| | Introduction to Meteosat images
Five identical geostationary satellites
Five identical geostationary satellites are placed in a constellation around the equator. There is Meteosat (ESA), GMS (Japan), INSAT (India), and GOES E and GOES W (USA). Together they produce new images of global weather conditions every half hour. They do not, however, cover the Polar Regions.
 | | | Meteosat Second Generation artistic view |
Meteosat rotates on its own axis, which is parallel to the Earth's axis. 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 pixels. After each rotation, the scanner mirror is tipped, and a new strip is scanned.
An image showing the entire Earth disk consists of 2500 strips, which are scanned in 25 minutes. After a five-minute pause, the process starts again for the next image. The continuous stream of data is sent to the control centre at Darmstadt in Germany, where the material is processed. The scan is performed in three channels: one visible and near infrared, one medium infrared, and one thermal infrared.
| | IR image | |
In the thermal infrared channel (IR), cold surfaces are light and warm surfaces are dark. Although clouds may appear the same in thermal infrared and visible channels, there are significant differences. In thermal infrared, the lightest clouds are the coldest, and must consequently be highest in the atmosphere, as temperature decreases with height above the Earth's surface.
The darker the cloud formation, the lower it is in the atmosphere. For IR 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 the Earth can be quite insignificant. Space is white in these images, as it radiates very little thermal infrared energy. In this exercise we will be using IR imagery.
| | | VIS image |
Visible channel images (VIS) display the amount of sunlight being reflected back into space by clouds or the Earth's surface. Cloud-free land and water will typically be dark, while clouds and snow will appear bright. Thicker clouds have a higher reflectivity and appear brighter than thinner clouds.
However, it is difficult to distinguish between low and high level clouds in a visible satellite image. For this scenario, infrared satellite images are more useful. Visible images cannot be acquired when solar radiation is not available.
| | WV image | |
Water vapour (WV) images display infrared radiation levels which are more apparent in cloud-free atmospheric conditions.
Water vapour images are useful for mapping regions of moist and dry air. Darker colours indicate drier air, while brighter whites depict more moisture in the air.
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