The limb cooling effect appears blue in the Airmass RGB images.
The blue colour in the Airmass RGB usually indicates polar air masses but can also be caused by radiation from full disk image edges.
The blue colour in the Airmass RGB stems from the so-called "limb cooling effect". If the whole disk of the Airmass RGB is plotted, one sees that the blue colour prevails at the image edge even in the tropical regions. Radiation from the image edge (full disk image) traverses a longer path through the troposphere until it reaches the satellite instrument. Therefore, the measured radiances, especially in the WV bands, are far smaller than from regions near the sub-satellite point (see image below).
Explanation of the blue colour of the limb cooling effect in the Airmass RGB (see also the recipe on the left side):
• High humidity values reflected in both the WV6.2 and the WV7.3 channel result in a low contribution of the red colour beam.
• The inclined viewing angle at full disk image edges are reducing the radiation measured at 9.7 micrometer, while the window channel at 10.8 micrometer remains largely unaffected. A larger (negative) brightness temperature difference results in a small green contribution.
• The main contribution comes from the blue colour beam. Because of the inclined viewing angle, brightness temperatures measured at 6.2 micrometer are rather low.