The AMSU's 23-GHz channel (21 GHz for SSM/I) lies within the weak water vapor absorption band at 22 GHz and is sensitive primarily to atmospheric water vapor and emissions from cloud liquid water. This makes the 23-GHz channel a key component in the retrieval of water vapor, cloud liquid water, and precipitation. The 23-GHz channel is also important in helping to remove the contamination by atmospheric moisture and precipitation in nearby window channels when extracting information about surface properties. Notice how oceanic water vapor and water clouds warm the brightness temperatures sensed from the underlying cold, low emissivity ocean surface.