Lesson 9: Background: Fog and Stratus

Differences in the radiative properties of clouds exist between the shorter wavelenght IR window (e.g. between 3.5 and 3.8 micron) and the longwaver IR window (e.g. between 11 and 12 micron). Clouds have a lower emissivity at the shorter wavelength. In the nightime, temperature difference image over fogs usually appear colder at night in the 3.7 micron channel then at 11 micron because the fog (and stratiform clouds as well) typically form within temperature inversions near the surface. The lower radiance from within and below the clouds is detected more in the shorter wavelengths while the 11 micron window senses primarily near the cloud top. Although the temperature difference between the two channels is usually less the 10 degrees, the difference is usually enough to discriminate fog edges. This is useful for forecasting as satellite images can provide information on the horizontal extent of the fog, which is difficult to determine from surface observations alone. A teim sequence of these difference images can be used to predict fog dissipation.

A night-time image product is made by subtracting the 3.9 micron scene temperatures from those at 10.7 mm (BT_3.9-BT_10.7), and scaling the results to show (-/+) differences, where negative difference indicate the presence of the fog. This technique is based on the principle that the emissivity of water cloud at 3.9 mm is less than at 10.7 mm (assuming the surface emissivity is similar at the two wavelengths). Evolution of night-time fog and low-level stratus clouds is easily observed by viewing the "fog product" image, as demonstrated below.

Notice in the GOES IR image below, the fog regions are have a higher brightness temperature than the near-by clear regions. This is sometimes refered to as a black fog, and results because of a temperature inversion near the surface. In the GOES-8 fog product image, red and orange regions are foggy.

The MODIS instrument is better calibrated and produces a smoother fog product, though only once an evening. Notice the fog that fills the valley.

Reflectence at 3.9 microns

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