Observing Weather from Space

This is a sample lesson page from the Certificate of Achievement in Weather Forecasting offered by the Penn State Department of Meteorology. Any questions about this program can be directed to: Steve Seman

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At the end of this section, you should be able to distinguish between geostationary and polar-orbiting satellites. You should also be able to describe their differences and roles in observing the earth, and be able to identify a satellite image as being collected by a geostationary satellite or a polar-orbiting satellite.

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Today, meteorologists have an ever-increasing number of sophisticated, computerized tools for weather analysis and forecasting. But, before 1960, meteorologists drew all their weather maps by hand and no useful computer models existed. Seems like the dark ages, right? Furthermore, before 1960, forecasters did not have weather satellites to afford them a birds-eye view of cloud patterns. The dark ages ended after NASA launched Tiros-I on April 1, 1960.

Photographic image of an early view of the earth from space taken by the Tiros satellite (pictured at right).
(Left) The first televised image from space captured by the TIROS-1 satellite (pictured right) on April 1, 1960.
Credit: NASA

Though the unrefined, fuzzy appearance of this image may seem crude and almost prehistoric, it was an eye-opener for weather forecasters, paving the way for new discoveries in meteorology (not to mention improved forecasts). Today, satellite imagery with high spatial resolution allows meteorologists to see fine details in cloud structures. For example, check out this close-up loop of the eye of Hurricane Dorian in 2019 (loop credit: Dakota Smith). We've come a long way, wouldn't you agree?

Two types of flagships exist in the select fleet of weather satellites that routinely beam back images of Earth and the atmosphere -- geostationary satellites and polar-orbiting satellites.

Geostationary Satellites

Artist's rendering of GOES-16 in orbit.
An artist's rendering of GOES-16 in orbit.
Credit: NASA

Geostationary satellites orbit approximately 35,785 kilometers (22,236 miles) above the equator, completing one orbit every 24 hours. Thus, their orbit is synchronized with the rotation of the Earth about its axis, essentially fixing their position above the same point on the equator (hence the name "geostationary"). In the United States, the National Oceanic and Atmospheric Administration's (NOAA) geostationary satellites go by the name of "GOES" (Geostationary Operational Environmental Satellite) followed by a number. To get an idea of what a geostationary satellite looks like, check out the artist's rendering of GOES-16 on the right.

Two operational geostationary satellites currently orbit over the equator at 75 and 135 degrees west longitude, respectively. The terms "GOES East" and "GOES West" are the generic terms for the operational satellites stationed at those longitudes. GOES-East is in a good spot to keenly observe Atlantic hurricanes as well as weather systems over the eastern half of the United States. GOES-West is in better position to observe the eastern Pacific and the western half of the United States. If you are interested in learning more about the current condition of any particular GOES satellite, you can check out the GOES Spacecraft Status page run by the NOAA's Office of Satellite Operations.

From their extremely high vantage point in space, GOES-East and GOES-West can effectively scan about one-third of the Earth's surface. Their broad, fixed views of North America and adjacent oceans make our fleet of geostationary satellites very effective tools for operational weather forecasters, providing constant surveillance of atmospheric "triggers" that can spark thunderstorms, flash floods, snowstorms and hurricanes (among other things). Once threatening conditions develop, the broad, fixed view of geostationary satellites is especially handy because we can create loops of geostationary satellite imagery, which allow forecasters to monitor the paths and intensities of storms. For example, this loop of GOES satellite images spans from 1345Z to 1745Z on September 27, 2017, and shows Hurricane Maria spinning just off the East Coast.

Geostationary satellites are far from perfect, however. Geostationary satellites don't have a very good view of high latitudes because they're centered over the equator. Therefore, clouds at high latitudes become highly distorted and at latitudes poleward of approximately 70 degrees, geostationary satellites become essentially useless.

I don't want you with the impression that the GOES program is unique, however. Other countries also own and operate geostationary weather satellites. For more on these satellite programs and some resources for accessing images from geostationary weather satellites, check out the Explore Further section below.

Summary: Geostationary satellites provide fixed views of large areas of the earth's surface (a large portion of an entire hemisphere, for example). The fact that their view is fixed over the same point on earth means that sequences of their images can be created to help forecasters track the movement and intensity of weather systems. The primary limitation of geostationary satellites is that they have a poor viewing angle for high latitudes and are essentially useless poleward of 70 degrees latitude.

Polar-Orbiting Satellites

Polar-orbiting satellites pick up the high-latitude slack left by geostationary satellites. In the figure below, note that the track of a polar orbiter runs nearly north-south above the earth and passes close to both poles, allowing these satellites to observe, for example, large polar storms and large Antarctic icebergs. Polar-orbiting satellites orbit at an average altitude of 850 kilometers (about 500 miles), which is considerably lower than geostationary satellites.

Each polar orbiter has a track that is essentially fixed in space, and completes 14 orbits every day while Earth rotates beneath it. So, polar orbiters get a worldly view, but not all at once! Like making back-and-forth passes while mowing the lawn, these low-flying satellites scan the Earth in swaths about 2600 kilometers wide, covering the entire earth twice every 24 hours. The appearance of a "lawn-mowing-like" swath against a data-void, dark background on a satellite image is a dead give-away that it came from a polar orbiter (as illustrated by this visible image of smoke sweeping over the Northeast States from fires in Quebec in early July, 2002).

During hurricane season, polar orbiters also send crucial imagery to forecasters at the National Hurricane Center in Miami, Florida. Spectacular images of hurricanes and tropical storms from polar orbiters are available at the weather website maintained by Johns Hopkins University.

A scaled drawing of earth, encircled by polar orbiting and geostationary satellites.
The orbits of geostationary and polar-orbiting satellites (drawn to scale).
Credit: David Babb

NOAA designates its polar orbiters with the acronym "POES" (Polar Orbiting Environmental Satellite) followed by a number. NOAA currently classifies the newest satellite as its "operational" polar orbiter, while slightly older satellites that continue to transmit data are classified as "secondary" or "backup" satellites. As a counterpart to the GOES satellites, the NOAA Office of Satellite Operations operates a POES Spacecraft Status page as well. NASA (Aqua, Terra) and the Department of Defense also operate polar orbiters. 

Summary: Polar-orbiting satellites orbit at a much lower altitude than geostationary satellites, and don't have a fixed view since the earth rotates beneath their paths. The benefit of polar-orbiters is that they can give us highly-detailed images, even at high latitudes. The main drawback is that they have a limited scanning width, and don't provide continuous coverage for any given area (like geostationary satellites do). A single image from a polar orbiter will often show a swath with sharply defined edges that mark the boundaries of what the satellite could see on a particular pass.

Data from satellites has truly revolutionized weather analysis and forecasting. Satellites can measure atmospheric temperatures, moisture, and winds, among other things. Roughly 80 percent of all data used to run computer forecast models comes from polar orbiting satellites alone, so satellites are a critical part of weather forecast operations around the globe! Now that you have some background about the different types of satellites providing crucial weather data, we'll turn our attention to interpreting basic types of satellite images.

Explore Further...

As I mentioned above, the GOES program is not unique, and other countries also own and operate geostationary weather satellites (check out this international perspective on geostationary weather satellites). You can access images from GOES or geostationary weather satellites operated by other countries at the University of Wisconsin's website. Or try NOAA's Geostationary Satellite Server, operated by the National Environmental Satellite, Data and Information Service (NESDIS). Another great site to bookmark is the GOES Imagery Viewer page. Check it out!

But geostationary satellites don't just cover weather. More than 200 geostationary satellites hover above the equator around the world! With the number of communications and TV satellites increasing, the "geostationary parking lot" is getting pretty crowded. If you look at the time-lapse photograph below, which was taken by a telescope atop Kitt Peak in Arizona between 0230Z and 11Z on March 19, 2007, the array of bright dots, which are geostationary satellites, shows that parking spots are at a real premium (this photograph captures only 9% of the total geostationary orbit, so you can see what I mean by "crowded").

Star trails on a long exposure photograph. Geostationary satellites are seen as points rather than streaks.
A time lapse of a small portion of the geostationary orbit taken from atop Kitt Peak in Arizona from 0230Z to 11Z on March 19, 2007. The lines represent star trails, while the bright dots mark the positions of geostationary satellites.
Credit: Dave Dooling, National Solar Observatory

How do I know those dots are geostationary satellites? Well, when photographers take time-lapse images of the nighttime sky, the stars leave star trails (check out this time-lapse photograph above Mauna Kea in Hawaii and note the awesome star trails; by the way, moonlight illuminated the mountain and sky). Of course, the stars don't move. Rather, the earth rotates about its axis and thus the stars appear to move. Now look closely at the time-lapse of the nighttime sky over Mauna Kea. Note that you don't see the stars themselves, only their trails. In other words, you don't see stars as fixed dots because the Earth rotates on its axis during the period of the time-lapse photography.

That means, of course, that the bright, fixed dots in the midst of the belt of star trails are in geosynchronous orbit with the earth (they obviously didn't move during the time-lapse photography). I emphasize here that there's no way that the light reflected by the geostationary satellites would be sufficiently bright to see them clearly on just a single snapshot, but the long exposure allows them to stand out on this time-lapse photograph.