Ever wondered what the Satellite weather image that we see so often mean? More often than not, we see the full disc image of the earth or images of geographical regions with white or colour impressions. Are the images direct reading types or is a deeper interpretation required.
Satellites can be either geo-stationary or polar orbiting;
Weather geostationary satellites are operated by EUMETSAT (Meteosat), the USA (GOES), Japan (MTSAT), China (Fengyun-2), Russia (GOMS) and India (KALPANA). They orbit in the earth’s equatorial plane at a height of 38,500 km. At this height, the speed of the satellite is the same as the earth’s rotation, and the satellite appears to be stationary over a certain point on the equator. This orbit allows the satellite to continually observe the same area: 42% of the earth’s surface. To get global coverage you need a network of 5-6 satellites. These satellites, however, do not see the poles at all.
There satellite images can be classified into three main categories:
- Visible images represent the amount of sunlight being reflected back into space by the clouds or by the Earth surface. Cloud free land and water will typically be dark while clouds and snow appear bright. Thicker clouds have a higher reflectivity and appear brighter than thinner clouds. However, it is difficult to distinguish among low and high level clouds in a visible satellite image. For this distinction, infrared satellite images are useful. Visible images can not be used when no solar light is available.
- Infrared images represent the infrared radiation emitted by the clouds or by the earth surface. They are actually measurements of temperature. For an infrared picture, warmer objects appear darker than colder objects. Cloud free areas will typically be dark, but also very low clouds and fog may appear dark. Most other clouds are bright. High level clouds are brighter than lower level clouds.
- Water vapour images represent the amount of water vapour in the mid and upper atmosphere. Water vapour images are useful for pointing out regions of moist and dry air, white regions on the imagery representing more moisture than dark regions.