GVI - Precipitable Water Index
The Precipitable Water Index (PWI), otherwise known as the brightness temperature difference, is the difference between AVHRR channels 4 and 5. PWI is a measurement of how much water vapor is in the atmosphere (lower K values denote higher concentrations of water vapor). Since AVHRR channels 4 and 5 are volume sampled from the top of the atmosphere to the surface, PWI, to an extent, measures the total column precipitable water in the atmosphere (Steve Olson, 1997).
Please note that PWI is not the same as relative humidity, which measures the percentage of water vaper over the amount of water vapor needed for saturation at a given temperature.