High-Flying Science, with Strings Attached

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.

Meteorologist Victoria Auld attaches an instrument package to a kite tether. C.J. Gilbert/British Antarctic Survey

To monitor atmospheric chemistry, University of Colorado researchers loft a kite behind a boat on the Peruvian Amazon. University of Colorado

By controlling the angle of the wing on this tram developed at the University of Colorado, researchers can send sensors up and down the kite’s tether without raising and lowering the kite. University of Colorado

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.

Complicated phenomena

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.

Plenty of chemistry

Beyond turbulence, mixing, and other physical phenomena in the atmosphere, Balsley says there’s plenty of atmospheric chemistry that kites can help unravel. He’s collaborated on various projects with John Birks, a chemist also at the University of Colorado at Boulder, and others to monitor atmospheric chemistry from Greenland to Peru.

Technology is making the job ever easier. In the early 1990s, Birks and his colleagues sent up flasks to collect air, which was later analyzed in a lab. Now, sophisticated electronic instruments can directly monitor ozone, carbon dioxide, water vapor, and other chemicals in the atmosphere and transmit the data to researchers on the ground.

Also, the scientists can now use a motorized tram that rides up and down the kite’s tether to carry the instruments to specific altitudes, instead of winching the kite in and out to chase moving layers of turbulence or wind shear.

In August 1993, for example, Balsley, Birks, and their colleagues used kites to examine ozone levels over Cape Sable Island, which is near the southern tip of Nova Scotia. Sensors showed that ozone appeared at ground level in concentrations of 20 to 40 parts per billion. At altitudes between 300 and 500 m, however, the scientists measured similar concentrations during one test but 90 to 130 ppb 2 days later.

By checking weather patterns, Birks and his group showed that the air that passed over the site during the earlier period had originated over southeastern Canada. The scientists were able to trace the later air, with more ozone, back to heavily industrialized areas of the northeastern United States.

In another project, this one in the Peruvian Amazon in 1995, Birks and others took measurements revealing low concentrations of ozone over sections of virgin rain forest (SN: 8/10/96, p. 93). They also monitored the amount of carbon dioxide in the air. They’re now conducting similar studies over the forests of northern Wisconsin and the Great Plains of Kansas and Oklahoma.

Bat frenzies

Kites provide surveillance of not only airborne chemicals but also the behavior of aerial creatures, such as the feeding frenzies of Mexican free-tailed bats. These bats, which migrate to their breeding caves in Texas each spring, serve as a brown, fuzzy 100-million-member Border Patrol that consumes about 2 million pounds of insects in a single night.

In the summer of 1997, researchers launched parafoil kites into the nighttime sky. Radiomicrophones were suspended beneath the kites at three different locations, each separated by 300 meters. This enabled the scientists to simultaneously eavesdrop at different altitudes.

Previous research with microphones suspended from free-floating balloons had revealed bats foraging at altitudes up to 750 meters, and ground-based radar had shown some bats and insects flying at heights approaching 3,000 m.

“Most people don’t appreciate that insects fly that high or that bats feed on them there,” says Gary F. McCracken, a professor of ecology at the University of Tennessee at Knoxville.

“The agricultural implications are pretty amazing,” he adds. Beginning in early June each year, billions of corn earworms, tobacco budworms, and cabbage loopers catch prevailing winds that assist their migration from the lower Rio Grande Valley into south central Texas. There they and their progeny cause more than $1 billion in crop losses each year.

McCracken has collected several hundred hours of bat calls, at altitudes up to 1,200 m, as the animals navigate, search for insects, and home in their meals during so-called feeding buzzes.

McCracken describes the kite-borne microphones as “spectacularly successful in monitoring the level of bat activity.” The bats were usually much more active in predawn hours than they were late in the evening, he says. The microphones often detected hundreds of calls and dozens of feeding buzzes per minute at altitudes of 400 to 600 m.

Recordings also revealed that the high-altitude calls of the bats were, in McCracken’s words, “interestingly different” from calls recorded at lower altitudes. The navigational chirps were frequently more than 20 milliseconds in length and were typically spaced more than 350 ms apart. This interval is longer than that usually measured at low altitudes. McCracken says that the high-flying bats may change their calls because they don’t have to avoid trees and other obstacles they encounter near the ground.

In a ground-based, companion research project, the researchers are also analyzing DNA from bat guano to identify just what insects the bats are eating.

Looking downward

In addition to kites’ value for getting data about the atmosphere and its inhabitants, they also serve as platforms for looking downward. In the late 19th and early 20th centuries, kite-based aerial photography was commonly used for such purposes as military reconnaissance and disaster assessment.

Researchers gradually abandoned the kite in favor of airplanes, but they have recently rediscovered it as a versatile, low-cost method of acquiring images.

Such photos can be either a supplement to satellite imagery or a source of low-altitude, high-resolution mapping data.

For example, Kansas scientists recently used photos from kite borne cameras to show that interpreters of Landsat satellite images probably underestimated the amount of leaf coverage in the forests near Fort Leavenworth.

Pictures taken from kites showed that the trees completely covered the ground when viewed from directly above, says Donald A. Distler, a biologist at Wichita State University. These images revealed that the taller trees were casting shadows on shorter ones. That lowered the amount of radiation reflected up to the imaging satellite, leading to the underestimate of foliage. This shows that corroboration of satellite data, using kites or other means, ought to be routine, Distler says.

He and his colleagues also have used kites to help map the intricate tapestry of erosion channels and potholes in a group of streams at a natural history reservation in south central Kansas. After scanning into computers more than 40 overlapping photos taken from kites, the researchers assembled a detailed map of the channel system.

Distler says that previous attempts to map this area from the ground or from conventional aerial photos had been unsuccessful because the streams’ geometry is so complex and thick, brushy vegetation covers the entire area.

Indispensable sources

Kites can be indispensable sources of high-resolution aerial photos for all sorts of investigations, says James S. Aber, professor of earth science at Emporia State University in Emporia, Kan. Such images can be used to map archeological sites or to inexpensively monitor areas where erosion, sedimentation, deforestation, or construction are rapidly changing the landscape, he notes.

For instance, Aber has traveled to Poland, where there are no high-quality aerial photographs of many parts of the country. Where photos are available, they’re typically more than 10 years old.

With a Polish colleague, Aber traveled the countryside to demonstrate that with a minimal investment—about $1,000 for a kite and camera gear and about $100 in travel costs—a two-person team could photograph a variety of sites within a few days. The resulting bargain imagery, which covers small areas in great detail, ought to complement higher altitude, more expensive satellite imagery that typically covers more area in less detail, Aber says.

The ascent of kites as data-gathering tools continues as innovative scientists find new ways that the airborne platforms can help them acquire information. As more researchers discover the advantages of using these toys with a high-tech edge, they gladly become members of a select group who smile when their colleagues tell them to go fly a kite.

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