A rain radar shows you where precipitation is falling right now — and where it is heading. Instead of relying on a forecast written hours ago, you watch the actual rain move across a live map. But how does a machine on the ground know that rain is falling twenty kilometres away? This guide explains the technology in plain language.
Overview
A weather radar is essentially a powerful spinning antenna that sends out short pulses of microwave energy. When a pulse hits raindrops, snowflakes, or hailstones, a small part of that energy bounces back to the antenna. By measuring how long the echo takes to return, the radar knows how far away the precipitation is. By measuring how strong the echo is, it estimates how heavy the rain is. One full rotation scans everything within roughly 200–300 km, and the results are painted onto a map as the familiar coloured blobs.
Step by Step: From Pulse to Picture
1. The radar transmits a pulse
The dish emits a microwave pulse lasting about a millionth of a second, then switches to listening mode. It spends over 99% of its time listening, not transmitting.
2. Raindrops reflect the signal
Water is very good at reflecting microwaves. Bigger drops and heavier rain return stronger echoes. The strength of the returned signal is called reflectivity, measured in dBZ — we cover this in detail in our guide to radar reflectivity (dBZ).
3. The echo is timed and mapped
Radio waves travel at the speed of light, so an echo that returns in one millisecond puts the rain at about 150 km away. Combining distance with the antenna’s direction pinpoints each rain shower on the map.
4. Motion is measured
Modern radars are Doppler radars: they detect tiny frequency shifts in the returning echo, which reveal whether the rain is moving toward or away from the radar and how fast. That is how radar can show storm rotation — read more in What Is Doppler Radar?
5. A composite map is built
A single radar cannot see the whole country, so national weather services merge dozens of radars into one seamless mosaic. The live rain map you see on RainyMap combines these feeds and refreshes every few minutes, with each colour representing rain intensity — see Rain Radar Colors Explained.
What Rain Radar Can and Cannot See
Radar is excellent at detecting rain, snow, and hail that is already falling, and at showing where it will drift over the next hour or two. It cannot see clouds that are not precipitating, fog, or storms that have not formed yet. The beam also travels in a straight line while the Earth curves away beneath it, so at long range the radar overshoots low drizzle — one reason the map occasionally misses very light rain. Mountains can block the beam and create permanent blind spots.
Frequently Asked Questions
How often does rain radar update?
Most weather radars complete a full volume scan every 5 minutes, and composite maps typically refresh every 5–10 minutes. The image you see is therefore at most a few minutes old.
How far can a rain radar see?
A single radar reliably covers about 200–250 km. Beyond that, the beam sits too high above the ground to catch most rain. National networks overlap many radars to close the gaps.
Why does radar sometimes show rain when it is dry outside?
The radar may be detecting rain that evaporates before reaching the ground (called virga), or the beam may be catching precipitation high in a cloud that has not fallen yet. Ground clutter — birds, wind farms, even insect swarms — can also produce false echoes.
Can rain radar detect snow and hail?
Yes. Snow reflects more weakly than rain, so it appears in the lighter colours, while hail reflects very strongly and shows up in the most intense colours on the scale.