| |  | | | Meteosat image in channel 1 taken on 21 December 1997 |
Meteosat
Five geostationary meteorological satellites have been placed in a ring around the Equator:
- Meteosat by ESA
- GMS by Japan
- INSAT by India and
- GOES E and GOES W by USA
Between them they produce updated images of global weather conditions every half hour, except for the polar regions.
| | Meteosat image in the thermal infrared channel, 21 December 1997 | |
Meteosat rotates on its own axis, which is parallel to the axis of the Earth. It rotates 100 times per minute. For each rotation it scans a 5 km wide strip from east to west. The strip is divided into 2500 scanning areas. For each rotation the scanner's mirror is adjusted so that a new strip can be scanned.
An image showing the entire Earth disk consists of 2500 strips, which are scanned in 25 minutes. After a five-minute pause the next image is started. The continuous stream of data is sent to the control centre in Darmstadt, Germany, where the material is processed. The scan is performed using three channels: one visible and near infrared, one medium infrared, and one thermal infrared.
| | | Meteosat image in the water vapour channel (2), 21 December 1997 |
In the visible and near infrared (channel 1), the albedo is measured for various surfaces. Clouds, snow, and ice have a strong reflection and are, therefore, seen as light grey. Dry and bare ground or sand is also light, while regions covered with vegetation have a slightly lower albedo and appear darker. Water surfaces have a very low albedo and are therefore dark. In channel 1 images the whitest clouds are the thickest, while thinner cloud formations appear greyish, because the surface of the Earth is partially visible. Space is black, as the empty space does not reflect light.
See the latest image from Meteosat.
These are albedo values for various types of surfaces. The value indicates the percentage of sunlight reflected from the surface.
| Surface | Albedo % | | Water | 5-10 | | Desert | 25-30 | | Fresh snow | 80-90 | | Old snow | 45-70 | | Green forest | 5-10 | | Green leaves | 5-25 | | Clouds (average) | 50-55 | | Clouds < 150 m thick | 25-63 | | Clouds 150-300 m thick | 45-75 | | Clouds 300-600 m thick | 59-84 | | Sea ice | 30-40 | | Bitumen road | 5-10 |
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In the thermal infrared (channel 3), cold surfaces are light and warm surfaces are dark. Like in channel 1, clouds appear as light areas, but there are significant differences. The lightest clouds are the coldest and must consequently be the highest in the atmosphere. Temperature drops with height above the surface of the Earth. The darker the cloud formation, the lower it is in the atmosphere. On infrared images it can be difficult to distinguish between low-lying clouds and cloudless areas, because the temperature difference between the cloud and a wet surface on Earth can be quite small. Space is white in these images as it does not radiate thermal infrared energy.
See the latest infrared image from Meteosat.
The water vapour channel is special, because it records at a wavelength where the atmosphere is not translucent, because the radiation is nearly 100% absorbed. The radiation received by the satellite in this channel comes from the content of otherwise invisible water vapour in the atmosphere. It is thus possible to analyse the transport of water vapour, which contains large quantities of latent energy. These movements are a significant factor in the distribution of energy throughout the atmosphere. The tropical rain forests assist this process by delivering enormous quantities of water vapour and energy to the temperate zones. The climatic significance of rain forest deforestation and the resultant impact on evaporation has yet to be clarified.
See the latest water vapour image from Meteosat.
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