Why Your Rain Gauge and a Rainfall App Disagree

Radar rainfall and a backyard gauge measure different things in different places. Here is where the gap comes from, and how to compare them fairly.

Published September 29, 2026 · Raindex

They are measuring two different things

A rain gauge catches the water that lands in a funnel a few inches across, at one spot. A radar-based total, like the one Raindex shows, is an estimate for a whole grid cell about 1 km on a side, built from radar echoes high above the ground and then corrected against gauges in the region.

NOAA's Multi-Radar Multi-Sensor system (MRMS) combines roughly 180 US and Canadian weather radars into a national mosaic at 1 km and 2-minute resolution (Zhang et al., 2016). That is extraordinary coverage, but a cell average and a funnel will rarely match to the hundredth of an inch, and they are not supposed to.

Where the difference comes from

Most disagreements trace back to one of five causes:

The beam is high up. The radar beam rises with distance from the radar, so far from a radar site it samples rain well above the ground. Rain can evaporate, drift in the wind, or grow on the way down.
Rain varies inside a kilometre. A thunderstorm core can put an inch on one end of a field and a quarter-inch on the other. A cell average smooths that out; your gauge sits at one point inside it.
Gauges under-catch in wind. Wind carries drops over the funnel's mouth, so an exposed gauge usually reads low in gusty rain. Trees and buildings nearby cause the opposite problem by blocking or funnelling rain.
The windows differ. Volunteer networks such as CoCoRaHS report the 24 hours to about 7 a.m. A rolling "last 24 hours" ending at 4 p.m. covers a different day's worth of rain.
The early number gets revised. The gauge-corrected radar estimate arrives roughly an hour and a half after the rain, so a figure read during or just after a storm can still move. Raindex labels those hours provisional.

How to compare them fairly

Match the window first: compare a gauge emptied at 7 a.m. with a total for the same 24 hours, not a rolling one. Then look at several rain events rather than one. A gauge that reads consistently 10% under the radar across a season is telling you something about its siting or the local radar; a single storm that differs by half an inch is usually just rain being uneven.

Finally, check the label on the reading. A total marked radar only had little gauge correction nearby and deserves more caution than one marked gauge-adjusted. The labels guide explains each one.

Which one is right?

Usually both, for what each measures. Your gauge is the best record of what fell on that spot, if it is well sited. The radar estimate is the better tool for places you do not have a gauge: the far field, the job site across town, your parents' yard. That is the gap Raindex is built for, and why it never publishes an accuracy percentage it cannot support.

See the last 24 hours for any ZIP code with the free rainfall lookup, or keep a labelled, day-by-day record for the places you care about in Raindex.

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Common questions

Is radar rainfall accurate?

It is an estimate for a ~1 km cell. Corrected against gauges, it usually lands close to what a well-sited gauge catches over a season, but single storms can differ noticeably because rain varies over short distances and gauges under-catch in wind.

Why did my rainfall total change after the storm?

The first estimate is made before most gauge reports are in. About an hour and a half after the rain, a later pass with more gauge data arrives, and the total is revised to use it. Raindex marks those early hours provisional and says when an hour was revised.

Should I trust my gauge or the app?

For the exact spot your gauge sits on, a well-sited gauge. For anywhere else, the radar estimate is the better information you have. Comparing both over several storms tells you how your spot behaves.