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Weather Underground provides 24 hour access to our satellite products. Satellite data is updated in real time as soon as updates from our data sources are received.
The satellite features are always just one click away. Look for the features toolbar near the top of all our web pages. Click the "Maps & Radar" tab and select "Satellite" from the pull-down list to open the satellite page. You will see our interactive WunderMap with a satellite image overlay.
You can use the controls to the right to modify the imagery, and you can use the zoom controls in the upper left portion of the map to change the area you are viewing.
The original image is taken in visible light from a satellite with 1 km resolution. We then remove everything we think is the ground, and replace the ground with a color topographic map we have (that does not include cities). The result is a map with clouds overlaid on top.
If you are seeing white dots on the satellite images, it is being caused by the satellite's image detector getting errors. There is nothing we can do to fix this, but they should go away after the next update.
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.