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Adapted from An Introduction to Satellite Image Interpretation, Eric D. Conway and the Maryland Space Grant Consortium, ©1997, Johns Hopkins University Press, Baltimore, 255 pp with Interactive CD-ROM.
For more information about this book and how to order copies go to the JHU Press On-line Catalog
When the world's first satellite pictures of the atmosphere were viewed, the most striking feature was the extensive cloud cover over large parts of the Earth. Even today, the first feature that one usually notices in a satellite image is the clouds. At first glance, these clouds may seem to be random in shape and distribution; however, they form as the result of very specific interactions between many different meteorological factors. When these factors interact in certain ways, different cloud types are formed. Clouds that form under similar conditions can be classified into individual categories on the basis of their appearance from the ground, and each cloud type will exhibit unique patterns in satellite imagery. Once the patterns are known, it is possible to identify the cloud types present in a satellite image.
In this activity, you will learn about the patterns that can help identify different cloud types and you will examine several visible - infrared image pairs to determine the cloud types at various locations around North America. When finished, you should be able to use visible and infrared imagery together to distinguish between low, medium, and high level clouds and between stratus, cumulus, and cirrus cloud types. This information will help form a base understanding that will lead to greater understanding of larger scale weather patterns as seen in satellite imagery.
NOTE: The images used in this section consist of two images of the same scene. The left-hand images are Visible and the right-hand images are taken in an Infrared channel.
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In general, the most effective method for identifying individual cloud types is to obtain a VIS and an IR image of the same scene. The VIS image can be used to identify cloud shapes, textures, organizational patterns, and thicknesses. In general, the thicker a cloud is, the higher its albedo and the brighter it will appear in visible imagery. Thin clouds are often very dark or transparent in visible imagery. Cloud texture refers to its appearance in visible imagery. Clouds with irregular tops will often exhibit shadowing, known as a "lumpy" textures, while layered clouds with flat tops will have a "smooth" texture. Windswept clouds that have a wispy appearance are often said to exhibit a "fibrous" texture.
Visible satellite data can then be compared to an IR image in order to determine the height of the clouds. In general, the higher a cloud is, the colder it is. In IR imagery, therefore, lower, warmer clouds will appear darker while high, cold clouds will appear brighter. When all this information is put together, it is usually possible to make a reliable assessment of what types of clouds are present in the image and the weather that is associated with each.
Cumuliform clouds are a classification of clouds characterized by irregular shapes that exhibit vertical development. Cumulus clouds are the classic fair-weather puffy clouds that often appear to resemble cotton balls in the sky. They are individual clouds that usually have lumpy tops. Towering cumulus are a type of cumuliform cloud and can grow to be very high and thick. In extreme cases, these clouds grow into cumulonimbus clouds and they take on a flat, anvil-shaped top as the cloud top reaches into strong upper-level winds. These are thunderstorm clouds and can reach heights of 12 miles.
In visible satellite imagery, cumulus, cumulonimbus, and towering cumulus will usually appear bright due to the thickness of these clouds. They often exhibit a lumpy texture caused by the shadows on the irregularly-shaped cloud tops. Cumulonimbus clouds will often have relatively flat, windswept tops where the cloud has grown into the upper-level winds, though in places the smooth anvil top will be interrupted by intense updraft regions known as overshooting tops. In these areas, the cloud will appear lumpy. In infrared imagery, the clouds will appear brighter when the tops have grown very high. Therefore, low-lying cumulus will appear darker gray compared to a taller towering cumulus. Cumulonimbus cloud tops will be extremely bright in infrared as they are very high.
Stratiform clouds are clouds that are characterized by a flat, layered organization. Stratus clouds are low-level clouds that appear to cover the entire sky, making it gray and overcast. Stratus clouds that form on the ground are commonly called fog. Middle level stratus clouds are called altostratus and high-level stratus clouds are called cirrostratus, which are so thin they often appear transparent from the ground and on satellite imagery. Low, middle, and high level stratus can be distinguished in infrared imagery where cloud top temperatures can provide clues about the relative height of a cloud deck. In some cases, low-lying stratus and fog will not be detected in infrared imagery as the temperature contrast between the cloud and surface are so low. In visible satellite imagery, stratus clouds appear as broad flat clouds with a very smooth texture. Higher stratus often appear darker as they are often more thin than lower stratus.
Cirriform clouds are high-level ice crystal clouds. The classic cirriform cloud is the cirrus cloud, which appears as a windswept, wispy cloud that is fibrous in nature. Cirrocumulus clouds are puffy high clouds and cirrostratus are very high flat layers of clouds. These clouds can be found above 25,000 ft where temperatures are extremely cold and water vapor is in low supply. Therefore, they are very thin and, at times, nearly transparent in visible imagery. The cumulus structure in cirrocumulus is usually too small to detect in satellite imagery, so these clouds will often appear to be smooth. In infrared imagery, cumuliform clouds will appear to be very bright because of the cold cloud top temperature.
This example consists of a visible image (left) and an infrared image (right) of the same portion of the US taken at the same time. The letters mark cloud features that will be discussed in this image.
A. In visible, these clouds are fairly bright and they seem to follow the topography of the Appalachian regions of Kentucky and West Virginia. In the infrared image, they cannot be detected, indicating that the temperature of the cloud tops is close to the temperature at the ground. This indicates very low warm clouds. In this case, feature "A" shows valley fog.
B. A broad, flat expanse of clouds covers much of Georgia in the visible image, but cannot be seen in the infrared. These clouds must be low, flat, warm stratus and possibly fog.
C. Off the North Carolina Coast, these clouds are lumpy in places, fibrous and windswept in others. They are bright in the visible image, indicating very thick, tall clouds. They are very bright in the infrared, indicating very high, cold cloud tops. Finally, in the infrared image, the clouds exhibit a wedge shape that indicates a classic thunderstorm cumulonimbus cloud.
D. These clouds are small, round clouds that appear to be individual cells in the visible image. In the infrared image these clouds do not show up at all, indicating warm, low clouds. These are fair weather cumulus clouds, often seen in areas of the country experiencing fair weather.
