A friend who moved from Boston a few years ago complains that his Sunday-morning softball games are no longer a pleasure. Washington summers, he says, make everything slimy -- the ball, the glove, even the bat gets slippery with sweat. Errors skyrocket, tempers flare, and by August the after-game beers in an air-conditioned bar are all that anyone looks forward to. It's not the heat, he says -- he could play ball at high noon in a desert. It's the God-awful humidity.

My friend is fooling himself. The astounding fact is Washington is no more humid than most big American cities and a lot less humid than some. Washington summers rarely sustain midday humidity levels of more than 60 percent -- as uncomfortable as that may feel -- and they never get above 70 percent, particularly when it's pushing 100 degrees. Common knowledge and sticky broadcloth aside, 93-degree days with 95 percent humidity just don't happen here. Humidity, that most maligned of weather measurements, is sorely misunderstood.

Humidity is a measure of the amount of water in the air relative to the amount of water it can hold. The warmer the air, the more water it can hold -- 90-degree air can hold about twice as much water as 70-degree air. Air that is cooled will increase in humidity because its overall ability to hold water is reduced. Air that is warmed will decrease in humidity.

The temperature at which air reaches 100 percent humidity is called the dew point. If the temperature falls below dew point, water is squeezed out in the form of rain or other precipitation. Typically, some dew comes out of the air after sundown unless cloud cover keeps the air from cooling sufficiently. By the way, meteorologists are emphatic about dew -- it does not fall, it condenses.

Humidity is measured with a hygrometer -- literally, a water meter.

Hygrometers come in all shapes and levels of sophistication, but a very simple and ancient one is based on the principle that many things expand when wet. One of these, a hair hygrometer, uses specially treated strands hooked up to a meter. The hair expands as the humidity increases -- by as much as 2.5 percent. (Head hair expands as well -- good news for those in need of a bit more coverage.) Hemp, catgut and other fibers can be used instead of hair. There are any number of other substances, not commonly used in hygrometers, that also expand with humidity. Among these are basement doors and human bones, both of which malfunction when the air gets damp.

Most of the humidity in the air comes from the ocean. In general, land tends to be dry and does not contribute to the water content of the air, even in Washington. (Rumors that the city was built on a swamp, and that water vapor rises from some fetid, malaria-ridden hot spring, are unfounded.) Air hovering above the mid-Atlantic tends to be cool, no more than about 70 degrees Fahrenheit even on a dog day. That air is, not surprisingly, packed with water -- its relative humidity commonly is 100 percent, the saturation point. Air flowing in from the ocean may heat up as it moves inland, but it will not pick up more moisture as it moves across the dry land. That means that its capacity to carry water may change, but the amount of water in it remains constant.

Hence, on a hot day in the city, the relative humidity of the air goes down as the temperature goes up. The time of highest humidity in the summer is actually early morning, when the temperatures are lower and the air, as a result, cannot hold as much moisture. By the time an ocean breeze hits downtown on a 95-degree afternoon, it is carrying no more than 60 to 65 percent of the water vapor it can potentially carry -- the humidity is 60 to 65 percent.

This is not to suggest that a 95-degree day with 60 percent humidity is anything to look forward to -- it's not. All the loathsome symptoms appear -- shirts stick to backs, hair goes limp, rugs curl at the edges. At 95 degrees, the body's ability to get rid of its own heat goes down dramatically, and it depends heavily on sweat to cool itself. But the moister the air, the less rapidly sweat evaporates and the less efficiently the body cools. Sixty percent humidity is moist enough to make an overheated body wish it were in northern Alaska.

Nonetheless, we can be grateful that the air in Washington is less damp than, for example, the air over Mississippi and southern Texas, where hot air masses sweeping off the Gulf of Mexico carry a veritable blast of steam inland. By contrast, dry towns like Phoenix, where the humidity has been known to drop to as low as 2 percent, can get very hot for the very reasons that there is no water in the atmosphere to cool them.

"Dry heat" may feel more comfortable, but it carries its own risks. Plants will die in very hot, dry air, even if their roots are immersed in water -- the roots can't get water to the leaves quickly enough. That is what happened to the Russian wheat in 1975 -- a hot desert wind called the Sukhevey threw the plants into moisture shock and wiped out nearly the entire wheat crop within 24 hours. Humans can also dehydrate on very dry days, even if they don't appear to be working up a sweat -- especially if they use window fans, which whisk away vital moisture as quickly as it is produced. The elderly are particularly susceptible to this and should be careful to drink as much liquid as possible when it's hot, especially before going to bed in an air-conditioned room.

Less obvious effects of climate on health can be equally dramatic. Larry Kalkstein, a researcher at the Center for Climatic Research at the University of Delaware, recently completed a study of the impact of extreme weather on the mortality of residents in 50 American cities (Washington wasn't one of them). He found that many cities have a threshold temperature above which people seem to die from nothing other than weather-related discomfort -- not heatstroke, he says, but something more like despair. For example, in Boston, where people tend to wear blizzard gear well into May, this threshold temperature is 85 degrees. Washingtonians are significantly less sensitive to the heat -- Kalkstein estimates the threshold here is the same as in Philadelphia, about 92 to 93 degrees.

This effect is not trivial -- every summer in St. Louis, there are 183 deaths that Kalkstein traces directly to weather. In New York, there are 139 additional deaths every summer, and in Philadelphia, there are 118 extra deaths. These people die of just about everything -- heart attacks, cancer, accidents. They seem, Kalkstein says, just to give up. Ironically, death rates in the hottest cities -- Dallas, New Orleans, Oklahoma City -- do not vary with temperature. Southerners, Kalkstein says, expect it to be hot. It is mostly people in moderate climates who die when things get steamy. Kalkstein says it is impossible to sort out whether high humidity or high heat has the greater impact on health, but it is known that a combination of the two is the most deadly.

The bottom line, though, is that humidity, like most things, is both a blessing and a curse. It buffers extremes of heat and cold, which is why places with too little, like deserts, are so hot during the day and so cold at night. On the other hand, too much water in the air prevents sweat from evaporating quickly enough to cool you off. Despite cocktail chatter to the contrary, however, it is not the humidity per se, but the coupling of very high temperatures with moderate humidity that spoils softball games and causes Washingtonians to flee -- or to wish they could -- in August. :: WETLANDS

DESPITE ITS SWAMPLIKE CHARACTERISTICS,

Washington is not the most humid place in the world; it's not even the most humid city in the country during July and August. With an average daily maximum temperature of 87 degrees in July and 85.8 degrees in August, the relative humidity in D.C. during those months is 87 percent in the morning and 56 percent in the afternoon in July and 89 percent and 56 percent, respectively, in August. Compared with several other locations, Washington seems dry.

ST. PAUL ISLAND, ALASKA, is the most humid place in the country, with an average relative humidity of 96 percent in the morning and 90 percent in the afternoon in July and 95 percent in the morning and 90 percent in the afternoon during August. Of course, it's cool there; the average daily maximum temperature is only 45.3 degrees in July and 43.7 degrees in August. Perfect weather for seals.

The town you might want most to bypass on your summer vacation is PORT ARTHUR, TEX., famous as the home town of rock 'n' roll superstar Janis Joplin and infamous for its heat and humidity. The average daily maximum temperature during July is 92.5 degrees, and in August, 92.2 degrees. Between the heat and an average relative humidity of 94 percent in the morning and 66 percent in the afternoon in both July and August, you can guess why Joplin's style ran toward the steamy.

For the driest summer ever, there is LAS VEGAS. The average relative humidity in July is only 29 percent in the morning and 15 percent in the afternoon. In August, it's 35 percent and 17 percent, making it the least humid city in the nation. Vegas is hot -- the average daily maximum temperature is 104.5 degrees in July and 101.9 in August -- yet heat hardly matters. No one stays outside long enough to even comment on the weather; all the casinos are air-conditioned.