As the graph on the left shows, scattering becomes most prevalent at higher frequencies and is caused by particles that are comparable in size to the wavelength of energy being measured. At 89 GHz for the AMSU (85 GHz for the SSM/I), this translates to a particle size of about 4 mm or greater and is an indication that precipitation is present. Within the precipitation layer, a mixture of snow, ice, and rain particles are the main cause of scattering. The scattering results in a decrease in brightness temperatures within the active raining regions. The brightness temperature vs rain rate plot shows how this cooling effect becomes more pronounced as rain rate increases. Dark or cool spots are easily noticeable over ocean and land when viewing 89-GHz channel imagery.