Channel 5 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 11.5 um and 12.5 um.

• Estimate brightness temperatures of land, sea and cloud tops.
• Detect cloud types (always compare with other channels to help determining cloud type.
• Channel 5 is slightly more sensitive to low-level watervapor than channel 4. You can use that aspect to do the following:

  • Use the differences to estimate the amount of low-level moisture in the atmosphere.
  • Take a linear combination of the two channels to remove the cooling effects of low-level moisture thereby retrieving more accurate sea surface temperatures. (e.g., 1.5(ch 5) - .5(ch 4)) = SST + adjustment factors
  • Clouds in temperature inversions (e.g., fog) have a different signal compared to clouds in other temperature profiles.

• Also use the differences to differentiate active convection from cirrus. Ch 5 is usually cooler than Ch 4 except above convective precipitating clouds where the differences are usually close to zero.
• Volcanic ash has a strong signal difference between Ch 4 and Ch 5.

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