The westward advancing surge and dry air of the Saharan Air Layer (SAL) are detectable using multi-spectral GOES-East infrared (IR) satellite imagery that tracks suspended dust silicates and dry lower tropospheric air. In the eastern North Atlantic, suspended dust in the SAL is clearly evident in visible satellite imagery as well. However, the dust becomes more diffuse and difficult to monitor as the SAL moves westward. The position of the dust and lower tropospheric dry air can be tracked more reliably using the 10.7 µm (Ch. 4) and 12 µm (Ch. 5) IR channels on the GOES-East imager.

Under normal atmospheric conditions, the 3.9 µm IR emissions originate closer to the Earth's surface than do those in the 10.7 µm IR channel, which has a slightly higher low-level water vapor absorptivity. Since it senses more effectively to the surface, the 3.9 µm channel signal usually appears slightly warmer than the 10.7 µm channel. However, suspended mineral dust absorbs some of the incident solar radiation and re-radiate it as longwave IR radiation. Dust emits more effectively at 10.7 µm than at 3.9 µm. Therefore, dust reduces the apparent 3.9 µm minus 10.7 µm temperature difference and in extreme dust events, can even reverse it. The exceptionally dry air in the SAL can also act to warm the apparent temperature in the 10.7 µm channel by reducing the amount of IR absorption by low-level moisture. We developed image enhancements to take advantage of these apparent temperature anomalies measured by the GOES-East IR channels to track the SAL (Figure 1).