Dealing With Ambiguities

MODIS false color RGB over California, 24 Dec 2009 with cirrus, low clouds and fog, and snow covered labeled

A nearly identical RGB is available from Terra and Aqua MODIS data. It looks similar to EUMETSAT’s natural color RGB but uses the 2.2 µm NIR channel in place of the 1.6 µm NIR channel. The MODIS false color product has the same color interpretation scheme and is used to identify the same features. However, it does a better job of detecting fires. MODIS false color products are available in near real-time.

In this winter example centered on Northern California, we can differentiate the cyan cirrus cloud near the coast from the whitish water cloud trapped in valleys over Oregon. But it’s hard to distinguish cirrus cloud from snow cover over the mountains based on color alone. Both features are cyan because ice crystals reflect strongly in the visible channels (which have been assigned to be green and blue) and poorly in the near-infrared channel (which has been assigned to be red).

Ambiguous situations like this can often be resolved in various ways, for example by:

  • Seeing if there is another RGB developed for the situation; in this case, it would be useful to check the Cloud Over Snow RGB, described in the Applications section.
  • Looping images to differentiate surface from atmospheric features.
  • Noticing that surface features, such as snow cover, are often tied to familiar topographic features, such as mountain ranges, while atmospheric features, such as ice clouds, are typically not. Do you see that effect in our example?