Wx Watch: Eyes in the Sky

Want to see satellite imagery? There are tons of Web sites that post satellite shots, including AOPA Online. But I especially like the link to “real time images,”on the one run by the National Oceanic and Atmospheric Administration’s Office of Satellite Operations. The NOAA site shows all three image views for the continental United States, Puerto Rico, Alaska, and Hawaii.

Another great Web site is the MODIS (moderate resolution imaging spectroradiometer) Rapid Response System. From the site, scroll down to the red squares to call up recent, very high resolution images from the MODIS Terra satellite. Be prepared to be blown away.

WXWatch

GOES satellites show more than clouds

By: Thomas A. Horne

Every pilot I know regularly checks out weather satellite imagery as part of his or her preflight duties, and for the most part, interpreting satellite photos is fairly intuitive. But there are some nuances to bear in mind.

There are two types of weather satellites up there, some of them in polar orbits and others in geostationary orbits. And, by the way, the United States doesn’t have a corner on the satellite-imagery market. Many other nations have their own satellite systems.

Polar orbiters, such as the United States’ NOAA-15 and NOAA-16 satellites, orbit north to south at altitudes of approximately 500 statute miles. Each satellite makes 14 orbits per day as the Earth rotates beneath them, making overlapping passes and creating mosaics of the weather below. In this way, each satellite maps the entire Earth every 24 hours.

Because of their low altitude, polar-orbiting satellites give close-up views of the weather below. They’re great for imaging hurricanes and other large-scale weather systems, and have excellent image resolution. But because these satellites have limited coverage, and never stay over any one place for long, they’re not very good for preflight planning. But low-orbit imagery can certainly be dramatic! The resolution can be so good that you can see contrails, smokestack emissions, forest fires—not to mention the smallest of rivers and lakes. The sounders on some polar-orbiting satellites have even been tailored to identify various types of vegetation.

Geostationary satellites orbit at altitudes of approximately 22,300 statute miles—so high that they rotate at the same speed as the Earth. This way, there is a constant view of the terrain and weather below. We’re served by two geostationary satellites—GOES-11 and GOES-12, also called GOES-East and GOES-West, respectively. That’s because GOES-East is parked at longitude 75 degrees West—over the eastern United States, and GOES-West is at longitude 135 degrees West, just off the U.S. West Coast. (GOES stands for Geostationary Operational Environmental Satellite.) With this arrangement nearly the entire hemisphere is covered.

When you call up satellite imagery from a weather briefing Web site, chances are that you’ll see GOES imagery. The GOES satellites, however, generate three different types of imagery, and it’s important to understand the strengths and weaknesses of each.

imagery is fairly straightforward. This is visible light reflected back to the satellites’ sensors. The greater the reflectivity of the objects below, the brighter it will appear on visible imagery. That’s why towering cumulus and cumulonimbus clouds show up brightest, along with snow-covered terrain.

Less dense clouds of lower reflectivity appear in shades of gray. So visible imagery can distinguish between thick and thin clouds. But therein lies a problem: Are we looking at high stratus clouds, or low, thin stratus clouds, or low-lying fog? The answer is that we often can’t tell. If there’s snow on the ground and you can make out terrain features, then you can pretty much rule out fog. Another drawback is that visible imagery only shows the tops of cloud layers; we have no idea if, or where, any lower clouds are present beneath them. They’re blocked by the higher clouds. Another drawback of visible imagery is that it isn’t available at night. No light, no reflectivity.

• Infrared imagery shows the amount of infrared radiation absorbed and emitted by clouds. The satellite detects this energy and assigns a shading and a temperature value. Cold, high cloud tops are depicted in enhanced shading (the color of the shading often varies) and lower, warmer clouds appear in gray shades.

  The problem with infrared imagery is that, as heat-seekers, they have a “leakage” problem. For example, the heat from warm terrain can penetrate high, thin clouds, tricking the satellite into thinking that the cloud layer is lower than it really is. And cold terrain can compete with colder cloud layers to give a false impression of cloud masses vaster than they really are. The satellite can “think” cold ground is a huge cloud mass! This is particularly true at night—the time when you’d most rely on infrared imagery—when it’s almost impossible to tell the difference between fog, low-level clouds, and the surface (because their temperatures can be so similar).

  A new fog-detecting feature is being used with infrared imagery at night, but at this point it remains a tool for meteorologists. This uses the near-infrared wavelengths between 3.7 and 11 micrometers to assign a whitish signature to ground fog.

• Water vapor imagery picks up on the radiation emitted by concentrations of water vapor in the middle levels of the troposphere—about 15,000 to 23,000 feet msl. That’s because this is the level where significant amounts of water vapor begin to drop off (most water vapor is closer to the surface). If higher-level moisture is stationed above lower moisture, its radiation will blot out any low-level water vapor concentrations.

  Concentrations of water vapor are given white and gray shades, while drier air appears in black. Again, different vendors may assign different colors to identify drier air—red is commonly used. It’s important to remember that bright whites on water vapor imagery do not always indicate clouds. There can be concentrations of water vapor without clouds being present.

  Similarly, black areas do not indicate zero water vapor, just lesser amounts. With this in mind, remember that low clouds may be present on infrared imagery, yet at the same location water vapor imagery can be solid black. Why? The radiation from the low clouds was absorbed by whatever (low) level of water vapor was present in the mid-troposphere.

Bottom line: Low clouds don’t show up on water vapor imagery.

Still, water vapor imagery is great for getting a general idea of the wind patterns aloft, locating the plumes of moist air ahead of cold fronts, and seeing the drier air that occurs behind them.

View all three of these satellite products, and you should have a pretty good idea of the moisture situation you face on your next flight.

As you’ve seen, no one image is a perfect rendition of the atmosphere’s vertical profile. But together with METARs, TAFs, radar imagery, winds aloft data, and other briefing products, satellite imagery can add immensely to the texture of your preflight picture.

E-mail the author at [email protected].