A Simple Example

MODIS True Color Product with Hotspot Info Overlaid

Perhaps the best known RGB combination is the true color product. It highlights atmospheric and surface features that are hard to distinguish with single channel images alone and imitates how the human eye might see the scene. Among current weather satellites, true color products are available from the Terra and Aqua MODIS imagers and the Suomi NPP VIIRS imager, since both have the requisite visible channels. Additional RGB capabilities will come online as new imagers are launched. Some of these include:

  • The Visible Infrared Imaging Radiometer Suite (VIIRS) on U.S. JPSS (Joint Polar Satellite System) polar-orbiting satellites
  • The Flexible Combined Imager (FCI) on the Meteosat Third Generation (MTG) geostationary weather satellites
  • The Visible and Infra-Red Radiometer on China's FY-3 polar-orbiting satellites
  • The Advanced Himawari Imager on Japan's third generation geostationary weather satellites
MODIS True Color Enhancement Color, Conceptual Illustration

The true color RGB is constructed from the three visible wavelengths that correspond to the red, green and blue components of visible light. The first spectral channel is assigned to be red, the second channel green, and the third channel blue.

In the resulting RGB, it’s easy to distinguish the small smoke plume in Australia from the large area of blowing dust. The suspended dust particles take on a light brownish appearance because they reflect more light at the longer visible wavelengths, the red and green regions of the spectrum. Smoke from burning vegetation appears gray, reflecting the red, green, and blue components of visible light in relatively equal amounts. In general, clouds are easily separated from suspended dust in true-color images.

The small group of red pixels at the western end of the smoke plume indicates hot spots or fires. These were inserted after the channel compositing took place, using information from the thermally sensitive shortwave infrared channels on MODIS. Later we will discuss the 'natural color' product, which has some similarities to true color as well as some important differences.