GOES Longwave IR (channel-4 minus channel-5) Difference

The longwave IR difference is generated as the change in temperature between the IR window channel (channel-4, 10.7 µm) and the "dirty" window (channel-5, 12.0 µm). This temperature difference is due to changes in atmospheric absorption or transmission between the two channels. The transmission differences are mainly due to clouds that allow different amounts of surface radiation to reach the satellite. The largest positive differences (dark areas) are generally due to differential transmission thru thin cirrus clouds. The absorption differences are mainly due to low-level water vapor when the dirty window has a lower radiative temperature than the "clean" window channel. Smaller positive differences (lighter gray areas) indicate changes in low-level moisture, which are most significant in the summer, with more moisture causing more difference between the channels (darker as opposed to lighter areas). Negative differences (the whitest areas) can occur when the surface is colder than the air above the surface, as in temperature inversion situations.