Rainfall affects earth-emitted microwave radiation in two ways: absorption by rain drops and scattering by ice particles. As shown in the absorption graph (top left), ice particles in fact absorb very little energy in the microwave so we can focus on scattering as an important process to be used for detection.

For the two plots on the left, compare the solid blue lines for water at 19 and 37 GHz. At the lower frequencies absorption for rain drops is dominant over scattering, making these frequencies ideal for the detection of rain. The emission by rain drops is particularly easy to detect over oceans. Brightness temperatures over the ocean are relatively uniform and relatively cold due to the low emissivities. The brightness temperature versus rain rate plot on the right highlights how quickly 18- and 37-GHz brightness temperatures over the ocean increase as cloud water increases for low rain rates.