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.

You can use this imagery all times of the day to:

- 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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