How to Read a Rain Radar Map: Colours, Timing and Nowcasting

Radar shows reflectivity — how much energy bounces back off raindrops — mapped to colour.

The colour scale Light blue and green: drizzle to light rain, under about 2.5 mm/hr. Yellow: moderate, roughly 2.5-10 mm/hr. Orange and red: heavy, 10-50 mm/hr. Purple or white: extreme, often hail.

Estimating arrival Play the loop and note how far the leading edge moves between frames. If it covers 10 km in 10 minutes it's tracking at 60 km/h; a cell 30 km away arrives in about half an hour. Systems usually keep direction and speed for 30-60 minutes, which is why radar nowcasts are strong inside an hour and weak beyond two.

Where radar misleads Beams travel in straight lines while the Earth curves, so at long range the beam overshoots low cloud and misses drizzle. Mountains block beams and leave shadows. Radar can also pick up birds, insects and even wind-turbine blades — those show as static speckle that does not move with the system.