Adding water to dry land significantly lowers the surface emissivity in the microwave. The effect is more pronounced at the lower frequencies. While a soil moisture product for the AMSU is still in the developmental stages, an operational product is available for SSM/I and shows the percentage of soil moisture over land. Values in excess of 70% are considered erroneous. The product is updated once per day at 4 AM EST.

Select the SSM/I Soil Moisture Index button above.
Another surface moisture product, an experimental "wetness index," is generated by the NESDIS Hydrology Team and is based on the differences between the SSM/I 85-GHz and 19-GHz channel brightness temperatures. These differences are selectively scaled to enhance extremely wet to flooded soil conditions. Images of five-day average wetness indices are produced daily with examples shown above. Indices are calibrated with respect to the average soil moisture calculated from pixel statistics based on SSM/I data for the period 1987-1996 (data for 1991 are excluded due to instrument problems). Negative anomalies were excluded, and positive anomalies are indicative of wetter than average conditions.