Met Office: More about rainfall radar

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More about rainfall radar

Radar works by sending out electromagnetic pulses and measuring how long they take to return from a target, often 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 radar sends out a pulse at a wavelength of 5.6 cm which is reflected by precipitation. This is compared to a number of rain gauges and adjusted accordingly.

Rainfall radar coverage across the British Isles

By the end of 2008, the Met Office hopes to have a total of be 19 weather radars operating in the UK, Jersey and Republic of Ireland*, each with a range of approximately 250 km. Each radar completes 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 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 5 km.

The weather radars show rain, hail and snow but drizzle can be hard to detect because the droplets are so small.

Processing at the Met Office normally removes:

permanent echoes or reflections from hills and buildings

anomalous propagation (anaprop) which often occurs in anticyclonic conditions

the strong echoes produced when falling snow starts to melt to rain.

*Radar data for the Republic of Ireland is supplied by Met Éireann in accordance with the regulations of ECOMET.