Rainfall radar

Weather radars send out electromagnetic pulses to measure the location and intensity of precipitation - including rain, hail and snow - in real time.

Radar works by sending out electromagnetic pulses and measuring how long they take to return from a target, e.g. an aircraft or a ship. It has been known for many years that other objects can also create a return, e.g. flocks of birds or precipitation. It was soon decided that being able to 'see' precipitation would be of great value, so much investigation was done to perfect this method. In simple terms, a weather radar sends out a pulse at a wavelength of 5.6 cm which is reflected by precipitation (this is then compared to a number of rain gauges and adjusted accordingly).

In operational use, weather radar is in fact the only means available for measuring the location and intensity of precipitation in real time. Data is used directly by weather forecasters, and is fed into forecast models. It is also crucial for aviation (both civil and military) and flood forecasting (in partnership with the Environment Agency in England and Wales, and the Scottish Environmental Protection Agency in Scotland) over large areas in critical rapidly responding catchments and urban areas, underpinning the protection of life and property in both instances. Weather radars observe rain, hail and snow, but drizzle can be more difficult to detect because the droplets are so small. Doppler radar functions are also used for the detection of dangerous wind conditions (e.g. wind shear) which constitute a significant hazard to aviation safety.

The UK and Ireland weather radar network composite operates 24/7/365 and is currently composed of a total of 18 weather radars*, of which 15 are operated and maintained by the Met Office. Each radar provides data out to 255 km, completing a series of scans about the vertical axis at different elevation angles every five minutes.

Data from the radars is sent to the Met Office HQ (at Exeter) for processing and a composite picture of the precipitation over the British Isles is produced. This picture is displayed using colours to denote different intensities, with a resolution of up to 1 km available. Processing at the Met Office normally removes:

  • permanent echoes or reflections from hills and buildings (known as clutter**),
  • anomalous propagation (anaprop) which often occurs in anticyclonic conditions,
  • the strong echoes produced when falling snow starts to melt to rain.

The Met Office safeguards its operational radio sites against such development that may adversely affect capability - see Safeguarding

*Radar data for Jersey and the Republic of Ireland also contributes to the UK composite, with data supplied by the Jersey Meteorological Department and Met Éireann in accordance with the regulations of ECOMET.

**Most clutter can be processed, but wind farms in line-of-sight of a weather radar can cause significantly detrimental impacts to operations.

Last updated: 16 May 2012