Why you're as changeable as the weather: there are reasons you feel bad when the weather gets worse.

A few years ago my sister and I vacationed in St. Louis to celebrate the twenty-fifth anniversary of the city's famous Gateway Arch. I was looking forward to enjoying time with my sister and just having some fun at the festivities; that is, until the weather changed my plans.

The temperatures in St. Louis - which had exceeded 90 degrees for days on end before our arrival - plunged into the 60s while rain drenched the city for the entire four days we were there. Much to my chagrin, I was incapacitated with agonizing arthritic back pain during the entire vacation, and my sister had to attend many of the festivities alone.

According to Stephen Rosen, M.D., author of Weathering: How the Atmosphere Conditions Your Body, Your Mind, Your Moods - and Your Health, healthy people are often affected by no fewer than 37 symptoms during changes in temperature, humidity, wind, and sunlight. The most frequent complaint is tiredness, followed by bad moods, disinterest in work, head pressure, insomnia, and headaches.

In his unique study several years ago, Joseph Hollander, M.D., professor emeritus of medicine at the University of Pennsylvania, constructed a special room called a climatron, where he controlled weather conditions. Whenever conditions in the climatron simulated a thunderstorm failing air pressure and rising humidity arthritis symptoms worsened in all but one of his 12 subjects. A steady, warm, and dry environment did not aggravate arthritis symptoms in these people.

Recently, Dr. Robert N. Jamison and colleagues at Brigham and Women's Hospital in Boston surveyed 558 chronic pain patients - those with backache, headache, arthritis, and the like - in four cities nationwide and found that weather affects pain no matter where you live. "The real culprit," explains Jamison, may be a change in barometric pressure, since patients are most likely to report an increase in pain in advance of weather conditions."

When the outside pressure falls, tendons, ligaments, muscles, and bones swell and readjust, pressing on sensitized nerves throughout the body. Headaches, particularly migraines, become severe when there is a drop in barometric pressure, and surgical scar tissue, which differs from original cells, is more sensitive when the weather is wet and stormy.

In the summertime high winds can fill the air with ragweed and various pollutants, leaving those who suffer from allergies or asthma gasping for breath.

People generally feel most comfortable when the temperature is about 70 and the humidity about 50 percent. Whenever the temperature changes significantly, your body struggles to regulate its internal temperature against the outside elements. If your body temperature climbs to 105 degrees or higher during a heat wave, brain damage can follow within minutes.

Body temperature that dips below 95 degrees in response to extreme cold weather is clearly dangerous; by some estimates no less than 35,000 deaths occur each year from cold stress. In addition, colder weather thickens the albumin-like fluid around the joints and tendons, allowing less movement.

Day-to-day and season-to-season weather changes not only affect physical health but also take a toll on behavior and moods. No one knows this better than Barb Baird, 45, a volunteer with the Michigan Salvation Army. During autumn the failing and molding of the leaves aggravate her allergies and weaken her immune system. "With the continued lack of rest comes orneriness," she says. "When I don't feel well, I lose my ability to care about others." Even worse, dry, warm winds, such as the foehn in Switzerland, the sharav in Israel, and the Santa Ana in California, have been associated with an increase in crime, murder, and assaultive behavior.

During the fall and winter months Neal Owens, of Gaithersburg, Maryland, watched his marriage collapse and almost lost his job because of depression. He volunteered for a medical study into seasonal affective disorder (SAD) - a mood disorder precipitated by decreased sunlight between October and April.

"This depression occurs at a particular time of year on a repeated basis," explains Paul Goodnick, Ph.D., director of the Mood Disorders Program at the University of Miami in Florida. "What distinguishes it from other forms of depression is that it fades almost spontaneously during the springtime."

The good news is that 75 percent of people affected by SAD find relief with the use of a medical light box that transmits a full-spectrum fluorescent light equivalent to midday natural light. The body doesn't know the difference between natural sunlight and the medical light boxes; both help the body to release two depression-fighting chemicals - melatonin and serotonin. With the improved light boxes in use today, patients get relief sitting in front of the light box from 15 minutes to two hours per day.

"But if you stop using the box," warns Goodnick, "you go right back to where you were before the light therapy." If you think you might be affected by SAD, discuss it with your doctor.

Ironically, the arrival of spring and sunshine is not an uplifting experience for everyone. Statistics show that suicides peak during March and April. The arrival of spring brings the spirit of renewal for all but depressed people, who feel anything but springlike inside. The newness all around them only makes their "winter of discontent" all the more obvious to them.

All this doesn't mean that if you're weather-sensitive, you're doomed to a life of fluctuating pain, bad health, and mood swings.

The most important thing, say the experts, is to remember that although you can't control the weather, you can control your responses to it. "So on those inescapable, cloudy, gloomy, damp days," says meteorologist Joel Sobel, of Accu-Weather, Inc., in State College, Pennsylvania, "try to keep an optimistic attitude and get through it, as opposed to letting life wash over you. Like Little Orphan Annie said, `The sun will come out tomorrow.,"