The radar total for your place and the water in your rain gauge will rarely match exactly, and that is expected. A radar estimate is an average over a square of ground about a kilometer across; your gauge measures one spot a few inches wide. Lean on the gauge for the spot it stands in, lean on the radar for everywhere you have no gauge, and compare the two over several storms to learn how far apart they usually run.
What does a radar rainfall estimate actually measure?
Weather radar does not catch rain. It sends out pulses, listens for the echo from raindrops in the air, and converts the strength of that echo into a rain rate. Added up hour by hour, those rates become a rainfall total for each cell of a grid.
In the United States, NOAA's Multi-Radar/Multi-Sensor system (MRMS) combines the national radar network into a grid of cells roughly one kilometer on a side, then corrects the radar-only numbers against readings from rain gauges on the ground. The result is an estimate for every square kilometer of the lower 48 states, including the many places where nobody has a gauge.
Two consequences follow. The number describes an area, not a point, so a heavy downpour that hit half the square shows up diluted. And the number gets better as more gauge readings arrive, which is why the first estimate for an hour can change once the second pass is out.
Why do radar and gauge totals disagree?
Some of the difference comes from the radar and some from the gauge.
On the radar side:
- Distance. The beam rises as it travels, so far from a radar site it can pass over low clouds and miss rain falling beneath it.
- Melting snow. Wet snowflakes melting into rain reflect strongly and can make the radar overestimate.
- Terrain. Hills and mountains block parts of the beam in some valleys.
On the gauge side:
- Wind. Wind carries drops past the opening, so gauges in exposed places catch less than actually fell.
- Siting. Trees, roofs and walls shelter a gauge; it should stand in the open, ideally at least twice as far from any obstruction as the obstruction is tall.
- Timing. A reading taken at 9 AM and a total that ends at 7 AM split the same storm differently.
And then there is the rain itself. A summer thunderstorm can drop an inch on one side of a road and almost nothing on the other. When that happens, both numbers are right about different things.
Which number should you use for what?
Use whichever one describes the decision you are making.
- Watering a lawn or garden next to the gauge: the gauge. It measured the ground you are about to water.
- Fields, pastures and job sites without a gauge: the radar estimate. It is the only measurement of that ground you have.
- Comparing places: the radar, because every place is measured the same way. Two gauges in different spots, sited differently, are harder to compare.
- Records you will look back on: both, if you have them. A season of side-by-side readings is how you find out whether the estimate for your place tends to run high or low.
How do you compare your gauge with the radar fairly?
A fair comparison takes a little routine.
- Read the gauge at the same time every day. Many volunteer observers in the CoCoRaHS network read theirs around 7 AM, which splits storms the same way day after day.
- Compare the same window. Match your reading to the radar total for the 24 hours ending at that same time, not to a calendar day.
- Use a proper gauge. A clear cylinder with a wide opening, such as the 4-inch gauges many observers use, reads more consistently than a small decorative one.
- Look across storms. One storm can be off by half an inch in either direction. Over five or ten storms, a pattern appears: perhaps the radar runs about ten percent low at your place, or matches closely except in summer thunderstorms.
Once you know the pattern, you can read the radar estimate for a place with no gauge the same way you would read your own.
How Pluvio puts the two side by side
Pluvio shows the radar estimate (NOAA MRMS) for the square kilometer around each place you save, with a gauge day that ends at 7 AM. If you keep a gauge, you can log its reading for the day; Pluvio shows it next to the radar's number, and after three readings on rainy days it tells you how the radar tends to run against your gauge at that place. A number that is still preliminary says so until the final pass arrives.
Neither Pluvio nor any radar estimate is an official measurement. For flooding and other safety questions, follow your local officials and the National Weather Service.