Sterling, Va. Just after dawn in an open field, Calvin Meadows tugs a cotton tether to send aloft a 6-foot helium balloon that will carry weather instruments more than 19 miles into the sky.
About the same time, technicians and scientists across the world do the same.
The practice, repeated twice daily in 92 locations in the United States and hundreds more worldwide, is part of a little-known, six-decade-old effort to predict where storms will strike.
Meadows, a government weather technician, has been releasing balloons for more than a decade. But the operation soon may become obsolete if a $20 million network of high-tech sensors quietly installed on some U.S. passenger planes succeeds.
The experiment promises to improve forecasts dramatically, warn pilots flying into dangerous ice storms or turbulence and even track hijacked flights.
The new sensors measure weather conditions every few seconds while planes are flying. In early tests with NASA and the National Weather Service, forecasters detected tornadic conditions earlier and predicted heavy storms, temperatures and fog more accurately.
"We've already seen significant improvements in forecasting," said Rich Mamrosh, a government meteorologist in Green Bay, Wis., who has used data from the new system to predict storms around the Great Lakes. The sensor network "can benefit pretty much all segments of society," he said.
The Bush administration has not decided whether to pay for the complete system, called TAMDAR, for "Tropospheric Airborne Meteorological Data Reporting."
Its future depends on an ambitious sales pitch to government and businesses by AirDat LLC, a small company in Raleigh, N.C.
The lunchbox-sized weather sensors that protrude near a plane's nose measure airborne moisture and other conditions.
Supporters say they will do more than just help picnickers predict when to fold their blankets and head for shelter.
An inaccurate forecast of pea-soup fog can shut down air travel throughout a region.
A flubbed temperature forecast can cause power outages because utilities anticipate electricity needs based on expected highs.
For instance, blackouts rolled across much of Texas for two hours in April after forecasters predicted highs near 90 - far cooler than the actual 98 degrees - leaving hundreds of thousands of customers in the dark and without air conditioners.
The new sensors also could prevent airplane crashes by detecting deadly ice buildups on wings - especially for regional commuter planes that fly at lower altitudes.
U.S. investigators blame a thin buildup of ice along the wing of Comair Flight 3272 for its crash near Detroit almost a decade ago that killed all 29 people aboard.
Investigators believe a nearly imperceptible ice buildup on the turboprop's wings caused it to fly too slowly, and the plane rolled left upside down almost immediately after the crew suspected trouble. It crashed in just 20 seconds.
"We have to give pilots better weather information to make better decisions," said Taumi Daniels, who led research at NASA on the new weather sensors.
How do the sensors work?
AirDat's devices transmit readings for humidity, pressure, temperature, icing, turbulence, location, altitude and time nearly continuously as hundreds of commercial flights zigzag across the United States.
The sensors also report immediately if any ice builds up.
Weather measurements are beamed via satellite, usually in less than 60 seconds, to AirDat's high-tech headquarters in North Carolina, where 64 powerful computers analyze the data to produce updated regional forecasts every hour.
AirDat says U.S. air marshals aboard planes could use the same satellite transmitters on the planes to send and receive messages while flying; they cannot do that now.
In the case of a hijacking, such communication would be valuable.
The National Oceanic and Atmospheric Administration expects within the next 60 days to pay for at least 30 weather sensors on planes in what is likely to be its first round of such purchases.
AirDat and rival ARINC Inc. of Annapolis, Md., are the lead contenders for the contract.