The 430 children, Bob Ryan observed, were crowded into the assembly hall like drops of water condensed into a cloud.

They had been waiting for him to come for weeks, months in fact, since October when the PTA arranged for the Channel 4 weatherman to visit the school. There was a 35- foot banner on the wall, reading "Weather We Like it or Not!" along with a number of smaller signs generated by computer: "Cashell Welcomes Bob Ryan."

Ryan, who had arrived at the Montgomery County elementary school with a film tucked under his arm, a handful of pictures taken from weather satellites and a box of his weather almanacs, surveyed the swelling crowd of little people. "You know it's been a good day," he said, "if one of them asks you how mh you make." And what does he tell them? "I tell them it depends on how often I'm right."

It also depends on how well Channel 4 does in the local Nielsen ratings, and that's been pretty well. Hiring Ryan off the "Today" show five years ago and adding sportscaster George Michael at about the same time (today is his fifth anniversary) coincided with WRC's renewed effort to build a respectable news presentation in the '80s and challenge market giant Channel 9. Anchor Bob McBride has been added since, along with Susan King, and in 1983 Four toppled Nine, temporarily, and its 11 p.m. news has generally remained competitive ever since.

In addition to the role he shares as ratings-builder, Ryan also is part of the continuing trend toward serious weather forecasting as part of news presentations, away from the days when the weather was given by someone who was strictly there for decoration.

Ryan, while decorative, smooth and attractive enough, is first of all a serious weatheran, a fellow who talks easily about the physics of weather and the complex, computer- driven systems for forecasting it. And, at 42, he's been predicting the weather for more than 30 years.

Ryan's recent appearance at Cashell in Olney, Md., was typical of the one-to-two presentations he makes each week at D.C. area schools. The Cashell PTA booked him last fall. He's now scheduling dates for the next school year.

The children, following brief remarks by Ryan and a 10- minute film on the complexity of predicting the weather, were alive with questions.

Why are you a meterologist?

"I was a bit strange when I was little. Weather was a hobby. I even made some instruments and compared my forecasts to the ones made on TV."

Is the weather different in Washington from other places?

"Yes. The mountains to the west and the Bay to the east complicate things here. They make for different patterns of weather."

"It's fun to go out to the schools and make the presentations and leave, " said Ryan. "I don't know what it would be like everyday . . . The second to sixth grade is an interesting group. They have an excitement for learning at that age. They're like sponges the way they absorb information." Older children, whose hormones have begun to flow, can be more challenging, said Ryan, recalling one junior high group that was "hanging off the walls."

Ryan recalls well the days when he too was a little sponge. He grew up in the Hudson Valley in New York, son of a military father who had once been a science teacher, and a mother who taught too. The teaching tradition runs deep in the family. Ryan's grandfather on his mothers' side was an elementary school principal.

The urge to learn about the weather and tell everybody else about it hit Ryan early.

"As a kid, I used to go to the library," he said. "There weren't many books on weather. I got one called Weather and How it Works and took it home."

With the book as a guide, he scavenged around the house for empty milk cartons and other neat kidstuff to build a barometer. What couldn't be scavenged was bought with his paper route money.

"I had a weather board with my own symbols and made my own forecasts," said Ryan. "I compared them to the forecasts made on television."

His hobby became widely known, and Ryan still has a clipping from his 7th grade school paper, urging students to talk to Bobby Ryan if they wanted to know anything about the weather.

Ryan is a member of the Sputnik generation of Americans who were in junior high or early high school when the Russians launched the first artificial earth satellite.

"I was in the 9th grade," he recalled. "I remember how startling it was." All over the country special programs were developed for children who showed promise in math and the hard sciences. "They tested kids," Ryan recalled. "My reports came back, 'Mr. Ryan shows an interest in science and an aptitude for math.' . . . I had visions of sending rockets to the moon."

What's lightning?

"It's sort of like a giant spark plug. All the swirling around in the atmosphere creates electrical energy in clouds, and it jumps, usually from one cloud to another."

What's thunder?

"Lightning is hot, so hot it causes the air around it to explode."

Ryan did not send rockets to the moon. After brief pursuits of the liberal arts and secondary education, he returned to science. He holds an undergraduate degree in physics and a masters in atmospheric science from the State University of New York in Albany. Then it was on to do consulting work with a company in Massachusetts.

"It was a fascinating place to be for a kid right out of school," said Ryan. "I joined a group doing some classified work for the government. We dealt with things like how to tell the difference from high altitude between a cloud and a missile plume."

When a new TV station went on the air in Boston, Ryan got an idea. "I called and asked if they would want a meteorologist for a weather man," he said. "I was there long enough to get the bug and decide that was what I wanted to do."

He went on to work for other stations in New England and while there met his wife, Olga. She of course was a teacher, at Boston University. They have been married 13 years and have a son.

Professionally, Ryan got a big break in 1978 when he won the weather reporting job on NBC's "Today" program. The hours were atrocious, though, getting him out of bed at 2:30 a.m., and his work day continued with 6 and 11 p.m. weather jobs in Boston.

Life is simpler in Washington, the station in Northwest Washington being an easy mid-morning commute from the family home in McLean.

Ryan excited the Cashell kids when he told them their group would be featured on his early evening weathe spot. "I hope you brushed your teeth this morning," he added, sending a fresh frenzy through the room.

A teacher winced and smiled at the same time. "That's it for the whole day," she said.

When his 40-minutes of film, rapid-fire Q and A and show-and-tell were over, a "We Excel at Cashell" button was pinned to his lapel, and he promised to show that at 5, too.

Ryan's day includes daily weather spots on WMAL radio. After the 5 o'clock news, weather and sports presentation on Channel 4, he goes home for a candlelight dinner with his family. A change of clothes, and it's back to town for the 11 p.m. presentation.

His weather reports are enhanced by the science's considerable paraphernalia and by some 200 pairs of weather eyes scattered in the region. They belong to Ryan's legion of weather watchers.

"I developed the idea at Boston," said Ryan. "I gave maximum-minimum thermometers to 10 or so people at the consulting firm who lived in the region, and I mentioned their names on the air and the temperatures they reported.

"My second or third day here in Washington I announced the same plan. There were 80 to 100 responses. We now have a hard core of more than 200."

The most famous among them is the one with the memorable name, Kitty Radcliffe.

"I guess because of the name people thought I had a honey tucked away in the Shenandoah," said Ryan. He recently introduced her to his TV audience and even displayed her art work. He also took her over to WMAL and let her chat with radio personalities Trumbull and Core.

Ryan has now added a science-technology feature to his presentation.

"I always approach television weather as doing it for myself," Ryan said. "I like to leave a sense of discussing what's happening with the audience and including the science behind it.

"I like to leave a sense of what's going on and why."

At Cashell, no one asked Ryan how much he makes. But one question was fairly pointed.

Do you ever make mitakes in your forecasts?

"No. The weather forecast is always right. Sometimes I get the days wrong."

The school visits aren't the only contacts Ryan has with children. "I get mail from kids." he said. "I can always tell when reports are due. My favorite letter read, 'I have a report due tomorrow. Please write and tell me all you know about the weather.'"