Clouds reflect one part of sunlight into space reducing the amount of solar energy which reaches the ground. But clouds don't always have this cooling effect on the Earth. Have you ever noticed that clear starry nights are much colder than nights when the sky is cloudy. Let us explain why!
Sunlight is made up of visible light (which, of course, we see!) and ultra-violet and infra-red radiation, both of which we humans can't see. Some animals can sense ultra-violet radiation (for example bees) whereas other animals such as snakes detect infra-red radiation. We humans can't 'see' infra-red radiation but we can feel it as heat.
1. Earth from space. Source: NASA.
Most of the harmful ultraviolet radiation emitted by the Sun doesn't reach the surface of the Earth. It is absorbed by the ozone layer in the stratosphere.
The atmosphere, the oceans and most of all the clouds, reflect part of the sunlight which reaches the Earth back into space (this fraction of reflected light is called the albedo). This reflected light means that astronauts can see the Earth from space.
Around 70% of energy from the Sun reaches the surface of the Earth and most of this is absorbed by the surface. Just as our skin warms up when the Sun shines on it, the Earth warms up and emits infra-red radiation and this heats the air above the ground. This infra-red radiation has a different energy to the sunlight absorbed by the surface of the Earth.
2. This image shows the average amount of heat (in Watts per square meter) that was emitted from the Earth back into space during a day in July 2000. Yellow colours show where most heat (or outgoing radiation) escapes out of the top of the atmosphere. Purples and blues show intermediate values and white colour shows the lowest values. Desert regions emit a lot of heat, while the snow and ice-covered continent of Antarctica emits very little heat. Source: NASA.
If all the infrared radiation emitted from the Earth's surface escaped directly back into space without being trapped, our planet’s temperature would be -18°C and only polar bears would be happy!
3. A baby polar bear and its mum. Source: USFWS.
The Greenhouse Effect and clouds
The average temperature of our planet isn't -18 oC and this is because of the Greenhouse Effect (see the Lower Atmosphere Unit for more details on the natural and enhanced Greenhouse Effect). The presence of clouds and greenhouse gases such as water vapour and carbon dioxide keep our average temperature much higher. Here we look just at the role of clouds.
Clouds cover about 50% of the sky and these clouds absorb radiation emitted from the surface of the Earth. The clouds then re-emit a portion of the energy into outer space and a portion back toward the surface. It is this portion which warms our planet and is why clouds have the capability for reducing temperature differences between the day and the night.
During the day the ground is warmed up by the sunshine. The fewer clouds there are in the sky, the more the surface of the Earth is heated by the Sun.
If there are no clouds during the night, most of the infra-red radiation emitted by the Earth goes back into space and the night is cold. If the sky is cloudy, part of the infra-red radiation from the Earth is trapped by the clouds. Some of this radiation is then reflected back to the Earth's surface and the temperature of the air above the ground is warmer that it would have been if the night had been cloudless.
5. a) The night is cold if there are no clouds in the sky. Source: J. Gourdeau.
5. b) Clouds send back infra-red radiation to the surface of the Earth.
There are huge differences in day and night time temperatures in deserts. Very few clouds form over deserts because the air is so dry. This means the surface heats up during the day and escapes rapidly at night. Night-time temperatures can be 35oC lower than daytime ones.
Climate forcing by clouds
To summarise, clouds can warm the Earth by trapping heat beneath them and this is called "cloud greenhouse forcing". This process tends to cause a heating or "positive forcing" of the Earth's climate. But clouds can also cool our planet by reflecting sunlight back into space. The balance of this opposing cloud albedo and cloud forcing determines whether a certain cloud type will warm our planet or produce a cooling effect.
6. Cloud Forcing from 1985 to 1986. The areas where clouds cause overall cooling are shown in colours that range from yellow to green to blue. In some areas, clouds cause warming and these areas are shown in colors that range from orange to red to pink. Source:NASA Langley Research Center.
High thin clouds like cirrus clouds contribute to heating, whereas low thick stratocumulus clouds tend to cool our planet.
Nowadays, scientists think that the overall global effect of clouds is to lower the temperature of the Earth.
Knowing whether clouds will warm or cool our planet as the Earth warms up is one of the major scientific challenges for the future. If global temperatures rise, more water vapour will be present in the atmosphere, therefore leading to more clouds. We don't know yet whether these clouds will reflect more sunlight back to space and reduce the temperature rise or trap more heat energy in the atmosphere and enhance warming further.
About this page
author: Dr. Justine Gourdeau - LaMP, Clermont-Ferrand, France scientific reviewer: Dr. Vincent Giraud - LaMP, Clermont-Ferrand, France last published: 2008-10-24
