The Rainfall Accumulation products attempt to estimate the amount of rainfall that has fallen in a given area under the radar's umbrella. NEXRAD does this by making certain assumptions about the number and kind of raindrops it detects. There are certain limitations involved with radar estimation of rainfall, which is a subject of current meteorological research, and there are plans to improve the way that NEXRAD produces its rainfall estimates. A given rainfall product should generally be compared with a product from another radar or with rain gage reports, if they're available.