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    Help - GOES Satellites

    A Geostationary Operational Environmental Satellite, or GOES Satellite, is a satellite which is placed on a geosynchronous orbit around the Earth's equator. This means that the satellite is orbiting the Earth at a rate equal to the rate of the Earth's rotation. Therefore, the satellite remains at the same position over the Earth at all times. There are two GOES Satellites used on this site: one positioned over the western US, and another positioned over the eastern US.

    GOES Satellites have several channels, each of which are used to measure different aspects of the atmosphere. The channels used here are visible, infrared (IR), and water vapor.

    The visible channel measures the intensity of solar radiation which has been reflected by terrestrial objects such as clouds, the ground, etc. The higher the intensity of a reflected solar radiation the whiter the image will appear on the map. Thick clouds have a higher reflectivity than thin clouds and will show up better on a visible image. Snowpack on the surface also has a high reflectivity and will thus make it difficult to differentiate between clouds and snow on the map. Because the visible image is based on reflected solar radiation it is only useful during daylight hours.

    The infrared (IR) channel measures the intensity of radiation emitted by terrestrial objects at a wavelength of about 10-11 micrometers. Radiation emitted at these wavelengths is known as "window IR" and is less likely to be absorbed by other atmospheric gases before reaching the satellite. Using blackbody radiation laws, the intensity of the radiation is translated into a temperature value. Higher temperature values are shown as darker colors on the map while colder temperatures are shown as whiter colors. The IR imagery can be used to determine cloud-top temperature and consequently can give a general idea of the cloud-top height. A lighter-colored cold temperature value on an IR image indicates a high cloud-top height. A darker-colored warm temperature value, on the other hand, indicates a low cloud-top height. The IR channel can only measure the radiation emitted from the top of the clouds or other terrestrial objects. Therefore, the thickness of a cloud layer on the IR imagery cannot be determined.

    The water vapor channel is also an IR image; however, it is sensitive to emitted radiation at a wavelength of about 6-7 micrometers. Water vapor in the atmosphere effectively absorbs radiation at these wavelengths. This absorption decreases the intensity of radiation that reaches the satellite and therefore reduces the temperature value sensed by the satellite. It can then be inferred that a higher temperature value on a water vapor image indicates a dry area and a lower temperature value indicates a moist area. On a water vapor image higher temperature values are indicated by darker colors and colder temperature values by whiter colors. The water vapor image gives a good look at the movement of air and inferred moisture content in the upper and mid-level portions of our atmosphere, including areas where clouds are not present. Low-level moisture content cannot be inferred from the water vapor channel because the radiation emitted at those levels is greatly or entirely absorbed by the time it reaches the satellite.