HOW COME: Seeing your breath in the winter

Why do you see your breath in the winter? -- reader Rivka ZimmIn cold weather, we all become cloud machines. Taking the dog for a walk on a wintry morning, we and Fido puff out a series of tiny white clouds, marking our progress down the road before they disappear behind us.

If the temperature is cool enough, we can see our breath in any season. And just like clouds in the sky and fog near the ground, our visible breath is actually a swirling mist of water vapor.

How does it work, and why don't we see our breath on balmy summer afternoons? It all depends on the air around us.

Earth's atmosphere is a mixed collection of (invisible) gases. Lurking among the nitrogen, oxygen and other gas molecules is a constantly varying amount of water vapor. Unlike liquid drops or solid ice, water vapor is H2O in its "gas" state. Like other gases in the air, water vapor is invisible.

Up to about 4 percent of the molecules in a parcel of very warm, humid air may be water; the average is about 2 to 3 percent. As air cools and gas molecules pack closer together, air reaches its saturation point. Then, like an overfull sponge, liquid water drips out of hiding.

Take, for instance, fog. Fogs often spring up at night. After the sun goes down, the Earth's surface radiates the day's stored heat into space, cooling the ground and the air above. If the cooling air just above the ground is moist, its water molecules may collect around bits of soil, salt or soot, condensing into a swirling mist of tiny liquid droplets. Presto: water, jumping out of hiding.

Breath fog forms when the temperature drops, too. On a warm day the water vapor you exhale (along with carbon dioxide) simply spreads out as H2O molecules into thin air. And your breath stays invisible.

But when the air outside your body is chilly-to-frigid, it can't hold as much water vapor as the nicely warmed air inside your lungs. Before you exhale, neither the cold air near your face nor the warm air coming from your lungs is completely saturated with water. But when exhaled water molecules hit the cold air in front of you, it suddenly becomes "supersaturated" with water.

Result: Water vapor immediately condenses into droplets, forming a visible breath cloud. But as the droplets spread out into the wider air, they evaporate, just as a jet's condensation trail fades into the sky.

Whether water vapor condenses into droplets depends on local air pressure as well as temperature. But we almost always can see our breath when the thermometer drops into the 40s.