Radar measurements of rainfall
Short pulses of electro-magnetic waves, which travel at the speed of light (approx. 186000 miles per second), are transmitted in a narrow beam for a very short time (typically 2 microseconds). When the beam hits a suitable target, some of the energy is reflected back to the radar, which ‘listens’ out for it for a much longer period (3300 microseconds in the case of Met Office radars) before transmitting a new pulse. The distance of the target from the transmitter can be worked out from the time taken by a pulse to travel there and back.The radars do not receive echoes from tiny cloud particles, but only from the precipitationsized droplets. Drizzle is generally too small to be reliably observed, unless close to the radar, but rain, snow and hail are all observed without difficulty.
Each radar completes a series of scans about a vertical axis between four and eight lowelevation angles every 5 minutes (typically between 0.5 and 4.0 degrees, depending on the height of surrounding hills). Each scan gives good, quantitative data (1 and 2 km resolutions) out to a range of about 75 km and useful qualitative data (5 km resolution) to 255 km.