Observations

There are many data sources used for weather forecasting include ships, aircraft, oil rigs, buoys and balloons, as well as manned land stations around the world. In recent years automation has often assisted or replaced the human observer and has provided information from inhospitable and remote areas. Information from remote-sensing equipment, both on the ground and in space, increasingly supplements and complements the older, conventional systems.

Surface Observations

Traditionally, meteorologists have relied upon observations taken near the Earth's surface using instruments (e.g. barometers, thermometers, anemometers and rain gauges) and visual observations (e.g. cloud and weather type). These surface observations are made at approved sites on land, and from ships at sea.

Standard types of instruments are used, with observations usually made at least every three hours, and in many cases hourly. Over land in the UK there are 33 key observing stations which are needed to define the broad-scale weather patterns. They are manned by professional meteorologists, with 12 making observations every hour, both day and night. The other 21 are manned during the daytime, thereafter switching to an automatic system. An additional 29 sites are manned by auxiliary observers such as coastguards, and there are more than 100 fully automated sites.

For weather observations at sea, the Met Office is indebted to the crews of 400 vessels of the UK Voluntary Observing Fleet and to observers on about 30 offshore drilling platforms. This is part of a much larger scheme officially involving around 6,500 ships from 53 nations, although the real number in use at any one time is closer to 3,500 ships. To fill in some of the gaps, there is a network of ocean buoys, most drifting, but some moored.

Upper-air Observations

Balloon-borne instruments offer important sources of upper-air information (known as radiosondes) including information about the pressure, temperature and humidity through the atmosphere. Wind can also be deduced from the track of the radiosonde. The radiosondes can reach a height of over 20 km (66,000 feet); they are released twice a day at the same time (midday and midnight UTC) all over the world.

Within the global network, the Met Office maintains six sites in the UK. Two of these are fully manned while the remaining four sites are equipped with autosondes, which are released remotely. There are also Met Office radiosonde sites in Gibraltar, St Helena and the Falkland Islands. Near the UK, there is one fully manned site in the Republic of Ireland and a variety of different sites in continental Europe. At sea, there are automatic systems that release radiosondes from the decks of merchant ships.

Aircraft reports (known as AMDARs) of wind and temperature along their flight routes, including take-off and landing, help boost the upper-air information.

Furthermore a type of radar known as a Doppler radar is used to measure the winds vertically through the atmosphere. When displayed over a period of type, this Windprofiler data shows the vertical profile of wind above the site and how it changes with time. At the time of writing, there are windprofiler observations made at six sites in the UK, two in the Irish Republic and one on the Isle of Man, as well as more in continental Europe.

A system for measuring the amount of water vapour in the atmosphere is being developed, which is known as the Ground-based GPS Network. This uses information from Global Positioning Satellites (GPS) and about 150 stations are envisaged. This data has been shown to be of great value in numerical modelling.

Radar

As well as the Windprofiler radars, there is a network of weather radars that provides a picture of the distribution of rainfall. From the radar it is possible to work out where it is raining and how heavy the rain is. The network includes sites provided by the Republic of Ireland and the States of Jersey and covers the whole of the British Isles. Extensive radar information from the continent is also available.

Radar pictures are often shown on television forecasts, and are also used by the Environment Agency for river management and flood warnings.

Satellites

Since the first meteorological satellite was fired into orbit in 1960, satellites have become essential tools for weather forecasters. The satellites used by meteorologists fall into two categories.

Polar-orbiting satellites provide pictures of clouds, and information about the temperature through the atmosphere.

Geostationary satellites remain over the equator, stationary with respect to the earth. This is achieved by having the satellite in orbit at a height of about 36,000 km. At this height it takes exactly 24 hours to complete one orbit, so it always views the same part of the globe.

Meteosat, the name given to the European geostationary satellites, like their US, Japanese and Indian counterparts, give sequences of cloud images. From these, the development and movement of weather systems can be followed and, of particular importance, tropical storms can be tracked. The motion of specified areas of cloud can also be followed to calculate the wind at various levels in the atmosphere.