Visible vs. Infrared Satellite
Visible satellite images can be thought of as photographs of the earth from space. Since they are like a photograph, they are dependent on visible light (brought by the sun). As a result, visible satellite pictures only work during daylight hours. This is the greatest drawback to using visible imagery. Also, since a visible satellite picture is basically a photograph, thicker clouds (which reflect the most sunlight) show up very bright, while thinner clouds (like cirrus) are hard to distinguish.
Infrared satellite technology works by sensing the temperature of infrared radiation being emitted into space from the earth and its atmosphere. Basically, all objects (including water, land, and clouds), radiate infrared light. However, our eyes are not "tuned" to see this kind of light, so we don't notice it. Weather satellites not only sense this infrared light, but they can also sense the temperature of the infrared emissions. The warmest emissions are displayed as dark greys on an infrared satellite image, while cold emissions are displayed as bright white (or sometimes other colors of the rainbow). In general, the temperature of the atmosphere decreases with height. Since clouds are often high in the atmosphere (about 10,000 feet), they are in air that is much colder than the earth's surface. Therefore, the rule of thumb is: the brighter the cloud in an infrared image, the higher the cloud. However, there are two major drawbacks to infrared satellite pictures. One, low clouds are almost impossible to spot since they blend in with the ground (remember, the image is based on relative temperatures... Low clouds are about the same temperature as the ground). And two, the resolution of the images is much lower. When zooming into an infrared picture, the image will look "blocky" much sooner.
|Usable when?||Only during daylight hours.||All the time.|
|High clouds appear as:||Not distinguishable from low clouds.||Bright white.|
|Low clouds appear as:||Not distinguishable from high clouds.||Very dark, sometimes blending in with the ground.|
|Thin clouds appear as:||Milky white.||Not distinguishable from thick clouds.|
|Thick clouds appear as:||Bright white.||Not distinguishable from thin clouds.|
|Resolution of cloud pixels:||1 km (high resolution)||4 km (lower resolution)|