Fog & Low Clouds, NexSat

Example of a NexSat Fog and Low Clouds RGB


This RGB helps in the detection of fog and low clouds at night, a task that is often difficult with single channel infrared images because features tend to blend into the thermal background. The most important input is the difference between the longwave and shortwave infrared channels.

Most current polar-orbiting and geostationary imagers have the necessary channels to make this product including the recently launched Suomi NPP polar orbiter. Future JPSS VIIRS, GOES-R ABI, Himawari AHI, and MTG FCI instruments will additional shortwave and longwave IR channels to improve the product and minimize false detections caused by low emissivity desert and other bare land surfaces.

Coverage: Nighttime only

Channels: Shortwave and longwave infrared on polar-orbiting and geostationary satellites

  • Red: 10.35 to 11 µm minus 3.5 to 4 µm SWIR BT difference
  • Green: 10.35 to 11 µm IR
  • Blue: 10.35 to 11 µm IR

Color scheme:

  • The land background is usually dark green, although it can be different shades of green or yellow
  • Fog and low cloud are shades of red, pink or orange
  • High cloud is cyan

Advantages: Enables the detection of low cloud at night when visible imagery is unavailable


  • Cirrus clouds can obscure the view of low clouds and fog at night
  • May not work well in regions of cold surface temperatures


Miller, S. D., J. D. Hawkins, J. Kent, F. J. Turk, T. F. Lee, A. P. Kuciauskas, K. Richardson, R. Wade, and C. Hoffman, 2006: NexSat: Previewing NPOESS/VIIRS imagery capabilities. Bull. Amer. Meteor. Soc., 87, 433-446.

Lee, T. F., and S. D. Miller, 2003: Improved detection of nocturnal low clouds by MODIS, Preprints, 12th Conf. on Satellite Meteorology and Oceanography, Long Beach, CA, Amer. Meteor. Soc., CD-ROM, P5.23, also available online at

Lee, T. F., S. D. Miller, C. Schueler, and S. Miller, 2006: NASA MODIS previews NPOESS VIIRS capabilities. Wea. Forecasting, 21, 649–655.


MODIS longwave IR image from 11 Jan 2008 over Lake Superior

There seems to be only cirrus in the MODIS longwave infrared image, while clouds are abundant in the fog and low cloud RGB (the orange-pink areas).

MODIS Contrail RGB from 11 Jan 08ODIS over Lake Superior

The MODIS fog and low cloud RGB is particularly valuable because the high-resolution, 1-km infrared channels produce a detailed view of low cloud features.


MODIS Longwave infrared over Florida, 28 Aug 2009

 Based on this longwave infrared image, where do you think the low cloud cover is? (Choose the best answer.)

MODIS Fog & Stratus RGB over Florida, 28 Aug 2009

The correct answer is C.

It is difficult to figure this out from the longwave infrared image alone, but the RGB makes it easy since low clouds appear as bright red or pink.