Weather-Satellite Images


The Geostationary Operational Environmental Satellites (GOES) and other satellites

Most of the satellite images of the Western Hemisphere that we see in class have been recorded by a pair of weather satellites called Geostationary Operational Environmental Satellites, or GOES, which were launched by the National Aeronautics and Space Administration (NASA) and are operated by the National Oceanographic and Atmospheric Administration (NOAA).

GOES satellites orbit the earth at an altitude of 22,000 miles above the equator once every 24 hours, keeping pace exactly with the earth's rotation. Consequently, the satellites appear to remain fixed above the same locations on earth, which is why they are called "geostationary". Each records several images every hour.

The U.S. has two GOES satellites. GOES-East orbits above Brazil's Amazon Basin, where it can see both the continental U.S. and the eastern tropical Atlantic Ocean near Africa, where hurricanes often develop in summer and fall. GOES-West orbits above the equator over the eastern Pacific Ocean, from where it can see (among other things) midlatitude cyclones developing over the ocean south of Alaska (east of Japan) during the winter.

Several other countries operate their own geostationary weather satellites. These other geostationary satellites include Europe's Meteosat, India's INSAT and METSAT, Japan's MTSAT, and China's FY-2. (For sample images from some of these satellites, see NOAA's Geostationary Satellite Server.

Full disk and sector images

GOES, GMS-5, and other geostationary satellite images that show an entire hemisphere of earth are called full disk images. The cameras and sensors on the satellites can also zoom in to record a smaller region in more detail. These images are called sector images.

Infrared and visible satellite images

GOES, GMS-5, Meteosat-7, and FY-2 carry sensors that can record wavelengths of radiation in several parts of the electromagnetic spectrum, including both visible radiation (light), which people can see, and longwave infrared (IR) radiation, which most features on earth emit but which is invisible to the human eye (because IR wavelengths are too long).

Emitted IR radiation with wavelengths lying in the portion of the electromagnetic spectrum called the atmospheric window passes unaffected through the atmosphere to space. In contrast, water vapor, carbon dioxide and other gases in the atmosphere absorb most of the outgoing IR radiation with wavelengths outside of the atmospheric window. (This behavior is responsible for the greenhouse effect.) However, these gases emit the same wavelengths of IR radiation that they absorb, and satellites can record this radiation to provide information about the state of the atmosphere. Water vapor images are an example.

Infrared satellite images record longwave infrared radiation with wavelengths in the atmospheric window that is emitted by cloud tops, land, oceans or ice and snow. While most things on earth emit longwave infrared radiation, warmer objects emit more than colder ones. Hence, the relative amounts of infrared radiation emitted by cloud tops, land, oceans and snow/ice gives information about their relative temperatures.

GOES satellites record longwave infrared radiation emitted by clouds, land, oceans and ice/snow and transmits the information to computers at the National Centers for Environmental Prediction (NCEP) in Washington, D.C. These computers are programmed to create images that display the warmest objects in black and the coldest in white. Objects at intermediate temperatures appear in shades of gray. The tops of high clouds are usually (though not always) colder than land or oceans, so the lightest parts of an IR satellite image usually show high, cold clouds, while the darkest parts of an image usually show land or ocean surfaces, which are usually warmer.

Visible satellite images record visible light (from the sun) that is reflected by clouds, land, oceans or snow and ice. There is an important difference between visible and infrared images. Since visible images record reflected light rather then emitted radiation, they tell how reflective an object is but nothing about its temperature.


Summary of Differences between
Visible and Infrared (IR) Weather-Satellite Images


Source of radiation coming from features on earth Intensity ("brightness") of radiation coming from a feature on earth
Visible Radiation (light) Light from the sun, reflected by oceans, land, cloud tops, snow/ice Intensity depends on:
  1. intensity of light from the sun striking a feature; and
  2. a feature's reflectivity (or albedo)
Infrared (IR) Radiation
(Invisible to human eye)
IR radiation emitted from oceans, land, clouds tops, snow/ice Intensity depends on temperature of feature (according to the second law of radiation)**

**Note: Since IR radiation is invisible, humans can't see differences in IR radiation intensity (IR "brightness") directly. Instead, a computer is programmed to translate different IR radiation intensities into lighter or darker shades on a satellite image. The translation is arbitrary, but here is how meteorologists have chosen to do it on black and white IR satellite images:

Colder temperatures (lower IR emission intensity)
<----------------------------->
Warmer temperatures (higher IR emission intensity)
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translated
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translated
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Lighter shades on IR satellite image <---------> Gray shades on IR satellite image <---------> Darker shades on IR satellite image

Different temperatures (i.e., different intensities of IR emission) could also be translated into different colors, to create a color enhanced IR satellite image. There are an infinite number of ways to do this--the translation is arbitrary.


Comparing features on infrared and visible images

On visible images, high and low clouds look equally bright (because all clouds have relatively similar reflectivities) and are therefore often indistinguishable. Similarly, clouds and snow-covered ground are about equally reflective and hence often look the same.

However, fog and low clouds are often easy to see on visible images because they reflect much more light than do nearby ocean or land surfaces.

Of course, visible images can only be recorded during the daytime.

On infrared images, in contrast, fog and low clouds are often hard to distinguish from nearby ocean or land surfaces because these features often all have similar temperatures (though not always--during the daytime, fog reflects solar radiation and so doesn't warm up much, whereas surrounding land surfaces tend to absorb more sunlight and warm up, creating a temperature difference).

However, snow, fog and low clouds are easy to distinguish from high clouds in infrared images because features near the earth's surface and clouds high in the troposphere are usually at different temperatures.

Since clouds, land, oceans and snow and ice never stop emitting longwave infrared radiation, infrared images can be recorded day or night. Land surfaces appear darker (warmer) during the day, lighter (colder) at night.