Fog & Stratus, NexSat

Example of a NexSat Fog and Stratus RGB

Description: This RGB makes it easy to detect fog and stratus at night, a task that’s often difficult with single channel longwave infrared images because the features blend into the thermal background. The most important input is the difference image between the longwave and shortwave infrared channels.

Coverage: Nighttime only

Channels: Shortwave and longwave infrared

Color scheme:

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

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

Limitations:

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

Reference:Miller, S. D.,Hawkins, J. D., Kent, J., Turk, F. J., Lee, T. F., Kuciauskas, A. P., Richardson, K., Wade, R., and Hoffman, C., 2006: NexSat: Previewing NPOESS/VIIRS Imagery Capabilities. Bulletin American Meteorological Society, 87, 433-446. http://www.nrlmry.navy.mil/NEXSAT.html

Example:

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 stratus RGB (the orange-pink areas).

MODIS Contrail RGB from 11 Jan 08ODIS over Lake Superior

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


Exercise:

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's difficult to figure this out from the infrared image but the RGB makes it easy since stratus appears in orange or pink.