Clouds may be classified by shape, content, or cloud height. For day 1, students will classify clouds by height, based on the appearance of cloud types in the polar-orbiter imagery. Infrared imagery is thermally sensitive, so areas of different temperatures display as different intensities on a gray scale (white is coldest, black is warmest, middle temepratures are shapes of gray).
The two basic shapes under which clouds may be classified are: stratus - layered and sheetlike and cumulus - puffy and heap-like Many clouds exhibit combinations of both traits. Cloud content may include water droplets only, a mixture of water and ice, or just ice. Cloud heights are generally described as low (under 2 km), medium (2-6 km), or high (6-12 km); these are average cloud heights for the mid-latitudes.
The temperature of the atmosphere generally decreases with height. The rate of decrease in air temperature with elevation is called the environmental lapse rate. An average value of this lapse rate is about 7 degrees Celcius per kilometer. The direct relationship is adiabatic - moisture helps control and decrease temperature.
In an infrared (thermal) image, temperature provides a quantitative measurement of cloud-top temperature with the coldest areas appearing to be the brightest. Low-level clouds, which are closest to the ground and therefore the warmest, appear dark gray and may be hard to distinguish from the ground. Mid-level clouds appear in medium (brighter) shades of gray due to their cooler temperature. High level clouds, the coldest, appear very pale gray or bright white on thermal images.
If you have an APT groundstation, you may wish to demonstrate how image processing can be used to help identify areas of differing temperature. Each pixel in the image represents a temperature value. Stretching the pixels (increasing the contrast) will make temperature variations more discernible. Students can readily see how the tops of cumulonimbus clouds appear dark on a white background, though they would appear all-white without the software manipulation of the image.
Figure 77.