Scientists are analyzing weather-satellite pictures to keep an eye on 100 dangerous, remote volcanoes along the Pacific Rim in Alaska and Russia. What they are looking for is excess heat that indicates a volcano is likely to erupt.

The method allows scientists to observe volcanoes when it is too expensive to install earthquake sensors to listen for signs of imminent eruptions.

Only 27 volcanoes in Alaska and Russian Kamchatka are monitored seismically because reaching these remote locations is difficult and costly -- long periods of winter darkness render solar-powered monitoring stations useless and the extreme cold makes batteries inefficient. So for the past few years, scientists routinely have examined infrared images taken by weather satellites.

"Using heat sensors on satellites, we can see changes at volcanoes days to months ahead of eruptions," said volcanologist Jonathan Dehn of the Alaska Volcano Observatorys Fairbanks office. "It allows us to monitor volcanoes we normally cannot due to their remote location."

The observatorys use of weather satellites to detect increased heat emissions before and during volcanic eruptions was outlined in the August issue of the journal Geology. The study was conducted by Dehn, satellite imagery expert Ken Dean and graduate student/technician Kevin Engle.

"These volcanoes pose a serious threat not only to local residents, but to commercial and cargo air traffic throughout the Arctic and sub-Arctic region about 25,000 passengers and more than $1 billion in cargo and equipment daily," they said.

Radiant heat emissions from volcanoes are monitored using the infrared-light imaging devices, called radiometers, aboard polar-orbiting NOAA 12, NOAA 14 and NOAA 15 weather satellites. Dehn said the devices were not designed for volcano monitoring, but rather for measuring cloud temperatures and altitudes, along with distinguishing clouds from the ocean.

He said a computer automatically scans incoming infrared images and, if it finds unusual heat emissions, "it sends an e-mail [to scientists] and says, Yo, dudes! Somethings up! "

If an eruption is impending, the USGS alerts airlines, air-traffic controllers, government and emergency officials, the National Weather Service and others.

"Five of the last seven eruptions weve had in the last three or four years were preceded by thermal anomalies" in which satellites detected increased heat, Dehn said.

In 1998, no excess heat was detected before eruptions of Sheveluch volcano in Kamchatka and Korovin in the Aleutians. Dehn said likely reasons were that a satellite was too far away to look down into Sheveluchs crater and bad weather obscured Korovin.

But Dehn said the satellite detection of unexpected heat from Russias Bezymianny volcano on December 4, 1997, prompted a successful USGS warning to the Russians. The volcano erupted December 6, sending ash 30,000 to 35,000 feet (9,150 to 10,700 meters) over the Pacific.

"We sent them e-mail and got them on the horn," Dehn said. "Flights were diverted."

Small earthquakes became more frequent in mid 1998 at Shishaldin volcano on Alaskas Unimak Island, and seismic activity accelerated that December. On February 9, 1999, satellite pictures began showing increased heat "a single pixel that was bright exactly at the summit of the volcano," Dehn said.

At first, the heat was detected only when the satellite was directly overhead, but later the satellite was able to detect hot material from greater distances, indicating "the hot source was rising in the central crater," Dehn said.

An April 1999 eruption sent ash 45,000 feet (13,700 meters) skyward, after possible small explosions in March. The detection of increased heat in early February "is the longest thermal warning weve seen from a volcano on the planet," Dehn said.

He said routine monitoring of heat from volcanoes was prompted by discovery of excess heat before the 1996 eruption of Alaskas Pavlof volcano. Satellites also detected increased heat emissions before eruptions of Alaskas Okmok in 1997, Russias Klyuchevskoy in 1994 and Karymsky from 1996 onward, although some other increases in heat were not followed by eruptions.

The heat-sensing satellites not only help predict eruptions; they monitor eruptions in progress. Scientists use the pictures to track volcanic ash plumes and recognize lava flows and lava domes.

After a 1992 eruption of Alaskas Mount Spurr, the satellites "were able to track the ash plume all the way through the lower 48 states and up into Greenland," Dehn said. "We had to reroute quite a few flights all through the country."

Volcanologists hope to use images from advanced infrared instruments on new U.S. and Japanese satellites to monitor volcanoes more often, he said. They also are developing a proposal for a joint USGS-NASA volcano-monitoring satellite named VOLCAM.