Radar detects objects by transmitting a pulse of radio waves and looking for signals reflected back from the object. By measuring the time taken for the pulse to reach the object and travel back to the radar, the distance can be calculated. By rotating the antenna and sending out a stream of pulses, the radar can build up a picture of objects.
Rain, snow and hail reflect radar waves. Using radar we can get a picture of the extent and intensity - the greater the intensity of rainfall, the stronger the signal returned - of the rainfall.
At longer ranges, typically over 100km, the accuracy falls off due to the curvature of the Earth. Sometimes the radar will not 'see' rain at long ranges or report rain at high altitudes that does not reach the ground.
Radars also 'see' the ground and while measures are taken to reduce echoes returned from the ground, some appear on the images. Frequently echoes can be seen from the Mourne and Galtee mountains even when no rainfall is present.
The images here are a composite of data from two radars - one at Dublin Airport and the other at Shannon Airport. The shaded area shows the coverage of the radars that are present. Data is updated every 15 minutes.