Rain radar is one of the most reliable tools in everyday weather, and also one of the most frequently doubted — usually by someone standing in dry air looking at a green blob on their phone. Both reactions are fair. Radar is very accurate at some things and genuinely limited at others.
Overview
For the question “is precipitation falling somewhere right now”, radar is highly accurate. For “exactly how much rain will land in my garden”, it is an estimate. The distinction matters, because radar never measures rainfall on the ground at all. It measures energy reflected by particles in the air, then converts that into an estimated rain rate.
The conversion uses an empirical relationship between reflectivity and rain rate, and that relationship changes with drop size, precipitation type and temperature. Different assumptions produce different numbers from identical raw data, which is one reason two radar apps can disagree.
The Three Reasons Radar Looks Wrong
1. Virga: rain that never lands
Radar detects raindrops in the air. In dry conditions those drops evaporate before reaching the ground. The radar is not malfunctioning — it really is raining a kilometre above you. This is extremely common in summer and in arid regions, and it is the single biggest source of “the radar is wrong” complaints.
2. Beam height and the curve of the Earth
A radar beam travels in a straight line while the Earth curves away beneath it. Close to the station the beam samples near the surface. At 150 km it may already be scanning at around 3,000 metres, sampling air well above your local weather. Shallow drizzle underneath goes undetected. This is why coastal areas and places distant from any station show gaps.
3. Resolution versus reality
A radar pixel typically covers a square kilometre or more. Summer convective showers are often smaller than that. One street floods while the next stays dry, and the map can only display an average of the two.
Other Things That Degrade Accuracy
- Beam blocking. Mountains, tall buildings and wind turbines block or scatter the beam, creating permanent shadows.
- Bright band. Where snow melts into rain, the melting layer reflects unusually strongly, producing a ring of exaggerated intensity around the station.
- Anomalous propagation. Under certain temperature inversions the beam bends downward and reflects off the ground or sea, showing precipitation where none exists.
- Non-weather targets. Flocks of birds, insect swarms, chaff and even wildfire smoke can return an echo.
- Attenuation. Very heavy rain close to the station absorbs the signal, so storms behind it appear weaker than they are.
Where Radar Is Genuinely Excellent
None of the above makes radar unreliable for its main job. Within roughly 100 km of a station, in the 0 to 90 minute window, radar outperforms any forecast model, because you are watching precipitation that actually exists rather than a prediction of precipitation that might.
- Detecting whether it is raining now: very accurate.
- Tracking direction and speed: very accurate. Extrapolating a band forward 30–60 minutes works well.
- Relative intensity: reliable. If one area is red and another green, the red one is genuinely wetter.
- Absolute rainfall totals: an estimate, typically within 20–40% of gauge measurements unless calibrated.
National meteorological services improve on that last point by adjusting radar against networks of ground rain gauges. Those gauge-adjusted products are what official rainfall statistics use — not raw radar.
How to Get the Most Out of It
- Watch the loop, not the frame. Movement carries more information than any single image.
- Trust it more at short range. Detail close to a station is far better than at the edge of coverage.
- Cross-check in winter. Near freezing, check the temperature map to know whether you are seeing rain or snow.
- Use lightning for severity. Radar shows precipitation, not electrical activity. The lightning map tells you if a cell is genuinely convective.
- Switch to forecasts beyond 90 minutes. Radar cannot show rain that does not yet exist. Use the 7-day forecast.
Frequently Asked Questions
How accurate is rain radar?
Very accurate for detecting whether precipitation is currently falling and tracking where it is heading over the next 30 to 90 minutes. Less accurate for exact rainfall totals, which are estimates derived from reflectivity and are typically within 20 to 40 percent of ground gauge measurements unless the radar has been gauge-adjusted.
Why does the radar show rain when it is not raining?
Most often virga: the rain is evaporating before it reaches the ground. Other causes include the radar beam scanning above your local weather at long range, showers smaller than a single radar pixel, and non-weather echoes from birds, insects or ground reflections.
Why does the radar show nothing when it is raining?
Usually because the beam is passing above shallow low-level rain, which happens at long range from the station or behind blocking terrain. Light drizzle from low cloud is the type most often missed.
How far can weather radar see?
Useful range is roughly 150 to 250 km from each station, though quality degrades with distance as the beam rises higher above the surface.
Do different radar apps show different rain?
Yes. They may use different source networks, different reflectivity-to-rainfall conversions, different colour scales and different smoothing. The underlying weather is the same; the presentation and assumptions differ.
Is radar or a forecast better for the next hour?
Radar. Within about 90 minutes you are observing precipitation that already exists, which beats a model’s prediction of it. Beyond that window, forecasts take over.