Sizzle to Fizzle : Romance: Studies show that passion heats up with temperatures between June and August. But as autumn emerges, love usually wanes.

He was her best friend’s big brother, and he hardly knew she existed. But, oh, the crush she had on him: that face, that laugh and that cute little convertible.

They were almost more than Eileen Mann could bear, those feelings she had for fellow camp counselor Marc Winger back in the summer of 1972. Then one August midnight, Winger finally noticed Mann and a love affair blossomed.

“He just asked me, ‘You want to start dating?’ It wasn’t very romantic,” said Mann--now married to Winger and living in Newhall with two kids--as she recalled the summer love 21 summers ago that became the love of her life.

Spring may be the time when a young man’s fancy turns lightly to thoughts of love, but summer is when those thoughts turn to action. As temperatures heat up, so do libidos, and starry-eyed singles across Southern California pair off. Sometimes the fireworks burn until the winter of old age, but all too often they die like the charcoal embers of a Labor Day barbecue.

The Beach Boys, Neil Diamond and countless other poets have long chronicled the travails and the triumphs of summer lovers. And statistics tend to bear out the bards’ assumption that passion runs high between June and August.

For instance, more Los Angeles County marriage licenses are issued in June than in any other month. Purveyors of lingerie say the summer season is second only to Valentine’s Day in the volume of such amorous accouterments as teddies and garter belts sold. And business at dating services is slow, suggesting singles are having better luck finding that special someone on their own.

CSUN speech communication Prof. Peter Marston said “it stands to reason” that people not only feel more frisky in the summer, but are more likely to find someone with whom to share the feeling.

“People have certain activities we associate with love,” Marston said. “It might be going to the beach or walking in the park. Those are the sorts of things we do more often in summer. You are much more likely to fall in love by engaging in leisure activities than by engaging in drudgery.”

Summer traditionally is a season when the normal rules no longer apply. In ancient times, fertility festivals encouraged men and women to couple freely to please the gods that provided their crops.

“It was the hunger of the stomach, but there was a hunger of the genitals as well,” said UCLA psychiatry Prof. Roderic Gorney.

Bits of those behaviors have survived to modern times. With longer and warmer days, more people are outside--at the beach, in parks or at sidewalk cafes. And they are generally more carefree, less concerned about the daily toils that put a damper on love.

Vacations tend to encourage romance. Marcus K. Joseph, senior case director for Nick Harris Detectives, said he gets many requests at the end of summer to investigate people with whom vacationers had a fling.

“A woman might want to know if the man she met is really a nice Jewish dentist from Wilmette, Ill.,” Joseph said, adding that many people also ask him for help in tracking down people they met at summer parties.

“People are more playful,” said Encino marriage counselor Debby Norden. “Heat does that to you. Think about it. The colors we associate with heat--colors like red--are the same colors we associate with passion.”

Nature, too, may play a role in sparking that loving feeling. Other animals--especially elephants and some primates--have sexual cycles that correspond with seasonal changes, assuring that their offspring will be born when the weather is fair and the food is plenty.

“No doubt there are similar legacies in human predispositions to make our behaviors synchronize with the calendar,” Gorney said. “But we have become so liberated from biological constraints that it is important to say that the biological impulses are very murky.”

They were loud and clear for David, 25, of Encino, who had a torrid summer romance several years ago with a fellow Universal Studios employee whose license plate frame read, “Ask me. I might.”

“I thought she was just fantastically sexy and beautiful and I would have done anything for her,” said David, who asked that his last name not be printed. “After all the pain and tumult and chaos of winter and spring, it looked like things were finally going to work out.”

They didn’t.

The relationship died two weeks before Labor Day, for reasons David still does not completely understand. He quit his job because he could not stand being around his former flame.

“She gave me a friendship bracelet,” David said. “I burned it on the barbecue.” He did, however, keep the ruby earring she gave him.

Such is the end of many summer flings.

Experts agree that once the leaves start to fall from the trees and people quit the activities that held these fair-weather romances together, love tends to wane.

“Everything that surrounded the relationship is gone,” Norden said. “You realize that maybe you didn’t have as much in common with this person as you thought you did.”

That’s when frustrated singles head to the dating services. Jeffrey Ullman, founder and president of Great Expectations video dating service, said business is slow during the summer but picks up dramatically after Labor Day.

“At the end of the summer people tend to look back and ask themselves, ‘Who did I spend it with? Did I really have a meaningful time?’ ” Ullman said. “Basically, people get caught up in a cycle of a bunch of first dates.”

Ullman continued: “There are two times a year in which there is always a mad dash for romance: right after Labor Day and right after the new year. Those are the two times of the year when you look back at what you did and forward to what the future might hold.”

Those patterns tend to agree with Marston’s theory that people make drastic relationship changes with the seasons. Divorces, for instance, also run slightly higher in the summer than in the rest of the year.

But Eleanor Lawson of Glendale suspects that maybe there is too much ballyhooing about summer romances. She should know. More than 50 summers ago, she fell in love with a boy.

When fall came, she saw the boy no more. She got married and lived a full life. Then, as a widow, Lawson was reunited with her summer sweetheart of half a century before.

They were married for four years before he died in 1989.

“I think any time is romantic,” Lawson said. “If it happens, it happens. I don’t think it has much to do with the season.”