How Come?: Cool air can't hold humidity

On a sweltering July afternoon, with no rain in weeks, the dry, hot grass in your backyard doesn't exactly invite a barefoot stroll. But venture out at midnight, and the same lawn may be covered with water, enough to make your shoes (or bare feet) wet on your moonlit walk through the grass. But don't thank invisible underground sprinklers, or hose-wielding garden fairies. It's the air that's doing the evening watering.

After the sun sets, the Earth's surface and the air above it begin to lose the heat stored up during the long day. As the earth radiates the day's heat into space, the air temperature drops. (Which is why a 95-degree afternoon can be followed by a 65-degree night.) And cool air, with its more tightly packed gas molecules, can't hold as much water vapor as hot air.

So at a particular temperature (and pressure), a parcel of air is holding all the water vapor it possibly can. At this temperature, called the dew point, the air is saturated, like a sponge full of water starting to drip on the floor.

When the evening air, brimming with water vapor, wafts against the chillier grass, the water jumps out of hiding. Encountering a cool green blade, the water vapor's temperature drops, and the vapor suddenly shrinks down into liquid drops. (These round droplets can also combine with water evaporating from the plants, forming bigger drops.) Soon, every blade, leaf and flower near the ground is covered with tiny beads of water, visible by moonlight or flashlight.

(And although we don't think of it as "dew," the drops that appear on your iced tea glass on a hot day, or on the cold pipes in your warm, humid bathroom, form the same way. So a "sweating pipe" isn't really perspiring water. It's collecting it from the air.)

On some summer nights, a stroll through the yard can leave shoes soaked, like walking through a rain puddle. On other nights, the grass seems barely damp. How come? Scientists say that more dew forms on clear nights, when heat can radiate freely from Earth into space, no insulating clouds in the way. And since a steady breeze can make dewdrops evaporate, calm air usually means more dew.

Ever wonder why leaves high up in trees look dry, while grass at the base of the trunk is soaking wet? As air makes contact with the cooler tree leaves, it cools and sinks, warmer air from above taking its place. When the cool, sinking air reaches the ground - or, say, the roof of your car - its moisture condenses into dewy droplets. Meanwhile, high leaves tend to stay dry.

Unfortunately, we can't rely on dew to save a drought-parched lawn. Dew rarely accumulates more than a measly 2/100ths of an inch during one night's condensation.