High-Flying Science, with Strings Attached - research with kites

Science News,  Sept 16, 2000  by Sid Perkins

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In the hands of scientists, a toy does serious data gathering

Before airplanes and hot-air balloons came along, early meteorological research was like child's play on a windy day: Scientists flew kites. In the mid-18th century, researchers studied the atmosphere by attaching well-padded thermometers to a kite's tether and sending them aloft. A slow-burning fuse later released the instruments, which plummeted to Earth. The scientists' big challenge was to quickly find the thermometers that survived the fall before they warmed up.

Conceptually, research with kites today remains unchanged--just hang some sort of sensor from a kite and send it up. However, modern designs and materials, sophisticated electronic instruments, and inexpensive, lightweight cameras transform a simple toy into a data-gathering system with broad applications.

Kites are light, portable, and easy to launch, and their only power requirement is a steady wind. They can soar at altitudes too high or in winds too strong for tethered balloons, and they can be flown at levels so low that aircraft would be impracticable or unsafe. They also can be sent up quickly with little support equipment, so they can be used almost anywhere--from desert basins to Antarctic ice, from Peruvian rain forests to U.S. corn fields.

Besides meteorologists, many other scientists have been employing these most simple aircraft for answering an ever more diverse range of questions. Atmospheric chemists, entomologists, agricultural scientists, geologists, and others now rival children in their love of kites.

To explain complicated atmospheric phenomena such as turbulence, scientists need to gather data at specific altitudes for extended periods. So, when researchers swept into the flatlands near Wichita, Kans., last October to take part in the Cooperative Atmosphere-Surface Exchange Study, some brought along rectangular parafoil kites.

While other researchers mounted instruments on towers to monitor conditions 10 meters and 60 m off the ground, Ben B. Balsley and his colleagues from the University of Colorado at Boulder used their kites to loft sensors several hundred meters to measure turbulence there. The researchers worked at night to avoid the extra turbulence caused by solar heating. Scientists presented some findings of the month-long project in August at the American Meteorological Society's 14th Symposium on Boundary Layer and Turbulence in Aspen, Colo.

Balsley's team measured temperature, wind speed and direction, and humidity at five closely spaced altitudes. They harvested data every 10 seconds during flights that lasted up to 16 hours. The researchers didn't have to rely on luck for their kites to end up in atmosphere layers of interest, such as ones with turbulence or wind shear, Balsley says. By monitoring the data as they were collected, the researchers could steer their kites directly to those heights.

Sensors suspended below the kites detected slow, regular cycles of up-and-down motion in the air about 400 m off the ground, which confirmed measurements made by ground-based laser instruments. Each cycle of motion lasted about 4 minutes. During a cycle, the boundary between a lower layer of cooler, denser air and the warmer air above it would slowly rise and fall like a swell on the ocean.

In less stable conditions, Balsley says, these waves in the atmosphere can become large enough to break, just like surf. This mixing helps transfer energy and materials--water vapor, other chemicals, and particulates, such as dust, smoke, and soot--between the two layers of air.

More than half a world away from Kansas, scientists with the British Antarctic Survey have conducted similar studies of the nighttime atmosphere. There's one big difference. At Britain's Halley Station on the Brunt Ice Shelf, the sun sets on April 30 and doesn't rise again until mid-August.

Victoria Auld, a meteorologist who spent winters at Halley Station from 1996 through 1999, studied the area's boundary between cold and warm air layers. Auld says this boundary remains stable during the dark winter months because there are no hot spots caused by solar heating. To examine the layer, she suspended sensors about 10 m beneath a rokakku kite, a hexagonal Japanese design that flies steadily even in light winds.

Some of her 60 kite flights over the 4 years rose to altitudes above 550 m, although Auld usually aimed the kite for much lower altitudes where a sodar, a sonic version of radar, had detected layers of turbulence and wind shear. In flights that typically lasted an hour or so, sensors measured altitude, temperature, and wind speed and direction.

These data, currently being analyzed by the Antarctic survey researchers, will help Auld and others understand how turbulence affects the amount of heat transferred across the boundary layer. This, in turn, may provide scientists with a better understanding of variations in the Antarctic climate and indicate how those changes might link to much larger climate patterns.