Channel 4 of the AVHRR is located within the part of the spectrum where Earth's emission dominates all the time. It is most sensitive to wavelengths between 10.3 um and 11.3 um.

• Estimate brightness temperatures of land, sea and cloud tops.
• Detect cloud types (always compare with other channels to help determining cloud type.
• Use the differences of channel 3 and 4 to determine the temperature structure within the cloud (at night).

  • See the section on emissivity for an explanation.
  • Clouds in temperature inversions (e.g., fog) have a different signal compared to clouds in other temperature profiles.

• Compare with channel 5 to indicate low-level moisture.
• Compare with channel 5 to spot volcanic ash clouds (more later).

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