Radar rainfall estimates vs. a backyard rain gauge

What a gauge measures

A rain gauge measures the water that enters one small opening at one exact location. That direct measurement is its great strength. If the gauge is level, unobstructed, correctly sized, and read at a consistent time, it can describe rainfall at that spot better than a broad regional product.

The same narrow footprint is also a limitation. A thunderstorm may soak one street and miss the next. Wind can carry drops over the opening, trees or buildings can block rain, splashing can add water, and debris can prevent collection. An automated tipping-bucket gauge may also undercount during very intense rain or when maintenance is overdue.

What radar and weather grids measure

Weather radar does not hold a container under the cloud. It sends energy into the atmosphere and interprets the returned signal from rain, snow, hail, and other targets. NOAA Multi-Radar Multi-Sensor products combine radar with gauges and other information to estimate precipitation across a grid. A grid cell represents an area, not a single yard.

Radar gives valuable coverage between gauges, but the estimate can be affected by distance from the radar, terrain, the height of the radar beam, the type of precipitation, and assumptions used to convert the signal into rainfall. Weather-model and reanalysis grids add consistent coverage over long periods, but they also smooth small storms and are not direct property measurements.

Why two reasonable numbers differ

The most common explanation is spatial variation: the gauge sampled one point while the grid represented a surrounding area. Timing matters too. A gauge may be read at 7 a.m., a calendar-day product resets at midnight, and a rolling 24-hour total ends at the latest completed analysis hour. Those are different periods even when each is labeled “daily rainfall.”

Small differences should not automatically be treated as errors. Large differences deserve a basic check: confirm the location, units, date range, time zone, gauge exposure, and whether snow or mixed precipitation was involved. Then compare several nearby gauges or official products rather than selecting only the number that matches an expectation.

A practical comparison method

Rain by Zip is designed for convenient local context. It combines current radar analysis, where available, with model-based estimates and clearly identifies important limitations. It should complement—not replace—a maintained gauge and official National Weather Service information.

• Use the same start and end times for both sources.
• Confirm that both values are rainfall or liquid-water equivalent in the same units.
• Check whether the grid point and physical gauge are actually near each other.
• Review hourly or daily values to find when the difference began.
• Use an official gauge record when a contract, insurance claim, engineering decision, or regulated measurement requires one.