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HOW COME? Water molecules turn to fog

What causes fog? ask Jason DiStefano of Commack?Fogs are like clouds that form near the earth instead of high in the summer sky. Fogs (and regular clouds) can spring out of the air because there are always water molecules zipping around to make them.

Water molecules jump into the air from lakes, rivers, oceans and plants, evaporating (turning to invisible vapor) like water left in the bottom of a glass, which mysteriously disappears overnight.

Warm air can hold a lot of water vapor. But as air cools, it reaches its saturation point like an overfull sponge about to drip. And then water molecules suddenly condense into visible droplets.

Water comes out of hiding by glomming on to particles of dirt or dust, salt or pollutants in the air. A liquid droplet forms around a tiny particle, like the grain of sand at the heart of a pearl.

High in the air, droplets form clouds, like the fluffy cumulus clouds of a July afternoon. But water droplets can also create a cloud that hugs the ground: fog. In fog, tiny droplets of water rise and sink, carried by currents in the air. Sometimes the droplets fall to the ground like rain, but new drops condense out of the air to take their place.

Droplets grow bigger as more and more water molecules collect together. And as water droplets grow -- and more form -- a light haze over a road can thicken to a dense fog.

The familiar fog that appears on cool nights, especially in the fall, is called a radiation fog. After the sun goes down the surface of the Earth and the air above it cool, radiating the heat stored during the day into space. If the air is moist and the cooling rapid, fog may form, hanging above fields and drifting across roads.

Chilly air tends to sink down hills and into low-lying areas, carrying heavy fogs with it. Drive down a dip in a country road at night, and you can find yourself in a thick fog, one that car headlights can't penetrate.

When the temperature plunges well below 0 degrees F. (say, to -40 degrees F.), we may see ice fogs, clouds of glittering ice crystals hanging in the air, like high cirrus clouds. Ice fogs form when water vapor freezes into crystals rather than condensing into droplets.

Freezing fogs can appear at higher (but still frigid) temperatures. These fogs are made of tiny droplets of supercooled water (with a temperature below 32 degrees F., water's normal freezing point). On cold surfaces the supercooled droplets suddenly freeze into ice crystals -- leaving a decorative coating of icy shards everywhere they touch.


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