Summer at its oppressive best is all around you. You run into a friend. "Hot enough for you?" says the friend. "It's not the heat," you reply. "It's the humidity." Or if you live in another part of the country, you reply, "At least it's dry."

This is boring, even for summertime conversation. But what if you could say instead, "Did you know that humid air is lighter than dry air?" Or, "Did you know that desert air has more moisture than North Pole air?" Wouldn't that wow your friends?

Probably not -- but at least you'd have something else to say. So here, for your seasonal reading pleasure -- "Humidity: A User's Guide."

Relative humidity has nothing to do with your Aunt Sally or your Uncle Norman, no matter how much they sweat. Relative humidity is the biggie, the one you always hear about, the one seldom explained. It's simple: Relative humidity is a measure of how much moisture is in the air, compared to how much moisture that particular air could be carrying.

Try this one at home: Take a shopping bag the size of six cantaloupes. Put six cantaloupes in the bag -- the bag is 100 percent full. Now take those same six cantaloupes and put them in a bag that can handle 12 cantaloupes. That bag is only 50 percent full. But you still have six cantaloupes. That's pretty much how it is with relative humidity; the same "absolute" amount of moisture can give you very different "relative" readings, depending on the air's changing capacity.The capacity of air to handle moisture varies greatly by temperature. A parcel of air at 95 degrees Fahrenheit can handle more than four times as much water vapor as the same parcel of air at 50 degrees. So, all other things being equal -- no new fronts moving in with additional moisture, for instance -- as the temperature rises, and the air's capacity for moisture increases, the relative humidity of that air will decrease.

That's why, generally speaking, the highest relative humidities occur in the early morning, the coolest part of the day, while the lowest relative humidities occur in midafternoon, when temperatures are highest.

"People say, 'It was so hot -- the temperature was 90, and the humidity must have been 100 percent,' " says Mike Mogil of "How the Weatherworks" Educational Weather Services in Rockville. " ... It is impossible."

Dehumidifiers make their living chilling out. Air that's drawn into a dehumidifier passes over cold pipes that lower the air's temperature and its moisture-holding capacity. When the air cools enough, the "excess" moisture condenses and is drained away. The drier air is then reheated and returned to the room.

If it's all the same cantaloupes, what difference does it make? Plenty of difference. The lower the relative humidity, the more easily the air can take on additional moisture -- cooling your body by evaporating sweat, for instance. The higher the relative humidity, the harder that is.

It's also -- that difference in evaporation -- one way weather-types figure out the relative humidity. They measure the different readings of "dry-bulb" and "wet-bulb" thermometers, the latter wrapped in a piece of damp, well-ventilated cloth. The lower the relative humidity, the more evaporation, and the cooler the wet bulb will be.

So "hot and humid" is the worst? A 1989-90 sampling of 44 football-related heat injuries by the NCAA's Injury Surveillance System found that 38 occurred under conditions reported as "hot and humid."

On the other hand ... An advisory issued by the Environmental Health Section of the Walter Reed Army Medical Center warns field commanders, "Don't be fooled by a "safe" WBGT index {the Army's method of measuring the combined effects of heat, humidity and solar radiation}, as most heat injuries occur during early morning training (especially physical training) when the air temperature is at its lowest point and the relative humidity is highest ... Individuals exercising at this time are less likely to drink adequate volumes of fluid prior to training."

Some things that happen when it's humid. The skin absorbs moisture. Joints and corns ache. Asthmatics usually feel better. (Asthmatics without corns, that is.) Hair expands in length by as much as 2 percent; blond hair expands more than red or brown hair. Curly hair gets curlier, straight hair goes limp. Weather-types, in fact, often use a "hair hygrometer" to measure humidity -- expanding and contracting strands of hair move a pen across a calibrated sheet of paper. There are books that tell you how to do this yourself.

Water your lawn when the humidity is high. Otherwise, lots of the water will simply evaporate.

The dew point and you. Another of the great unexplaineds, the dew point is the temperature to which the air would have to be cooled to bring it to saturation.

If the current temperature and the dew point are far apart, the relative humidity is low. If they're close together, the air is nearly saturated already. If they're the same, reach for your umbrella or your fog lights. Air that's cooled to the dew point, by the way, is the major reason clouds form.

Find your own dew point. Fill a metal pitcher part way with water that's warmer than the air around you. Slowly add ice chips to the water, stirring the mix and keeping track of the resulting water temperature. Eventually, the cooling pitcher will start to "sweat," as a thin layer of surrounding air reaches saturation. Measure the water temperature again -- that's your dew point.

Caution: Don't try this in a desert or anywhere else so dry that the dew point is likely to be below freezing; it makes for a long afternoon.

Temperature and humidity don't tell the whole discomfort story. Ventilation (a breeze, for instance) speeds evaporation, and makes you feel cooler despite the numbers. Solar radiation makes you feel warmer; that's why 92 in the sun feels even worse than 92 in the shade. Then there's pollen, glare, tight-fitting clothes ...

How bad was it? When Phoenix topped out at 122 earlier this summer, the relative humidity was only 11 percent, which "didn't do a heck of a lot to the temperature," says KPNX-TV weatherman Bill Austin. Meanwhile, in Washington just a few days later, the temperature was only 97 -- 25 degrees cooler than in Phoenix -- but the 60 percent humidity made it "feel like" 114.

Absolutely humid, absolutely not. The most humid part of the United States (in absolute, not just relative terms) is the Gulf Coast region, from Texas to Florida. In summertime moisture-filled breezes from the Gulf drive the humidity up for much of the Northeast and Midwest as well.

In winter, things change. The Gulf Coast is still moist, but that "tongue" of Gulf air is stuffed before it gets much further inland by a cold, dry "tongue" of even stronger Canadian air, pushing well beyond the Great Plains and as far south as New Mexico and northern Texas.

Looking West ... And why, you wonder, isn't Pacific air nearly as moist as Gulf air? The biggest reason: The water's much colder, so the air passing over it cools down, too, and its moisture capacity drops.

Since you asked ... The average relative humidity for an August afternoon in Washington is 55 percent.

"I can pretty well guarantee, if you put men in a hot, humid climate, you're going to get skin problems." Skin infections put more American soldiers in Vietnam out of duty on a daily basis than any other cause, says David Taplin, professor of Epidemiology and Public Health at the University of Miami School of Medicine. "We found up to 50 percent of combat forces totally covered with ringworm," says Taplin, who worked with a Green Beret medical research team there.Taplin suggested that soldiers be issued shower sandals, and wherever possible wear shorts without underwear and no shirts. "It completely eliminated the problem," he says.

Women have fewer problems than men do with fungal infections of the feet. They wear more open-toed shoes, which drops their "personal" humidity considerably. Taplin (quoting someone, he says): "Man is obsessed with incarcerating his pedal appendages in the dried carcass of a cow."

It can rain even when the relative humidity is less than 100 percent. That's because relative humidity measures the moisture capacity of surface air; there could be a mass of saturated air higher up, in effect "raining through" the drier air below.

It doesn't always rain when the relative humidity is 100 percent. Moisture can condense without precipitating in air that's been cooled to the saturation point. A mystery? Nope -- fog.

Strange But True: Desert air is more humid than polar air. Polar air has very high relative humidity; it's carrying almost all the moisture it can -- but that's not very much. (It's cold, remember?) Hot desert air, by contrast, doesn't come close to reaching its capacity, so its relative humidity is low. But when you measure how much water vapor is actually in the air in each of the locations, desert air is often more humid. Not very humid, just more humid.

Research suggests that the healthiest relative humidity indoors is 40-60 percent. Bacteria, viruses, fungi and mites thrive at low or high humidities. Likewise, respiratory infections, allergic reactions, chemical interactions and ozone production all occur more frequently at the extremes.

More Strange But True: Moist air is lighter than dry air. You'll win a lot of bets with this one. Whenever you add water vapor to the air (as long as the temperature and pressure stay the same), you're replacing heavier molecules of nitrogen (atomic weight 28), oxygen (atomic weight 32) and carbon dioxide (atomic weight 44) with good old light 'n' lively water (atomic weight 18).

So is it the heat, or the humidity? Says Mike Mogil of "Weatherworks": "It's clearly both."