If the microwaves hit an obstacle like rain they are reflected and detected by the radar antenna. The time between the transmission and echo reveals the location of the rainstorm, and enables meteorologists to work out where the storm is heading. The intensity of the echo reveals the severity of the storm. But intensity is also influenced by the raindrop size distribution, which depends on the type of storm and geography. The distrometer measures raindrops from 0.25 mm to 5 mm in diameter.
Small drops are spherical but the big ones flatten out into an oblate shape.
We need to know the typical raindrop size distribution for Sydney so that we can calibrate our radars to compensate for this effect. The relationship between the frequency of raindrop sizes and energy reflected by the rain is one of the big uncertainties. If we don't measure this, we have difficulty in telling how heavy a downpour is from radar measurements. The project will run from September 1999 until November 2000, a period taking in the Olympics.