You can't watch the evening news or The Weather Channel without seeing satellite images, and the Web has made these graphics easily available to any pilot with a modem and notebook computer. But what does it all mean? That depends on what flavor of satellite image you're looking at: visible, infrared, or vapor. Jack Williams and Chris Cappella of Flight Training magazine help you break the code.
November 28, 1998
|About Flight Training ...
Flight Training is the preeminent monthly
publication for student pilots and flight instructors.
An annual subscription costs just $21.95 (higher if mailed outside the U.S.), with
longer-term subscriptions available at significant savings. You can subscribe
online and help support continued free access to
AVweb and AVflash in the process.
Rember, a good pilot is always learning!
After you've looked at the weather maps and read the observations
and forecasts for a flight, a quick glance at the latest satellite images can help bring
even the cloudiest weather picture into focus. Knowing how to interpret the images can
also help keep you away from dangerous weather.
Satellites are the weather eyes in the sky that meteorologists use to see major rain
and snow storms, thunderstorms ahead of warm fronts or along cold fronts, high clouds and
fog, and even jet stream winds.
These images come in a variety of forms and sizes, from global composites that resemble
snapshots of the Earth from space to tight close ups of individual thunderstorm tops, day
or night. But, in one form or another, all images show the same thing - water as clouds or
vapor in the atmosphere.
By the way, "images" is a much better word than "photo" to describe
the pictures from satellites because they show things no ordinary camera could ever
capture, such as invisible water vapor or the temperatures of cloud tops.
When meteorologists saw the first images of Earth from NASA's Tiros 1 spacecraft in
1960, they quickly realized that satellites would play a vital role in understanding and
forecasting weather. More than three decades of fine tuning have sharpened the resolution
of these images. Today's sensors can capture details as small as one-half mile across, and
technological advances have increased the number of ways of probing the lower atmosphere.
Satellite sensors detect and record the radiation reflected and radiated by clouds and
water vapor in the troposphere. Radiation sensed in the visible part of the spectrum
(ordinary light) allows satellites to reproduce the Earth's cloud cover as if they were
taking a picture of Earth from space. Infrared radiation emitted by clouds reveals their
temperatures, which can then be used to calculate cloud heights.
The National Oceanic and Atmospheric Administration uses two kinds of satellites:
Geostationary Operational Environmental Satellites (GOES) orbit the Earth from a
position 22,380 miles above the equator. At this position they orbit at the same speed
that the Earth turns, so they remain over the exact same spot on the equator - hence, the
name geostationary - and scan the same geographic area all the time. The images you see on
TV weathercasts or the Internet are almost always from GOES.
Polar Orbiter Environmental Satellites (POES) circumnavigate the Earth nearly across its
poles. Cruising at altitudes of 300 to 500 miles, POES can reveal details about cloud
cover and water vapor in higher latitudes, where GOES images are usually distorted because
of Earth's curvature. As the Earth revolves beneath their orbit, these satellites scan a
new 900-mile-wide strip of land and ocean and the atmosphere above them on each pass.
Figure 1 Visible images depict cloud cover by measuring the amount of
solar radiation the clouds reflect. As the table below shows, thick clouds and fresh snow
reflect more light than thin clouds, so they appear brighter in the images. Thin clouds
appear gray, and areas with no shading are generally clear.
| Large thunderstorm
| Fresh snow
| Thick stratocumulus
| Thin stratus
| Water surfaces
Scientists and forecasters make mini movies using consecutive GOES images, looping them
to see a storm's motion during the previous hours and days. The U.S. operates two
geostationary satellites, GOES 8 covers the eastern U.S. and Atlantic, GOES 9 the West and
part of the Pacific. The Europeans and Japanese operate similar satellites that cover most
of the rest of the world.
The close imaging from POES and the lack of easily accessible images mean they are not
very useful to pilots. We'll focus on GOES imagery because it's easier to understand and
is much easier to find.
The image in Figure 1 is a visible GOES image. You can use such images as if they were
photographs. Visible images depict cloud cover by measuring the amount of solar radiation
the clouds reflect. As Table 1 shows, thick clouds and fresh snow reflect more light than
thin clouds and appear brighter in the images. Thin clouds appear gray and areas with no
shading are generally clear.
But, there's some danger in relying upon visible images alone to determine cloud cover.
Because low, middle, and high clouds reflect about the same amount of sunlight, a visible
image can be misleading, often not telling pilots anything about cloud heights. Areas that
appear clear on a visible image might be covered by a thin veil of clouds or fog that
could obstruct your view of an airport, making a VFR landing impossible. To get a truer
reading of clouds, you should also look at infrared images.
Figure 2 Satellites that scan the infrared (IR) wavelength measure
cloud top temperatures rather than reflected light. In this image, the highest clouds
(with the coldest tops) appear bright white. Middle clouds have warmertops and appear in
shades of gray. Low clouds and fog show up as the darkest shades of gray in this IR image.
Satellites that scan the infrared (IR) wavelength actually measure cloud top
temperatures rather than reflected light, as in visible satellite images. Because
temperatures in the lower part of the atmosphere decrease with increasing altitude,
high-level clouds are cooler than low-level clouds. Low-top clouds with warmer
temperatures appear darker in IR images than taller, colder cloud tops.
In Figure 2, the highest clouds, probably cirrus, off the mid-Atlantic and New England
coasts appear bright white. Middle clouds over the central U.S. have warmer cloud tops and
appear in shades of gray. Low clouds and fog show up as the darkest shades of gray in this
IR image, such as over east Texas and Louisiana.
One obvious advantage pilots have when viewing visible and infrared images together is
that dangerous thunderstorms are very apparent. In both Figures 1 and 2, you can see
thunderstorms north of the Bahamas and east of Florida. Cumulonimbus (thunderstorm) clouds
are always thick, and usually are the tallest clouds. They reflect a lot of sunlight, and
their cirrus anvils are very cold. Thunderstorms show up as bright white clusters in both
visible and IR satellite images.
Another obvious advantage of IR images is that they are available at night, while
visible images are only useful during daylight hours. But pilots need to be careful when
using satellites to identify areas of low clouds and fog, especially at night.
Because low clouds and fog are often the same temperature as the ground, an IR image
might show both as dark gray, not distinguishing between the clouds or fog and the ground.
A visible image during daylight should reveal the low clouds and fog. At night, you should
check weather observations and pilot reports for fog or low clouds.
Jet streams are rivers of upper-air winds that pilots of high-flying planes can use to
their advantage when flying from west to east, or that they would want to avoid when
flying east to west. Jet streams are not very obvious on either visible or IR images. This
is where satellites' special water vapor imaging is useful.
Water Vapor Channel
Figure 3 This is a water vapor image. It shows humid air feeding into
an immense storm over the Pacific Northwest. Dry air is indicated by the black areas, and
areas of moderate moisture appear as intermediate shades of gray.
Today's satellites offer a third type of imagery. They can sense and measure humid air
using part of the IR scanning range called the "vapor channel." Vapor channel
images show water vapor as shades of gray to white - the whiter the shade, the more humid
the air. Dark shades of gray indicate drier air, with black shading for very dry air. GOES
water vapor imagery can be animated to reveal the swirling motions of middle and upper
level winds as they move the water vapor around.
Upper-air jet streams, which often transport high-level moist air, are readily visible
on water vapor images, as are mid-level storms. Such mid-level storms, or upper air
disturbances, can often create clouds and precipitation in areas with no surface storms
around. They can also help trigger surface storms.
Figure 3 is a water vapor image. It shows humid air feeding into the southern end of an
immense storm over the Midwest. Dry air is wrapping into the back side of the same storm
as indicated by the black areas penetrating the storm's center from the west. A streak of
light gray across the bottom of the figure shows the subtropical jet stream racing from
west to east and energizing thunderstorms over the Gulf of Mexico. A faint swirl in the
water vapor over Utah indicates a mid-level storm, which is probably bringing rain and
snow to the Rockies.
Don't expect any kind of satellite image to give you all the information you need for a
safe flight. Even forecasters who are expert in reading satellite images use them in
conjunction with other information to make predictions. Still, satellites with their view
of weather from much higher than any airplane can fly, can help you understand the big
picture of what the atmosphere is doing as you prepare to fly.
Not long ago, you needed a special, expensive receiver to get satellite transmissions.
This is still the best way to get the latest transmissions, but you can also get satellite
imagery free on the World Wide Web. As with any information on the Web, make sure the
satellite images you're looking at are up-to-date. Satellite images have their date and
time in Zulu time, usually at the top or bottom margin.
Because the National Oceanic and Atmospheric Administration (NOAA) is the main user of
satellite data, its Geostationary Satellite Browse Server (http://www.goes.noaa.gov) is the one stop for the
latest visible, IR and water vapor images for the U.S.
The Cooperative Institute for Research in the Atmosphere current weather page has links
to visible, IR (long wave) and water vapor images for the eastern half of the U.S.
(GOES-8) and the western half of the U.S. (GOES-9) at http://www.cira.colostate.edu/Special/CurrWx/currwx.htm.
NOAA's Aviation Weather Center has lots of products for pilots. Its main page has links
to eastern and western U.S. visible satellite images at http://www.awc-kc.noaa.gov/awc/aviation_weather_center.html.
The University of Wisconsin-Madison's Space Science and Engineering Center has links to
a variety of visible, IR and water vapor images of the U.S. and the Western Hemisphere at
The NOAA National Environmental Satellite Data and Information Service (NEDIS) Forecast
Products Development Team enhances satellite imagery to create special products such as
winds in the atmosphere, nighttime low cloud and fog images, and experimental aircraft
icing products. You'll find this at http://ns.noaa.gov/NESDIS/NESDIS_Home.html.