Fog

Fog is a thick cloud of tiny water droplets that floats close to the ground. It forms when air near the surface cools down and the water vapor in it turns back into liquid. Fog is really just a cloud that touches the earth. If you have ever walked through fog, you have walked through a cloud.

The air around you always carries water, even when you cannot see it. This invisible water is called water vapor. Warm air can hold a lot of water vapor. Cool air cannot hold as much. When warm, damp air cools down to a certain point, the vapor turns into tiny droplets. Those droplets are what you see as fog. The temperature where this happens is called the dew point.

Fog forms in different ways depending on the weather. Radiation fog forms on clear, calm nights when the ground loses its heat to the sky. The air just above the cool ground chills, and a low fog settles in. Advection fog forms when warm, wet air slides over a cold surface, like cool ocean water. This is why San Francisco Bay is so foggy in summer. Warm air from the land drifts over the cold Pacific and turns to mist. Valley fog gets trapped between hills, and it can last for days.

Fog and clouds are made of the same stuff. The only difference is height. A cloud sits high in the sky. Fog touches the ground. If you climbed a mountain into a cloud, the people below would say you were in a cloud. The people on top of the mountain would say they were in fog.

Fog can make travel dangerous. Drivers cannot see far ahead, and pilots sometimes cannot land. The deadliest fog in history was the Great Smog of London in 1952. Cold air trapped coal smoke and fog over the city for five days. Thousands of people died from breathing the polluted air.

But fog also gives life. In the Atacama Desert in Chile, almost no rain falls, yet plants and animals survive on moisture from coastal fog. Some villages even hang giant nets to catch fog and turn it into drinking water. A single net can collect hundreds of gallons in a year.

Next time you see fog drifting past a streetlight, look closely. You are watching a cloud go for a walk.