Above the Equator 36,000km up in space sit weather satellites launched by various countries. Because these satellites orbit at the same speed at which the Earth rotates, from the ground they appear to be fixed, and are therefore known as stationary satellites. The western United States is covered by the GOES-W (Goes West) satellite, the east by GOES-E (Goes East), Europe and its vicinity by METEOSAT, western Asia by METEOSAT-5, and east Asia including Japan by MTSAT (more commonly known as Himawari).
Location of the world's weather satellitesImages relayed by these five satellites can be combined to show the cloud situation across the whole planet. At the Japan Weather Association, these global cloud images are assembled automatically by computer four times a day, i.e. every six hours. The cloud images used in this edition of Earthrium are taken from this data.
Computer-synthesized cloud image © Japan Weather AssociationThe main task of weather satellites is to observe cloud movement. The data obtained is used to ascertain movement in the Earth's atmosphere, identify changes in weather conditions, monitor and provide forecasts for typhoons, heavy rain, and climate change etc., and supply information for the safe navigation of shipping and aircraft.
To photograph clouds, broadly speaking satellites are fitted with two types of sensor: visible light and infra-red. Recent satellites have infra-red sensors subdivided further into channels. For example, the Japanese weather satellite Himawari has four infra-red channels. The images used on the globe to the left were shot on infra-red channel 1. Infra-red 2 is used to track more detailed phenomena such as the movement of fog and volcanic ash, by identifying differences between its images and those shot using infra-red 1. Infra-red 2 can also detect the temperature of the ocean surface. Infra-red 3 meanwhile reveals the distribution of water vapor from the mid to upper layers of cloud, showing clearly where areas of low pressure will emerge, and the path traced by jetstreams. Infra-red 4 can observe nighttime fog. It is also becoming increasingly possible to find out about the three-dimensional structure of the air, for example the difference between higher and lower layers of cloud.