November 28, 1998 Interpreting Weather Satellite Imagery

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by	Jack Williams and Chris Cappella
	This article first appeared in the February 1998 issue of FLIGHT TRAINING magazine and appears here by permission.

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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.   Visible Image Reflectivity Large thunderstorm 92% Fresh snow 88% Thick stratocumulus 68% Thin stratus 42% Water surfaces 9%

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.

Visible Images

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.

Infrared Scans

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.