Water Vapor Imagery
Water vapor is transparent to radiation at visible and 10-12 micron wavelengths. This is why visible and IR satellite imagery are used to observe surface features and clouds. However, water vapor is a very efficient absorber and emitter of radiation with wavelengths between 6.5 and 6.9 microns. So, satellite radiometers measuring the amount of radiation emitted by the atmosphere at these wavelengths can be used to detect water vapor in the atmosphere. The water vapor satellite image displays the water vapor concentration in the atmospheric layer between approximately 200 and 600 mb (or about 15,000 to 30,000 feet). Black indicates low amounts of water vapor and milky white indicates high concentrations. Bright white regions correspond to cirrus clouds. In the middle latitude regions, zones with strong contrast in the amount of water vapor often indicate the presence of a jet stream
The activity below helps you explore how cloud altitude and upper tropospheric humidity affect the water vapor imagery.
Instructions:
The image to the left is an example of an water vapor image made from a radiometers flown on several geostationary satellites. A piece of this image has been extracted and appears as a square below the larger satellite image. This portion of the image contains a cloud and some clear sky areas. You can change the appearance of this extracted image by changing the cloud altitude (and its associated ambient air temperature) or the relative humidity of the upper troposphere (approximately 200 to 500 mb). You do this by moving the sliding scales (scroll bars) accompanying the picture on the right. The vertical scale changes the cloud altitude and its temperature. The numbers on each side of figure show the altitude and corresponding temperature, assuming standard atmospheric conditions. You can modify the amount of water vapor in the atmosphere the horizontal sliding scale.
Questions to consider:
- What cloud types can water vapor imagery best observe?
- Can cirrus and clear sky conditions always be discriminated in the imagery?
- How does changing the humidity affect the image?
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This example Java applet developed by Tom Whittaker and Steve Ackerman of the Department of Atmospheric and Oceanic Sciences and the Space Science and Engineering Center at the University of Wisconsin-Madison.
Created on ... February 14, 2005