What holds fog together?

Photo Credit: Lida Rose

You can think of fog as a cloud that touches the ground. It may form when relatively warm, moist air cools quickly. When warm, moist air is carried over cooler land, its temperature is lowered by contact with the cool land surface. The relative humidity of the air then rises. The cooling condenses the water vapor into tiny droplets of liquid water – fog. That’s how the famous fogs of London, San Francisco, and Newfoundland form.

And even though the water droplets that make up fog may seem suspended, they’re actually falling. They’re just not falling very fast. As in most clouds, in fog banks there are a wide variety of forces all acting together, such as buoyancy forces, updrafts, and downdrafts. It’s possible that some of these forces – combined with the droplet’s small size – help slow its descent.

You can get a closer look at fog with a magnifying glass – and the help of a spider. As fog drifts over a spider web, some of the liquid sticks to the fine filaments, leaving a necklace of fog droplets.

May 17, 2008

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