Using satellite imagery

The first meteorological satellite, TIROS, was launched in 1960 by the USA. Initially satellite images were treated purely as qualitative pictures, which were manually viewed and interpreted by meteorologists. Satellite imagery provides a picture of the current cloud conditions and is a familiar sight on TV weather forecasts. However, satellite imagery can also undergo various types of quantitative processing to obtain information on variables such as wind vectors, cloud height, cloud cover, surface temperature, sea-ice cover and rainfall.

Satellite imagery can also capture the development of transient features such as areas of fog, dust storms and plumes of volcanic ash. Slower changes in surface features, such as snow cover and vegetation, can also be inferred from the imagery.

How imaging works

Radiant energy from the earth is measured by a satellite radiometer and stored as digital values in two-dimensional arrays of pixels, which make up the image. The radiometer measures at different wavelengths which provides complementary information about the atmosphere and surface:

• Infrared radiation tells us about the temperature of emitting bodies, such as cloud tops or the surface in cloud-free regions. Infrared images are good for viewing high clouds and surfaces, at any time of the day or night. Water vapour (WV) images show radiation in the water-vapour absorption band and are good for viewing clouds and upper-level water vapour distributions in cloud-free areas
• Visible radiation shows low and high clouds, but only by reflected sunlight, so no images are produced at night. However, the pixel size of these images can be finer than for the infrared images, allowing more detail to be seen

How the Met Office uses satellite imagery

• By assimilation of satellite imagery products into NWP models to improve the forecast
• Through development of products for presentation to the forecasters in the form of enhanced imagery and movie loops

Satellites used by the Met Office

Satellite imagery used in meteorology is produced by instruments on board two types of satellite.

Polar orbiters are positioned about 900 km above the surface of the Earth, in a sun-synchronous orbit, which means they see the same part of the Earth at the same time each day. Polar orbiters make about 14 orbits a day and can view all parts of the atmosphere/surface at least twice a day. Although their frequency is limited, there is finer detail (typically around 1 km size pixels) since they are relatively close to the Earth's surface.

The main polar satellites used are the NOAA and Metop series, although several research satellites also provide data. Imagery from the polar orbiters is from the Advanced Very High Resolution Radiometer (AVHRR) sensor on the NOAA and METOP satellites and the Moderate Resolution Imaging Spectroradiometer (MODIS) data on the NASA Terra and Aqua research satellites.

Geostationary satellites are positioned about 36,000 km above the equator in a geostationary orbit, which means they are fixed in position above one part of the Earth. These satellites scan the same area continuously, so give more frequent images (15-30 minutes), but the detail is coarser (typically 3-10 km size pixels).

Geostationary satellites used by the Met Office are Meteosat, at longitudes of 0° E and 57° E, MTSAT at 140° E and GOES at 75° W and 135° W. The Meteosat Second Generation satellites — the first of which was launched in 2002 — provide greatly enhanced imagery over Europe and Africa. Work continues to fully exploit these data.