Over the next 10 years, the Federal Aviation Administration will spend billions of dollars on new weather radar systems, sensors to detect weather conditions and computers to process the information. If it all works, it will revolutionize aviation weather forecasting, and in the bargain, people on the ground will know more about the weather sooner.

Every time an airliner lifts off the runway, weather is a potential enemy. Thunderstorms and their treacherous wind shears can doom the largest airliner. In the winter, ice on the wings and fuselage can lead to such disasters as the Air Florida crash here in 1982. And, from nowhere, clear air turbulence can turn a smooth flight into a roller-coaster ride.

Even at their most benign, rain and snow and fog cause delays, costing airlines millions of dollars a year and robbing passengers of millions of hours. Roughly 70 percent of airline delays are weather-related, and the problem is growing worse as the airways grow thicker with planes and major airports approach capacity even in good weather. The aviation system is so tightly interlinked that bad weather in one major airport can cause a chain reaction of delays throughout the country.

"The uncertainty of weather looms larger and larger as a factor," said Martin T. Pozesky, the associate administrator in charge of the FAA's project to upgrade the national airspace system.

Airlines could save millions of dollars in fuel alone each year if weather forecasters could just tell them precisely what weather will face each aircraft and each airport, not as far in the future as, say, tomorrow but just in the next few hours.

The problem is that no matter how sophisticated short-term weather forecasting has become, it is still not good enough, for example, to tell a plane leaving Denver that it can land for sure in Washington five hours later or that a thunderstorm will form in its path in what is now clear air over Illinois.

"Existing forecast products are so general in their geographical coverage as to make them useless," said Robert Massey, chairman of the Air Line Pilots Association's Aviation Weather Committee in testimony to the House aviation subcommittee.

John McCarthy, director of the research applications program at the National Center for Atmospheric Research (NCAR) in Boulder, Colo., likened current weather forecasting to a stone dropped into a pond. Only after meteorologists see the stone hit the water can they predict that ripples are coming. "The future system will say we're expecting a stone to fall in an hour," McCarthy said.

Most of the weather projects, part of a larger FAA plan to upgrade the National Airspace System, have been in the planning and experimental stages for more than a decade. The first systems are now being deployed.

Initially, the emphasis of the FAA weather program, most of it being developed in concert with the National Weather Service and the Defense Department, is on doing better what the aviation weather system already does: observe. But down the line, as more new equipment is plugged into high-capacity computers, meteorologists talk of such unheard-of short-term forecasts as accurately predicting the location of thunderstorms and tornados an hour or more before they form.

The FAA's Pozesky said the $4 billion-plus aviation weather enhancement program has three goals: improve weather observation, improve forecasts and improve methods of getting the information to pilots.

The ultimate purpose of the projects is to produce "nowcasts" -- extremely accurate localized one-hour forecasts of violent storms and other items of intense interest to pilots: icing conditions, heavy snow, wind shear, turbulence, winds aloft and fog. Beyond nowcasts, meteorologists want to produce accurate airport-to-airport forecasts of up to six hours, the maximum time it takes a jet airliner to cross the country.

Pozesky said he is likely to recommend approval of a powerful new computer project recommended by McCarthy and other meteorologists, called the Aviation Weather Products Generator. As envisioned by these meteorologists, the project would provide high-speed computing capacity to allow aviation to cross the line from weather observation to accurate short-term forecasting.

The first step, however, is to deploy new weather-observing equipment. After more than a decade of planning and development, the first elements of the new generation of equipment are just going into the field.

A new terminal doppler radar has been deployed at Denver's Stapleton Airport to detect wind shear -- sudden wind shifts blamed for more than 650 deaths in the last 25 years. McCarthy said the radar has already saved at least one aircraft and possibly others.

Lincoln Laboratories of Boston also installed a doppler radar at the Orlando airport this summer to test its effectiveness in a climate far more humid than Denver's. James E. Evans of Lincoln Labs said the radar was disabled for three weeks by a particularly violent thunderstorm during which 3,000 lightning bolts hit the ground in an hour, one of which "ended up frying some computer chips."

Nonetheless, the experiment operated long enough to record numerous thunderstorm-spawned microbursts, a particularly treacherous form of wind shear that begins as a rapidly falling column of cold air that fans out in all directions as it hits the ground. Evans said the experiment drew thanks from a pilot who said he might have flown into one especially violent microburst had he not been warned.

A doppler radar essentially sees air movement. When a microburst hits the ground and begins growing near a runway, a red circle appears on the radar screen and grows larger. In conjunction with other sensors at the airport, the air traffic controller can immediately warn a pilot of the problem and say how great an airspeed loss the plane will suffer if it flies through the microburst.

The FAA has contracted for 47 terminal doppler weather radar systems to be installed between 1992 and 1995 at airports in the eastern two-thirds of the country, with an additional 55 to be deployed later.

A new air traffic control radar, the ASR-9, is also being deployed with a weather channel that will allow more accurate weather observations in a wider area around airports.

Another type of doppler radar called NEXRAD -- for NEXt-generation RADar -- has been tested and is scheduled to be deployed at 115 locations, beginning in the storm-prone Midwest. This Weather Service radar, which will cover a wider area, is designed to pinpoint tornados and severe thunderstorms far more accurately than is possible now, with fewer false alarms. For instance, current weather radar may not detect wind movement as small as a tornado, relying instead on observations of larger masses of air that indicate only that a tornado may be present.

Each NEXRAD will be the centerpiece of 115 new Weather Service Warning and Forecast Offices. Fifty-two current forecast offices and 197 smaller weather offices are to be closed when NEXRAD becomes operational. A new Automated Surface Observing System (ASOS) is to be placed at 160 to 320 locations to automatically report temperature, winds, ceiling, precipitation and other local weather conditions, a function now performed by humans.

The FAA plans to expand this automated observing system with 750 or more Automated Weather Observing System (AWOS) installations, more sophisticated than ASOS in that they can differentiate between types of precipitation. ASOS is now being delivered and one is operating in Alaska, while AWOS is in the development stage.

However, the National Weather Service Employees Organization, a union of meteorologists, has charged NEXRAD has performed poorly in tests in Oklahoma City, and that test results on ASOS have also been discouraging. Until the systems have been fully tested and approved, the Weather Service should not cut back on stations staffed by people, the organization said.

A mass of new weather information from the upper atmosphere is expected to flow from two other programs being developed: a new upward-pointing doppler radar to measure winds aloft, and a new aircraft communications system that will continuously report wind conditions from the thousands of airliners that cross the country every day.