Part 1—Learn About GOES Images

Satellites Track Weather from Space

Geostationary Operational Environmental Satellites (GOES). Click on image for larger view. Source: NASA

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.

GOES full disk image of Hurricane Irene captured August 26, 2011. Click image for larger view.

More Information About the GOES Satellites

Based on a NASA image.

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.

Compares the distance from Earth of the GOES satellites 35800 km with the International Space Station 360 km.
Based on a NASA image. (NOTE: In this image, the relative distances are reasonably accurate but sizes of the GOES satellite and the International Space Station have been exaggerated.)

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.

Note: Just as there is a great range in the wavelengths of visible light, accounting for the different colors of light, so too is there a great range of infrared wavelengths. Thermal infrared radiation, as well as the shorter wavelength near infrared radiation are both emitted by the Sun, but this investigation addresses just the thermal infrared radiation emitted by Earth, clouds, and atmospheric water vapor. You may have seen satellite images of Earth in which all vegetation has a red color. It is the near infrared radiation emitted by the Sun that reflects well off chlorophyll, that can be captured on special film, and that produces those images showing vegetation in shades of red.

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.

GOES Images

Visible Light Images

Scientists have a special name to describe reflected light: it is called albedo. A surface that reflects most of the light that falls on it, such as snow-covered ground, has a high albedo. A surface that absorbs most of the light that falls on it, such as water, has a low albedo. In this image, the brightest clouds, those with higher albedo, are the thicker clouds that hold more moisture. Thinner clouds, such as those in the northwest, have lower albedo, an indication that they hold less moisture than the clouds over parts of the south. Meteorologists can learn a lot about the approaching weather by studying cloud types, and viewing a series of GOES images to determine cloud speed and direction. Notice that the Great Lakes, as well as the oceans, appear dark.

Infrared Images

This image was acquired at approximately the same time as the image above. The instrument that acquired it is sensitive to a portion of the infrared spectrum that is not scattered or absorbed by the atmosphere. Therefore, the image provides thermal or temperature information about Earth's surface as well as lower and higher clouds. The brightest areas of this image are the coldest, and since the coldest clouds are the highest, the image also indicates cloud height. The colder the cloud, the more likely it is to produce rain. Sometimes meteorologists color infrared images to more easily interpret them. Those colored images will have a key explaining the significance of the colors. Notice the darker (warmer) ground in Mexico. This image type is also used to monitor sea surface temperature. Infrared images are available both day and night because they do not depend on solar radiation.

Water Vapor Images

This image was acquired at approximately the same time as the two images above, and it was also acquired by thermal infrared radiation. However, the instrument that acquired it is sensitive to the wavelengths of thermal infrared radiation that are largely absorbed and scattered by the atmosphere. As a result, the image is acquired by thermal infrared radiation emitted by water vapor and clouds that are high above Earth's surface, 6 to 10 km in elevation, which is also the elevation of the jet stream. The brightest areas have the most atmospheric water vapor. Dark areas indicate drier air, including the jet stream and high-pressure systems. Water vapor images are available both day and night because they do not depend on solar radiation.

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.

Sketch showing the sweep-step pathway of the sensors

Source: NOAA
The GOES instruments have regular schedules for scanning Earth. The image below shows the boundary of each GOES East scan. The instruments move through a routine of scanning North America (CONUS), an extended Northern Hemisphere scan, a (partial) Southern hemisphere scan, and finally what is called a Full Disk scan, which takes in the entire visible hemisphere. GOES West has a similar map and schedule.

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.

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