Water, water, way up high.

The middle layer of the atmosphere--a remote region far above Earth's surface--has grown significantly soggier since 1992, according to measurements made by satellite and ground-based instruments.

Atmospheric researchers say they were surprised to find water vapor concentrations climbing by 2 to 3 percent per year at altitudes of 40 to 60 kilometers--the upper stratosphere and lower mesosphere. Gerald E. Nedoluha of the Naval Research Laboratory in Washington, D.C., and his colleagues report their discovery in the Feb. 20 Journal of Geophysical Research. The water vapor readings come from spectrometers located in California and New Zealand and from a sensor on board the Upper Atmosphere Research Satellite.

The stratosphere and upper reaches of the atmosphere are far drier than the lowest layer, the troposphere, home of Earth's weather systems. Water vapor from the troposphere cannot easily leak upward into the stratosphere because the boundary between the two regions, known as the tropopause, is extremely cold. This frigid barrier causes water vapor to condense and fall back into the troposphere.

Researchers think the additional water vapor comes from several sources. A small fraction results when methane is converted into water vapor in the stratosphere, says Nedoluha. Methane concentrations have been climbing in recent years because of pollution.

Another, less certain source of water vapor is the 1991 eruption of Mount Pinatubo. Volcanic gases lofted into the stratosphere helped warm that region, causing it to rise faster than normal. Since the eruption, the rising motion has slowed somewhat, thereby allowing a larger share of the available methane to be converted into water vapor. That change, coupled with increasing methane pollution, could account for about half of the extra water vapor, he says.

To explain the rest, the researchers suggest that the tropopause over the tropics may have warmed, allowing more water vapor to sneak from the lower atmosphere into the stratosphere. Now they are investigating what could have caused this warming. Pinatubo appears a prime candidate, Nedoluha says.