ERBE space observations and simulations from general circulation models. This figure shows that forcing values have opposite signs for the visible and infra-red frequencies (from Potter and Cess, 2004). |
In order to improve our understanding, we have to find a better way of quantifying the radiative impact of clouds. However, in order to evaluate a cloud's radiative properties, we have to know its macrophysical characteristics (altitude and temperature of the base and the top, spatial extension and cover rate for multi-layer clouds), its water and ice content, its microphysical characteristics (phase [water, ice or mixed], size of droplets, shape and orientation of ice crystals), and its environment, i.e. the nature of the layer beneath the cloud.
Ice clouds are currently the least well-known type as they are difficult to observe. Cirrus clouds, which are thin and non-homogeneous, are cold and have weak reflectivity, which makes them difficult to detect using classic radiometers operating at visible and infra-red frequencies. In addition, the naturally great variability in size (1 to 1,000 µm), shape (spherical, hexagonal platelets or columns, polycrystals) or orientation (random or horizontal) of the crystals of which they are comprised can cause the cloud's radiative properties to vary by a considerable amount. In order to penetrate the secrets of this type of cloud, new observation techniques have now become necessary.
