Background Information about Weather-Satellite Images

(Provided by San Francisco State University's California Regional Weather Server.)

U.S. Geostationary Weather Satellites

The visible and infrared satellite images displayed by SFSU's California Regional Weather Server (and by most other WWWeb servers in the U.S.) are recorded hourly by one of two weather satellites, GOES-West (currently GOES-15) or GOES-East (currently GOES-13), which are in geostationary orbit about 22,000 miles above the equator. ("GOES" stands for Geostationary Operational Environmental Satellite.)

The time and date appearing on most satellite images is in Universal Time Coordinates (UTC), formerly known as Greenwich Mean Time (GMT). (See comments about time labels for more information.)

GOES-15, the newest of the U.S.'s operational geostrationary satellites, was launched in the spring of 2010 and became operational in December, 2011. It orbits above the eastern equatorial Pacific Ocean (at 135 degrees west longitude) and provides images of the western U.S. and eastern Pacific Ocean.

GOES-13 was launched in 2006 and became operational in spring, 2010. It orbits above Brazil (at 75 degrees west longitude) and provides images of the central and eastern U.S. and western Atlantic Ocean.

GOES-West and GOES-East were launched and are maintained by NASA (the National Aeronautics and Space Administration) and are managed by NOAA (the National Oceanographic and Atmospheric Administration).

Visible Images

Visible satellite images record visible light from the sun reflected back to the satellite by cloud tops, land surfaces and ocean surfaces. These black-and-white images show what your eye would see from space (if you were color blind) and hence represent nothing fancier than ordinary black-and-white photographs of the earth from space. The brightness of any feature on a visible satellite images depends on (1) how directly light from the sun strikes it, and (2) how reflective the feature is. Cloud tops and snow and ice surfaces tend to reflect visible light best, so they tend to be the brightest (i.e., whitest) features on a visible satellite image. Ocean surfaces tend to reflect the least visible light, so they tend to be the darkest features.

Infrared Images

Infrared satellite images record invisible infrared radiation emitted directly by cloud tops, land surfaces or ocean surfaces. The warmer an object is, the more intensely it emits radiation, so the intensity with which a feature on earth emits infrared radiation tells us about that feature's temperature.

Using a computer, we can take the intensities of infrared radiation recorded by a weather satellite and translate them arbitrarily into different shades of gray and/or different colors, constructing images that humans can see. On the infrared images available at most WWWeb weather sites, including SFSU's California Regional Weather Server, darker shades of gray represent relatively warmer features, and lighter shades of gray represent relatively colder features. (For examples of gray-scale infrared satellite images, see below).

On the color-enhanced infrared images available at SFSU's California Regional Weather Server, colors other than gray are assigned to some of the coldest temperatures. (For examples of color-enhanced infrared satellite images, see below). Far from the polar regions, features this cold invariably comprise the tops of clouds in the upper troposphere or lower stratosphere and are often associated with thunderstorms, hurricanes or midlatitude cyclones. Hence, color-enhanced infrared images help highlight storms of various types.

A scale along the lower left-hand edge of some color-enhanced infrared images (for example, see large-scale North American overviews) relates colors and shades of gray to relative temperatures. Each shade of gray or distinct color represents a range of 5 degrees Celsius (9 degrees Fahrenheit). In particular, colors are assigned to the following temperatures:

Temperatures colder than -70 degrees C are assigned a very light shade of gray and would generally appear surrounded by one or more of the colors listed above.
Example of a Gray-Scale Infrared Satellite Image: [Note that no temperature scale appears on this particular type of gray-scale infrared satellite image.]
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Examples of Color-Enhanced Infrared Satellite Images:

[Note that a temperature scale appears only on the second of these two color-enhanced infrared satellite images.]
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(Page last modified 12/28/11.)