Part 1—Learn About GOES Images
Satellites Track Weather from Space
Today's meteorologists use data collected by an extensive set of instruments as they develop their weather forecasts. Some of those instruments are just outside the buildings in which they work. Others, such as the Geostationary Operational Environmental Satellites (GOES), pictured on the right, hover 35,800 meters (22,300 miles) above Earth's surface. (Geostationary means that the satellite always remains positioned above the same point on Earth.) From this altitude, GOES satellites monitor large sectors of Earth's surface. The primary mission of the GOES program is to help scientists provide early warning of developing storms, such as thunderstorms, tornadoes, and other severe weather. In addition, the satellites help scientists develop estimates of rainfall and snowfall, predict stream flooding and flash floods, and track the movement of hurricanes, smoke from forest fires, volcanic ash, and even sea ice.
The image below, showing the Western Hemisphere, was composed from data collected by a GOES satellite. This so-called "full disk" image is only one of the image types produced by the GOES satellites; GOES satellites also produce more detailed images showing smaller portions of Earth's surface. White outlines of the landmasses (and sometimes of states and counties) are superimposed on satellite images to help users pinpoint the areas being affected by weather.
More Information About the GOES Satellites
GOES satellites, like all geostationary satellites, orbit Earth in the plane of Earth's equator. There are two GOES satellites that monitor the United States and the adjoining oceans: GOES East is positioned above the equator at longitude 75 degrees west (see the black line crossing the equator ); GOES West is positioned above the equator at longitude 135 degrees west (see the white line crossing the equator). GOES East provides a reasonable view of the U.S. except for the western states, Alaska, and Hawaii. GOES West provides a better view of the western states, includes Alaska and Hawaii, and monitors a large area of the Pacific Ocean.
Geostationary satellites orbit the Earth exactly once each day. This is very different than the International Space Station, for example, which orbits the Earth approximately 15 times each day. In order to rotate at exactly the same speed as Earth, geostationary satellites need to be very far away from Earth.
How GOES Satellites Monitor Earth
GOES satellites monitor Earth by detecting two different types of electromagnetic radiation and sending that information back to Earth. The information is then used to form images that reveal conditions in the atmosphere.
One type of radiation monitored by GOES satellites is the visible light that comes from the Sun and that is reflected off cloud tops and Earth's surface. This is the same radiation you need to take regular photographs: no reflected visible light means no photograph. Therefore, GOES visible light images are available only during daylight hours. The sunlight that reflects off cloud tops allows meteorologists to identify cloud type, track cloud movement, and provide early warning about developing severe weather. Visible light images also show the portions of Earth that are not cloud covered. Snow, ice, and light-colored sand reflect the greatest amount of visible light from the ground and appear bright in visible light images. Water absorbs most of the sun's light and appears dark.
The other type of electromagnetic radiation detected by GOES satellites is the thermal infrared radiation that is emitted by Earth. Thermal infrared wavelengths are longer than visible light wavelengths and are not visible to our eyes. Thermal infrared radiation is actually another name for heat. It is produced by the motion of atoms and therefore is radiated from everything that has a temperature above absolute zero (-273 degrees C). Everything on Earth and in its atmosphere is much warmer than absolute zero and therefore emits thermal infrared radiation. Scientists use thermal infrared radiation to determine cloud temperature and to highlight atmospheric water vapor that does not reflect visible light.
The purpose of this note is just to help avoid any possible confusion between the solar-emitted infrared radiation and Earth-emitted thermal infrared radiation.
Not all of the thermal infrared radiation that Earth emits can reach the GOES satellites. This is because the oxygen, carbon dioxide, ozone, methane, and water vapor in Earth's atmosphere absorb the longer wavelengths of thermal infrared radiation. Only the shorter wavelengths of thermal infrared radiation pass through the atmosphere. Scientists have figured out how to take advantage of this situation. To monitor cloud tops and water vapor in the upper atmosphere, they use instruments sensitive to long-wave infrared radiation. At those elevations there is not enough atmosphere to absorb the longer wavelengths. And to monitor the ground and low-level clouds, they use instruments sensitive to short-wave infrared radiation, since short-wave radiation can pass through the atmosphere.
In addition to Earth's surface, other sources of infrared radiation include the water vapor in the atmosphere and clouds. Since some atmospheric water vapor and cloud tops are very high in the atmosphere, most of the thermal infrared radiation they emit can be detected by GOES satellites.
Visible Light Images
Water Vapor Images
Imager and Sounder
The two primary weather instruments on each GOES satellite are named Imager and Sounder. Each instrument is capable of forming visual light images as well as a set of infrared images of various wavelengths. Neither instrument captures a single image at one moment in time. Both instruments form composite images in the same way: the sensors sweep across Earth in an east-west direction, collecting data from an 8 km wide path, then step north (south) and sweep in the opposite direction to collect another 8 km wide path. They continue to do this until they complete the image being developed. For more information about Imager and Sounder, visit their web pages.
The frequency at which an area is scanned can be changed if conditions call for that. For example, an area of severe weather will be scanned more frequently in order to track changes and keep local meteorologists better informed.