By TIM GNATEK
Published: December 30, 2004

GREG TRUTA works long hours as an accountant, but that hasn't stopped him from developing a sideline as the neighborhood weatherman in Broomfield, Colo., a suburb of Denver.

Mr. Truta, 33, was so fascinated by the weather moving off of the Front Range of the Rocky Mountains that he mounted a personal weather station in his backyard to monitor it. Soon he decided to share his data by posting it online. He connected his instruments to a computer and began uploading live readings, along with National Weather Service forecasts and even a Webcam view of the mountains to his Web site, www.broomfieldweather.com.

His site has become a resource for those in his area and beyond, logging more than 98,000 page views in the last 18 months. ''I've gotten e-mails from all over Colorado, even on the East Coast,'' he said. For those hooked on Broomfield weather, Mr. Truta offers an e-mail system that sends out weather updates three times a day.

His audience includes professional meteorologists, like those at Denver's ABC affiliate, KMGH, who have used his station data in their forecasts. ''They told me they needed a temperature for northwest Denver, and were using my temperatures on the 5 o'clock news,'' he said.

Mr. Truta is not alone. Thousands of armchair sky watchers are pairing computers and consumer-grade meteorological equipment to share their observations of local conditions online. Posted on personal Web sites or community weather pages, the data is helping neighbors and beginning to have a larger impact on meteorology, by shaping a more detailed view of weather patterns than was previously available.

''We use those reports,'' said Mike Nelson, KMGH's chief meteorologist. ''It's been useful for television to get more reports from all kinds of locales, compared to just the airport. The old joke goes, no one lives out there.''

Even without building Web sites, backyard meteorologists can contribute to the professional weather world. They can send their data to a number of organizations that aggregate the information and post it online.

One such group that has achieved official recognition is the Citizen Weather Observer Program (www.cwop.net), an association of weather watchers who collect information, share it online, and forward it to outlets like the National Oceanic and Atmospheric Administration's giant pool of freely available weather research data, the Meteorological Assimilation Data Ingest System. Information from 1,900 of the 4,000 personal weather stations registered with the observer program makes its way to the National Weather Service forecast offices, the Kennedy Space Center, the National Center for Atmospheric Research and university research programs, among other destinations.

''The data goes to 80 percent of the weather forecast offices in the U.S.,'' said Russ Chadwick, the senior engineer for the National Oceanic and Atmospheric Administration's forecast systems laboratory in Boulder, Colo., and volunteer administrator of the citizen observer program's network.

''It's helpful in places where there are fewer official data points, and where we have quite a number of stations,'' he said. ''The conventional weather network might have 70 kilometers between observations, and sometimes we have stations in those areas that are just 10 kilometers apart.''

James Scarlett is a weather professional who takes advantage of the observer program's network. As the warning coordination meteorologist for the National Weather Service in Billings, Mont., Mr. Scarlett was in a position to recognize the shortage of official observations from the southeastern part of the state. He mobilized amateur radio operators to collect weather data around Forsyth, a rural town where there were no official weather stations.

Now, information from nearly a dozen personal weather stations channels into findu.com, an affiliate of the observer program for amateur band broadcasters, and Mr. Scarlett can analyze the data before issuing weather alerts and wildfire warnings for the northern Rockies.

''We've always struggled to get information from out there, and it's been great,'' Mr. Scarlett said. ''The more data we get from the field, the better our forecasting.''

A number of other Web sites collect personal weather station information and offer it online for use by amateur and professional meteorologists.

Anythingweather.com and weatherforyou.com members share personal weather data, which is displayed alongside official reports in a series of interactive state maps.

They will be joined next year by WeatherBug, an advertising-supported software program that, its publishers say, is installed on over seven million desktops. WeatherBug plans to augment data collected from its network of 7,000 school-based weather stations with personal weather data in early 2005. Seventy-seven TV stations use WeatherBug data on neighborhood conditions in their local weather reporting.

The neighborhood reports have helped meteorologists like Justin Derk, of WMAR-TV in Baltimore, zero in on difficult forecasts like predicting the often vague rain and snow lines during winter storms.

''I was able to pick up four stations and see a temperature difference between the north and south side, and draw that line,'' he said. ''For the people watching us that day, knowing what street they live on, we were nailing precisely who was getting rain and who was getting snow.''

Perhaps the most popular site for budding meteorologists is Weather Underground (www.wunderground.com). For nearly three years, the site has allowed weather watchers to add their stations to a growing regional list. Weather Underground reports a worldwide membership of 12,092 personal weather stations; more than 3,200 provide daily uploads to the site.

For some hobbyists, monitoring the weather means more than merely predicting fair skies or foul.

One volunteer in the observer program, David Helms, a National Weather Service scientist in Silver Spring, Md., who also operates a personal weather station, became interested in the hobby for national security reasons.

''I joined C.W.O.P. soon after Sept. 11, 2001,'' he said. ''There were lots of reports of terrorists wanting to use crop-dusters to spread chemical and biological agents, so I started researching how one could contribute weather data that might help with urban dispersion forecasts.''

Regardless of how it is used, the personal weather data displayed on these sites provides a perceived element of control over something technology has yet to master. Despite meteorologists' efforts, there is only so much they can do about the weather.

''We have the couch-potato philosophy on storm chasing,'' Mr. Helms said. ''You don't chase it. It chases you.''

At least amateur analysts have the satisfaction of knowing that their on-demand information is often more up to date and more adaptable than what is available through traditional news sources.

''Why let the TV man control the graphics?'' Mr. Helms said. ''You can control the data right from your couch.''

Where to Start
Knowing More Than Which Way the Wind Blows

STARTING a weather station for yourself or for an online group is a matter of finding the right instruments, the right computer software and a site where you can post your data.

Personal weather instruments are available for a wide range of budgets. These are among them:

VANTAGE PRO2 from Davis Instruments ($595 at davisnet.com, but available for less elsewhere online) includes a full range of sensors and connects wirelessly with its indoor console reader.

WMR968 from Oregon Scientific ($500 at www2.oregonscientific.com, but available for less elsewhere online), another wireless weather station, includes a thermo-hygrometer, rain gauge and anemometer.

ULTIMETER 100, the basic model from Peet Bros., ($200, peetbros.com), takes most common weather measurements and adds an adjustable flash-flood alert. The station also includes three serial-port modes to send weather data by phone, modem or radio link.

Add-on software packages like these coordinate the transfer of data from instrument to computer, provide graphics that lend an analog feel to the digital information, use graphs to chart weather trends and ease uploading to a personal Web site or a community weather service:

VIRTUAL WEATHER STATION by Ambient Weather (ambientweather.com) is a popular weather program that converts data into on-screen bar graphs and meters. It is compatible with many weather instruments. The Internet Edition allows information to be uplinked to Web sites and costs $105.

WEATHER DISPLAY ($70, weather-display.com) supports many manufacturers' weather devices and offers numerous real-time, scalable and historical graphs.

WEATHER VIEW 32 (weatherview32.com) adds Internet displays like satellite images to personal weather station data. It can send weather alerts to e-mail addresses and pagers. The home edition costs $50; a professional, broadcast-licensed version with slide-show ability costs $1,000.

Weather watchers who want to share their findings can post data and images to these community weather services:

WEATHER UNDERGROUND, which is at www.wunderground.com, lists more than 12,000 personal stations in its collection.

ANYTHINGWEATHER.COM allows contributors to post the data they collect free to pages the service is host to, using a simple template that comes with its basic software package.

CITIZEN WEATHER OBSERVER PROGRAM (www.cwop.net) is better known for aggregating contributed data and providing it to government meteorologists rather than as a destination itself. The association verifies data before sending it to weather services, augmenting official readings in outlying areas.

WEATHERMATRIX members trade information through e-mail messaging and postings at a Web site (weathermatrix.net) that also provides a free desktop program that uses contributed data to provide weather alerts, radar images and current conditions. Tim Gnatek