Weather Radars (also called weather surveillance radar and Doppler weather radar) can detect precipitation (i.e. rainfall) in real time.
RADAR stands for RAdio Detecting And Ranging. It uses radio waves to detect location and quantity of precipitation in the atmosphere. The basic principle is similar to an echo that you experience if you shout out aloud in a cave and hear your voice bounce back off the wall and call again. Likewise, a RADAR unit sends out radio waves that are reflected and scattered back and based on how much of the echoed waves are received, the RADAR unit can detect where and how much precipitation exists.
This reflectivity of radio waves was discovered by German Heinrich Hertz in 1887, and has been used extensively ever since to understand more about the atmospheric makeup of the earth.
Forty years earlier, in 1842, the Austrian physicist Christian Doppler discovered what we now know as the Doppler Effect. The Doppler effect is the phenomenon that objects moving towards us have a higher frequency than objects moving away from us. Think of when an ambulance with its siren going drives past: it sounds high when it’s approaching (it has a high frequency) and drops to a lower pitch (a lower frequency) when it’s driving away. The same principle can be translated to Radio waves, and in this way, Weather Radars can detect position and intensity as well as the speed of precipitation. Moreover, since rainfall generally moves with the wind, Radar technology can also detect wind velocity.
The Bureau of Meteorology collects and publishes Radar information every 10 minutes, and also uses Radar information to generate localised weather warnings (e.g. severe thunderstorms in Sydney CBD). The image below is a typical RADAR image from the Bureau of Meteorology, where precipitation is superimposed over a geographical map. The colour code of 15 colours indicates how intense the rainfall is, from very light in white, right through to extremely heavy or hailstones in red and black.