The Summer Soup - NYTimes.com ON summer days, do you feel like you are in a soup? Well, you are. Water vapor is not the only thing in that sopping summer air. It also contains aerosols - both solid and liquid. They may have condensed from the gases that emerge from your tailpipe or from a factory chimney, or they may have risen into the air from the ground: tiny particles of silicon, organic matter, threads, starch, spores, bacterial cells, tire rubber. One of the most common aerosols in New York City air, thanks in part to the booming restaurant scene, is fat. The aerosols and the water vapor together make summer soup. A halo of water condenses upon each bit of stuff, and this spicy mixture sloshes against your skin, leaving behind a bit of fat and water, while picking up a few of your skin cells for its own purposes. Before the industrial age, even at the height of summer, there were times when there were not enough aerosols to mix with all the water in the air. So a small amount of stuff would quickly attract all the extra water, making drops that were heavy enough to fall from the sky as rain. But nowadays, we put so much more stuff in the air that most every tiny vapor puff has an aerosol partner, making for a lot of little wet airborne parcels, each too small and too light to fall. There is on average 5 percent more water in the air now than there was during World War II. This is the summer soup. It can stay put for a long time. It envelopes us with its miscellany of flavors, solids and liquids. It is reluctant to rain. Indeed, in the 1970s, scientists studying air pollution theorized that the airborne filth might cause droughts, because the large quantity of stuff in the air would lock up the water into drops too light to fall. We would have cloudy skies but no rain. This has not turned out to be our experience. Today it doesn't just rain, it pours. We get not the "the small rain down shall rain" of the medieval lyric, but cloudbursts. The soup hangs around us for days. Then, suddenly, there is a change. A cold front or converging air or hot rising air pushes the soup up higher into the atmosphere. There, it gets colder and the rest of its vapor condenses. One drop falls into another, picks it up, and slams into another. Big thick drops come down. The soup falls on our heads. In the old days, in Spain, when a woman threw the wash water (or worse) from an upper story window, she shouted, "Agua va!" Perhaps we should add the phrase to the forecaster's list? William Bryant Logan is an arborist and the author of "Air: The Restless Shaper of the World."