Second Input: A Difference Image

2 images differenced to create an input to a dust RGB [12.0 - 10.8]

In addition to building RGBs from single channel inputs, we can also use difference images, where the calibrated pixel brightness temperature values of one image are subtracted from those in another image. For dust imaging, these differences often bring out the dust signature that cannot be observed easily on single channel images.

Therefore, we will use the 12.0 µm IR minus 10.8 µm IR brightness temperature difference (BTD) for our second input.

Depiction of land with a dust layer vs. cirrus cloud above and arrows depicting absorption differences between the 10.8 vs. 8.7 micrometer channels

The effectiveness of the BTD stems from the interaction of upwelling energy from the surface of the Earth with the dust cloud. Infrared energy passing through a dust layer has a colder brightness temperature at 10.8 µm than 12.0 µm because dust is more sensitive to and absorbs more energy at 10.8 µm. In effect, dust blocks more upwelling radiation from reaching the satellite at this wavelength.

This differential sensitivity of dust leads to a positive brightness temperature difference and bright shades in imagery.

Conversely, cirrus clouds are less sensitive to energy at 10.8 µm than 12.0 µm, which produces a negative difference and black shades on images. This straightforward channel difference provides a powerful way of differentiating higher clouds from dust.

Enhancement table with a portion stretched for use in creating a dust RGB

By differencing the 12.0 µm IR and 10.8 µm IR channels and scaling the difference from -4 to +2 Kelvin, we get a sharp depiction of the dust clouds on the brightness temperature difference image, which is perfect for input into the RGB. Notice how the difference image shows the dust cloud in white.

2 images differenced to create an input to a dust RGB [12.0 - 10.8]