The radar's computer determines the direction of the rain from the orientation of the antenna, and the distance to the rain from the time taken for the radar signal to return to the receiver. In summary, the display produced by the radar's computer gives a horizontal map of where rain is falling and an estimate of how heavily it is falling. The estimated rainfall intensity usually appears as light to medium because of the smaller raindrop size produced in such rain bands. The estimated rainfall intensity can be medium to heavy owing to the high rainfall rates from such clouds. Heavy Precipitation from Thunderstorms Radar echoes from the rain and hail produced in thunderstorms are very sharp-edged cells with intense cores indicating heavy rainfall.
Hailstones produce particularly intense echoes because of their large size.
Thunderstorm precipitation cells can appear as isolated cells or in clusters or lines. Each cell tends to last for 30 minutes or more. Also a very slow moving cell or the repeated passage of a number of cells over a particular location could indicate potential for flash-flooding. Tropical Cyclones Tropical cyclones produce widespread heavy rain. Of course, many tropical cyclones are so extensive that they are rarely seen entirely by a single radar.