Mountains give rise to perplexing plumes.

Taking advantage of the new world order, U.S. scientists have finally gathered hard evidence to explain the Bennett Island plumes, a mystery that remained unsolvable during the Cold War.

But the long-sought explanation has proved less exciting than experts had hoped. The Bennett Island plumes apparently result from airstreams passing over low mountains on the island, reports Russell C. Schnell, an atmospheric scientists who coordinated the recent aircraft experiment above the island.

Scientists would have liked to find evidence to the contrary. "Deep in my heart of hearts, I still hope they're not orographic [mountain-caused] clouds," says Schnell, director of the National Oceanic and Atmospheric Administration's monitoring station in Hilo, Hawaii.

First spotted on satellite images in 1983, the plumes are extremely thin cloud trails stretching hundreds of kilometers downwind of tiny Bennett Island, located in the East Siberian Sea. Atmospheric scientists put forward several explanations for the ephemeral contrail-like plumes, including the possibility that they resulted from Soviet activity in what was then a militarily sensitive area. At the time, western investigators could not hope to visit Bennett Island to test their ideas. In the absence of direct data, they settled on the hypothesis that the plumes originated from leaking deposits of frozen methane, known to exist beneath the floor of the Arctic Sea (SN: 3/28/87,p.204).

Such methane plumes might have economic implications if they signaled the presence of large natural gas reserves located close to the ocean floor. But the leaks could also have an ominous message, suggesting that global warming had started melting the frozen subsea stores of methane in the Arctic. Because methane is a powerful greenhouse gas, the thawing of such reserves would accelerate the global temperature rise caused by carbon dioxide pollution, scientists warn.

To test the methane hypothesis, Schnell and colleague Anthony D.A. Hansen of the Lawrence Berkeley (Calif.) Laboratory hired a Russian airplane and crew to collect air samples downwind of the island. In late April, the plane flew through one plume and in the vicinity of several others. Anaylsis of the gas samples using extremely sensitive instruments reveals normal amounts of methane, says Schnell. "It doesn't appear that methane leaks at Bennett Island are causing what we see," he says.

Instead, Schnell believes, the clouds form through a more prosaic process as air masses saturated with water vapor pass over the mountains on Bennett Island. As the airstreams rise, they cool, causing water vapor to condense and form clouds that spread out downwind. Scientists have seen similar types of clouds develop downwind of other mountains, including peaks on the Novaya Zemlya islands north of Russia. Using the mountain theory, Schnell successfully forecast the appearance of a plume, lending credence to that hypothesis.

But the Bennett Island plumes have not yielded all their mystery. Meteorologists must now explain why the plumes form at an unusually high altitude, more than 3 kilometers above the mountaintops, says Schnell.