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 dis-
play as different intensities on a gray scale (white is coldest, black is warmest, middle
temperatures are shades 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 aver-
age 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 for this lapse rate is about 7 degrees Celsius per kilometer. The direct
relationship is adiabatic(page 48)—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 cold-
est, appear very pale gray or bright white on thermal images.
If you have an APT groundstation, you may wish to demonstrate how image process-
ing can be used to help identify areas of differing temperature. Each pixel in the image
represents a temperature value. 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.
BA C K G R O U N D: CL O U D S
High Clouds
These clouds are generally found between 6 and 12 kilometers. Composed primarily of ice, they
appear bright white on infrared imagery. Two examples are towering cumulonimbus and cirrus.
Cumulonimbus appear as bright white splotches and are often associated with thunderstorm
activity. Cirrus clouds appear as bright white streaks — as if they had been painted by a brush.Middle Clouds
Heights are generally between 2 and 6 kilometers. On infrared images, they appear as a lighter
shade of gray than the surface or low-level clouds. Two examples are altocumulus and altostratus.
The former will have a mottled texture while the latter will appear more sheet-like. Be aware that
the middle level clouds are usually the hardest to determine because they are often shielded by
higher level clouds.Low clouds
Heights generally do not exceed 2 kilometers. Two of the lowest clouds are stratus and fog. On
the image, they appear a uniform dark gray shade and lack any texture. Often they are hard to
distinguish from the ground because of similarities in temperature and the resulting similarity of
the gray shades.12 km6 km2 km