When Hurricane Sally was approaching in September, weather observation websites were in heavy demand. The various images shown there serve a variety of different purposes.
GOES (Geostationary Operational Environmental Satellite) images come to us from a pair of satellites high above the equator. Unlike satellites that supply, say, Google Earth images, they don’t orbit around the globe. Their location over the equator is fixed at an altitude of 22,300 miles and they duplicate the earth’s rotation in order to remain there. For this reason they’re called geostationary. The U.S. Oceanic and Atmospheric Administration (NOAA) operates two GOES units, one over the Eastern and another over the Western states. Images come in “visible” and “infrared” flavor. “Visible” is how the human eye would see clouds from above. That’s normally the least useful mode because it makes the earth’s surface look murky, and just looking at the clouds doesn’t take in the true structure of storms. It’s better to use infrared, which one can imagine as a set of colors that our eyes aren’t equipped to see. There are many different types of infrared, from one that makes a nice, sharp distinction between water, land and clouds to one that pictures heat differences and cloud heights.
Infrared images are great for researchers who analyze the structure of storms and build models of their behavior. But for the rest of us, who want to know “where is it raining right now, and which way is the storm moving?”, ground-based radar is the most useful. All the weather radar stations in the U.S. are networked together, so we can get a current image from anywhere on the continent. There is even a magnificent NOAA web page that shows, in full animation, all of the weather simultaneously in the 48 contiguous states (search for “mosaic radar loop”).
In essence, weather radar is a way of shining a giant flashlight at the sky. Radar “light” is another color that the eye can’t see. Where there aren’t any clouds, nothing reflects, and the radar image produced by the computer remains black. The greater the cloud density, the more intense the reflection. This is how the weather images show the severity of storms. And because modern radar is of the “Doppler” type, it can also animate the image so we can observe storm movement as a video clip.
Rudi Kiefer, Ph.D., is a professor at Brenau University, teaching physical and health sciences on Brenau’s Georgia campuses and in China. His column appears Sundays and at gainesvilletimes.com.