Sensing the surface with water vapor imagery
As a cold, dry arctic air mass moved across the western Great Lakes on 06 February 2018, portions of the land-water boundaries of Lake Superior, Lake Michigan and Lake Huron were very distinct on GOES-16 (GOES-East) Low-level (7.3 µm) Water Vapor images (above). The motion of low-altitude lake effect clouds were also apparent in the imagery.Plots of weighting functions for the three GOES-16 ABI Water Vapor bands (7.3 µm, 6.9 µm and 6.2 µm) are shown below, calculated using rawinsonde data from Green Bay, Wisconsin and Gaylord, Michigan. With cold air and low values of Total Precipitable Water at these 2 sites (1.53 mm / 0.06 in and 1.88 mm / 0.07 in, respectively), the height of their weighting functions was shifted to significantly lower altitudes compared to what would be observed in a standard atmosphere. This enabled the contrasting thermal signature of the land/water boundaries to easily reach the satellite sensors, passing through what little moisture existed within the atmospheric column. While the peak of the violet 7.3 µm weighting function plots descended to the 879 hPa pressure level at both sites (which was approximately 1.2 km above the surface), a significant contribution could be seen originating from the surface itself.
![Weighting function plots for the three GOES-16 Water Vapor bands, calculated using rawinsonde data from Green Bay, Wisconsin [click to enlarge]](https://cimss.ssec.wisc.edu/satellite-blog/wp-content/uploads/sites/5/2018/02/180206_12utc_kgrb_wv_wf.jpeg)
Weighting function plots for the three GOES-16 Water Vapor bands, calculated using rawinsonde data from Green Bay, Wisconsin [click to enlarge]