By viewing weather systems over whole regions of the planet – especially regions such as oceans where data was previously very sparse – satellites give forecasters new advantages and new insights into weather and climate. Tiros - the first weather satellite - was launched by the USA on 1 April 1960, just over three years after the Russians launched Sputnik in October 1957.
The design of weather satellites has to take into account two key things; what kind of imagery is required and which part of the world it is required for. In terms of image type, satellite sensors can take images in different parts of the electromagnetic spectrum. The most useful parts of the spectrum for weather forecasting are visible light and infrared.
Comparing weather images
Visible light sensors
Visible light sensors provide detailed visual data on cloud shapes, textures, patterns and thicknesses, clearly discriminating between cloud and land or sea. Unfortunately, these sensors only provide data during daylight hours and the appearance of features varies with the angle of the Sun.
Infrared sensors: Temperature
The infrared spectrum is very sensitive to temperature and channels are visible both day and night. Different tones on infrared temperature sensor images indicate detailed temperature data of land, sea or cloud surfaces. Combined with other data, cloud height, thickness and likely nature of precipitation can be estimated. Unfortunately, the physical structure of weather systems can be difficult to make out on infrared sensor images, since air temperature rather than cloud is being observed. Features such as low stratus and fog can be difficult to see.
Infrared sensors: Water vapour
Certain ranges of the infrared spectrum are strongly absorbed by water vapour; images in this part of the spectrum therefore show the presence or absence of water vapour in the atmosphere. Water vapour infrared sensors provide data on water vapour content of the air even if no clouds are visible. This allows forecasters to see circulation patterns which might influence future weather conditions. As with infrared temperature sensors, the physical structure of weather systems can be difficult to make out, though in this case moisture rather than cloud is being observed.
By combining different sensors on the same satellite a range of different images – each providing slightly different information – can be obtained.