Infrared Imagery: Color Enhanced

You may have noticed the odd pattern near the bottom of the image, or the "smudge" near the left edge. These patterns are produced by clouds. Clouds also absorb and emit infrared radiation. Because the temperature in the atmosphere usually decreases as you go up, clouds are typically cooler than the underlying surface of the Earth. Thus, in this image, the clouds show up as being darker. The especially dark smudge on the left is caused by a high cloud: this cloud is effectively blocking the strong radiation from the Earth and is emitting much weaker radiation of its own. The temperature of the cloud is probably close to 50C colder than the temperature of the ground.

Since meteorologists look at infrared satellite images primarily to detect cloud patterns, they reverse the colors so that the clouds look white rather than black. Brains seem to have an easier time with white clouds than with black clouds. So here's the same image, as it would normally be displayed. Now the clouds are white, and the warm Gulf Stream is dark.

Here's a full-sized infrared image, displayed in raw format:

And in conventional, reversed-video format:

In the raw image, notice that the warmest (brightest) areas are the Mexican deserts and the tropical, cloud-free oceans. Meanwhile, the clouds are dark! In the conventional infrared image, the clouds are light, and Mexico is dark. The lightest clouds are the ones that are coldest (generally, highest in the atmosphere).

Of course, whoever's displaying the image is free to display it an any color they want. Frequently, certain radiation intensities, corresponding to particular temperatures, are assigned special colors if they are especially significant. Very cold temperatures, for example, often indicate the tops of tall thunderstorms, and a color scheme is used which makes those very cold temperatures stand out. Also, by using a wide range of colors, subtle differences in radiation intensity are made easier to distinguish. Here's the same image as before, but with an enhancement scheme that makes the variations in intensity of infrared radiation much more prominent. The hot areas are red, the cooler areas are blue, and the really cold areas, presumably corresponding to middle and high level clouds, are shaded white to black. Notice how well the Gulf Stream stands out in this enhancement, as does the really deep convection over the Caribbean.

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