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.
| By the end of 2008, the Met Office hopes to have a total of 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
are 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.
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Key to rainfall radar imagery |
Rainfall radar coverage across the British Isles |
