Satellite Images and Products

Fog and low stratus can be inferred from multispectral RGBs imagery and derived products and from visible channel imagery during daytime. The following tables describe the satellite products that are useful for identifying fog and low-level clouds. Review the information, then we will look at some of the key products for the case.

SINGLE CHANNELS AND DIFFERENCES

COVERAGE

USE/INTERPRETATION

Water vapour 6.2 µm

 

24 hrs

  • Good for detecting mid- to upper-level moisture over cloud-free areas

  • Useful for upper-level diagnosis of atmospheric circulation features and related dynamics

  • Provides information about the 200 to 600 hPa layer when the atmosphere is moist, and 250 to 700 hPa when atmosphere is dry

  • Drier troposphere appears in darker shades of grey

  • Moist troposphere appears in lighter shades of grey

  • Low clouds and land surfaces are not visible

  • Dark grey areas indicate a dry mid- to upper-level troposphere (areas of low RH) and presence of enhanced low-level radiative cooling

Ch 4 minus 9 (MSG IR 3.9 - 10.8 µm) temperature difference ("fog product")

24 hrs
  • Useful for checking for areas of possible fog or low-level water clouds
  • At night, when applying a standard grayscale enhancement, dark areas indicate possible fog or low water cloud
  • During the day, solar energy contaminates the 3.9 µm SWIR channel and the difference product is not useful for fog and low cloud detection
  • At dawn, fog is difficult to distinguish from the surrounding areas due to solar contamination in the 3.9 µm SWIR Ch.4

EUMETSAT's Meteorological Product Extraction Facilities (MPEF) produce a range of meteorological products derived from satellite data. One of the most useful derived products for fog detection is the Cloud Analysis Image or CLAI. The CLAI product is produced every three hours (corresponding to the ECMWF analysis/forecast periods) and helps distinguish different layers of clouds.

PRODUCT

COVERAGE

USE/INTERPRETATION

MSG Cloud Analysis Image (MPEF CLAI)

24 hrs
  • Distinguishes different layers of clouds based on temperature and height. Lighter colours mean lower levels.
  • Low clouds are pink; mid-level clouds are ochre-brown, high clouds are blue/cyan; and vegetated ground appears in various shades of green

The following RGBs also provide useful information about fog and low cloud.

RGB PRODUCT

COVERAGE

USE/INTERPRETATION

Fog / Low Clouds (Night Microphysics) RGB
(IR 12.0-10.8, IR 10.8-3.9, IR 10.8 µm)

Nighttime only for cloud microphysics, monitoring of fog and low stratus

Day Microphysics RGB
(Vis 0.8, IR 3.9, IR 10.8 µm)

Daytime only for cloud microphysics; cloud classification, convection, fog, snow and fires

  • The colour depends on cloud thickness and droplet size

  • Low clouds with large droplets are more magenta

  • Small droplet clouds are more whitish-yellowish

24-hour Dust Microphysics (IR 12.0 – IR 10.8, IR 10.8 – IR 8.7, IR 10.8 µm)

24-hour dust and cloud classification

  • The colours are somewhat similar to the Night Microphysics RGB

  • Not as good a product as the Night Microphysics RGB for tropical conditions (because of the sensitivity of 8.7 µm channel to water vapour)

Natural Colour RGB (1.6, 0.8, 0.6 µm)

Daytime only
  • Fog and low clouds appear milky white