Tropical airmasses with higher upper tropospheric humidity appear green in the Airmass RGB images.
The green colour in the Airmass RGB indicates ozone-poor or more precisely warm/tropical air masses with high upper level tropospheric humidity.
Primarily responsible for the greenish colour is the brightness temperature difference (BTD) of channels IR9.7 and IR10.8 on the green colour beam (see recipe). The smaller the difference between both BTD, the stronger the green contribution. The channel IR9.7 is responsive to ozone gas concentration in the lower stratosphere. Tropical regions are characterised by a high tropopause level and, hand in hand, by a higher altitude of the ozone rich stratospheric air. As the air mass is ozone poor in the levels where IR9.7 has its contribution maximum, the resulting BTD is small.
Due to high upper level tropospheric humidity, the BTD WV6.2-WV7.3, which determines the red colour beam, is large. The scale for the red colour beam is chosen this way, that large BTD lead to small contributions of the red colour. Hence, the red contribution is not 0, but certainly low.
The contribution from the blue colour beam is rather small, if not zero. The temperature scale for channel WV6.2 is chosen such that warm air masses give little contribution when clouds are not present.
The image below shows an extended tropical airmass over the Saharian region of Africa on a winter day. The ochre boundary in northern Africa as well as the red stripe near the equator and the Arabian peninsula delimit the moist tropical airmass.
Explanation of the colour of moist tropical airmasses in the Airmass RGB (see also the recipe on the left side):
• Tropical air masses are characterised by their green colour in the Airmass RGB. This results from a combination of a high tropopause level (warm airmass) and a low ozone concentration in the level of highest instrument sensitivity.