HOW COME? Why we blow hot and cold air

Why is it that when you blow air out of your mouth with pursed lips, the air is cool to the touch, but when you exhale with your mouth wide open, the air is warm? asks Samir El-Sawaf, a student in Brookville.Have you ever found yourself "blowing hot and cold" on something (or someone)? You love the idea of a vacation in Florida (the beach! Disney World!). No, wait, you hate the idea (the bugs! the alligators!). Changing your mind seems easy. But adjusting the temperature of your breath, actually blowing hot and then cold? That seems downright impossible.

And yet, we've all experienced the paradox: coming inside on a chilly autumn day and breathing on your cold hands to warm them. Taking off your jacket and sitting down to a bowl of hot soup. And then blowing on the soup to cool it.

For such a simple, everyday phenomenon, there's a surprising amount of debate about why it happens. But everyone agrees the temperature of your breath is the same -- around 98 degrees F. -- whether it exits through your open mouth or pursed lips.

But try blowing on your hand about 3 inches from your face. Wide-mouthed breathing feels warm. Blowing on your hand as if it's a lit candle? Downright chilly.

How come? One popular explanation is that nozzle-lipped breath comes out quickly and forcefully, blowing away the warmed air layer just above your skin. Room-temperature (say, 72-degree) air replaces it, and you sense coolness. But a gentler, "yawning" breath simply wafts in with less disturbance, enveloping your skin in momentary warmth.

However, experiments show that there's actually more going on. Imagine breathing on a cold window in winter, or using your breath to fog a mirror. It works with your mouth open, but fails with whistle-shaped lips.

Earl Zwicker, a retired physicist who taught at the Illinois Institute of Technology, conducted informal experiments with fellow scientists to solve the mystery. Their conclusion: It's all about evaporation.

Blow through pursed lips, and your jet of streaming breath -- a bit of dry room air "entrained" along the way -- loses some of its own moisture. As with a blowing fan, the rushing air lowers the humidity at the surface of the skin. The result: Moisture evaporates like sweat on a dry day, and the skin feels cooler.

But yawn out a breath, and the warm, moist air from our lungs emerges in a kind of cloud, with little mixing-in of dry room air. Humidity near the skin's surface rises. Evaporation from the skin temporarily decreases, and heat loss drops. And our skin feels warmer, like on a steamy day.