Weather radar transmit electro-magnetic waves.
The radar detects water droplets of diameters exceeding approximately 1 mm. Due to the millions of water droplets contained within the radar sample volume, the reflected power from these droplets can be measured. The larger the diameter of the droplets and the more numerous, the higher is the reflected power from the droplets back to the radar antenna. The returned power is related to a radar reflectivity value , namely dBZ, which is a logarithmic derived value from the returned power of the droplets sampled. Due to the fact that a relationship exists between the droplet received power and the sizes of the droplets, an estimated rainfall intensity can be derived.