Understanding clouds and fog

Clouds are usually the most obvious feature of the sky. They both reflect weather patterns and play a role in what the weather does. In addition to their obvious role as sources of precipitation, clouds also can affect the temperatures of the places below them. Clouds not only block incoming sunlight during the day, which cools the air, but they can also block outgoing radiation from the Earth, which can warm temperatures. Many unanswered questions about Earth's climate revolve around the roles of clouds. Clouds also create some patterns of light in the sky such as halos.

Fog forms in the same way as clouds. In fact, fog is a cloud that is on the ground, or with its bottom very near the ground.

Different kinds of fog, where fog forms

Advection fog forms when humid air flows over cold ground or water.

Radiation fog forms on generally clear, cool nights.

Precipitation fog forms when rain or snow falls.. As precipitation falls into drier air below the cloud, the liquid drops or ice crystals evaporate or sublimate directly into water vapor. The water vapor increases the moisture content of the air while cooling the air. This often saturates the air below the cloud and allows fog to form.

Upslope fog is very common along large hills and mountains. It forms when winds blow up the side of a hill or mountain, which cools the air.

Valley fog forms in mountain valleys during winter and can be more than 1,500 feet thick. Often, the winter sun is not strong enough to evaporate the fog during the day. When the air cools again the following night, the fog often becomes thicker, which makes it even harder for the sun to burn it off the following day. These fogs can last for several days until strong winds blow the moist air out of the valley. The tendency for cool, dense air to pool at the bottom of valleys also enhances valley fog.

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