HOW COME? Putting the pressure on wind

How does wind start? asks Rachel Miller, a student in Lisa Drennan's class at Hewlett Elementary School in HewlettWind whips flags, whistles through wires, ruffles your hair, and pushes you down the sidewalk like a strong hand at your back. Yet despite all the commotion, wind is invisible, seeming to come out of everywhere and nowhere at the same time.

Of course, it's actually the air around us, moving this way and that, that may become a wind, whirling dry leaves like a tiny tornado. Surprisingly, it's our home star, the sun, that is the force behind the winds of Earth. Sunlight warms the ground, which, like an electric heater, warms the air above it. Warm air rises; cooler air rushes in to take its place. This flow of air is what we call wind.

How does it work? Warming air expands and thins. As its gas molecules spread further apart, a parcel of air weighs less, creating less pressure on objects beneath it. But cold air shrinks together, so a parcel of colder air presses down with more force.

Like water flowing out the nozzle of a hose, air tends to flow from an area of high pressure to an area of low. In their attempt to equalize the pressure, air molecules rush into spaces where air is thinner, creating a breeze.

But winds don't just blow by your front door; they also blow around the whole planet. Rising air from Earth's hot equator regions meets cold, sinking air from the north and south poles, causing global winds to blow. But our planet's winds don't simply blow from high- to low-pressure regions. Since the Earth and its atmosphere are always rotating, global winds are forced to follow a curving path.

Smaller, local winds aren't much affected by the Earth's turning. Winds along coastlines, for example, blow in from the sea during the day, but stream from the land out to sea at night. Why? Imagine a hot, sunny day, beach sand warming fast. Air over the beach heats and rises, and cooler ocean air flows in to replace it -- creating a welcome sea breeze.

But at night, coastal land radiates heat into space more rapidly than does ocean water, which tends to hold onto heat. So as air cools above the beach, it flows out to fill in the thinner, warmer air above the water. Out on a boat at night, you may feel a breeze blowing from shore.

The greater the pressure difference between two regions of air, the faster air flows between them. Encountering a pocket of especially low pressure, air may suddenly gust in, taking the hat right off your head.