WV Image Interpretation
Relative Humidity
According to Soden and Bretherton (1993)* and Vern Suomi before them, 6.7-µm water vapor
data can be interpreted as mid- to upper-level relative humidity:
- In dark areas, the satellite measures more radiation than in bright areas.
- More radiation reaches the satellite for one of two reasons:
- Dark areas are warmer at mid- to upper-levels than bright areas, or
- In dark areas, there is less water vapor, so the weighting function is
lower, which means that the radiation reaching the satellite comes from lower,
warmer portions of the troposphere.
- In either case, the relative humidity of the dark areas is less than in the bright areas.
Vertical Motion
Because rising motion moistens air, and sinking motion dries air, contrasting dark and bright areas
in WV images can be interpreted as areas of downward and upward motion, respectively. This is
particularly evident at fronts, and the contrast across a front in WV imagery may indicate the strength
of the front. Unfortunately there are no strong fronts in the images presented.
Upper-Level Flow
Water vapor moves with the wind. Because water vapor is quite variable, patterns are observable in cloud-free
areas in WV imagery. Ridges and troughs can be deduced from still WV images.
WV image loops can be used to track
these WV patterns--just as clouds are tracked in visible or 11-µm imagery--to produce wind vectors.
*Soden, B. J., and F. P. Bretherton, 1993: Upper tropospheric relative humidity from the GOES 6.7 µm
channel: Method and climatology for July 1987. Journal of Geophysical Research, 98, 16669.