How rainfall radar works - and how to read it properly

Rainfall radar is the single most useful weather tool ever put in the public's hands - and one of the most misread. Here's what the map is actually showing you, why it sometimes shows rain that never arrives, and how to read a radar loop the way a forecaster does.

Published 13 July 2026 · WeatherZ Guides

What a weather radar actually measures

A weather radar doesn't "see rain falling on the ground". It sends out pulses of microwave energy and measures how much bounces back off precipitation in the air. The strength of that returned signal - reflectivity - is converted into an estimated rainfall rate. The UK is covered by a network of Met Office radars whose scans are stitched into a single national composite image, typically updated every five minutes.

Two details matter for reading it correctly:

• The radar beam is measuring precipitation hundreds or thousands of feet up, not at the surface - and the further from the radar, the higher the beam sits.
• The colours are estimates of rate (light drizzle through torrential downpour), not guarantees of what reaches your garden.

What the colours mean

Almost every radar map, including the one in WeatherZ, uses the same convention: blues and greens for light rain, yellows for moderate, oranges and reds for heavy, and purples/whites for intense downpours or hail cores. The exact palette varies by provider, but the rule of thumb holds: if a yellow-or-stronger cell is heading for you, expect a proper soaking; if it's red or purple, expect thunder to be possible.

Why radar sometimes lies

Rain that never reaches the ground (virga)

In dry air - common after a warm spell - rain can evaporate completely between the radar beam and the surface. The map shows rain; your street stays dry. This is virga, and it's the most common "the radar is wrong" complaint.

Drizzle the radar can't see

The opposite also happens: shallow drizzle can sit below the radar beam, especially far from the nearest radar site. You're getting wet; the map shows nothing. Coastal and hilly areas see this most.

The bright band

Where snowflakes melt into rain, the half-melted flakes reflect unusually strongly, painting a ring or stripe of apparently heavy rain that's really moderate. Winter radar images often exaggerate intensity for exactly this reason.

Ghost echoes

Unusual atmospheric conditions can bend the beam into the ground ("anaprop"), and ships, wind farms, flocks of birds and even insect swarms can all leave marks on the map. If an echo doesn't move like weather, it probably isn't.

Reading a radar loop like a forecaster

1. Watch the loop, not the frame. A single image tells you where rain is; the animation tells you where it's going and whether it's growing or decaying.
2. Track cell motion, not the band. Showers often travel along a line's axis - individual cells can "train" over the same spot, which is how flash flooding happens.
3. Growing or dying? Cells that brighten frame-on-frame are intensifying. Fading edges usually mean the rain is running out of energy - or evaporating into dry air.
4. Respect the nowcast limit. Short-range radar extrapolation ("nowcasting") is excellent for the next 30–60 minutes and increasingly unreliable beyond that, because it projects motion rather than physics. For anything past an hour or two, you need a forecast model.

How WeatherZ uses radar

WeatherZ renders its UK radar from Met Office open data, with an automatic fallback to the RainViewer global composite if our pipeline is ever behind - every frame is timestamped so you can always see how fresh the picture is. Full details, including limitations and attribution, are on our data sources and methodology page. And when the radar and reality disagree, that's precisely where community ground-truth reports come in - see how to report severe weather.