A sound way to sense the sky.

With massive loudspeakers aimed skyward, engineers are testing a new technique for taking the atmosphere's temperature - an approach that could eventually help meteorologists improve weather forecasts. The sound-based system provides temperature readings every six minutes and can fill gaps between the twice-daily measurements made by weather balloons, according to the National Oceanic and Atmospheric Administration (NOAA), which is developing the system.

The technique relies on a combination of loudspeakers and special Doppler radars that gauge the speed of winds in the atmosphere. By bouncing the radar off ascending acoustic waves, the system can judge the speed of the rising sound up to 1.5 miles in altitude. Because sound waves move faster through warmer air, the sound's speed provides a measure of atmospheric temperature.

The system does have some drawbacks. To get the most radar reflection possible, the speakers must emit a frequency that falls in the middle of the range of human hearing. This siren-like sound can be heard for several miles-a factor that precludes the use of this technique in populated areas, says

Daniel Law, a NOAA engineer in Boulder, Colo.

NOAA engineers have installed the acoustic system at five of the 31 sites that house wind-profiler radars. Currently, they are running only two of the sound thermometers full-time; the others sit too close to farms to permit nighttime use. The team is evaluating the temperature data collected by the acoustic systems, but meteorologists have not yet had a chance to incorporate the information into their forecasts.