Weather 101: How is fog formed?

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We have a great question this week about something we see a lot of in this area! Eric asked how fog is formed. There are five main ways fog forms, but some types are more common in certain locations or at different times of the year.

The first type is radiation fog. As the land cools overnight, the air right above the ground starts to cool, causing condensation and fog to form. This type of fog usually forms on calm clear nights, and most often in the winter.

The second type of fog is evaporation fog. This happens when cool air moves over a warm body of water or moist ground. Some of the warm water evaporates into the air above, causing it to rise and cool, which creates condensation and fog.

The third type of fog is upslope fog. When the wind blows air up a mountain, the air cools as it rises, causing the moisture to condense, which leads to fog.

The fourth is valley fog. This forms as cold, dense air – which is heavier than the surrounding air – sinks into a valley and condenses, forming fog. This happens very often in the Shenandoah Valley. The air gets stuck between the mountains, which can sometimes cause it to last for several days.

The last is advection fog. This happens as warm, moist air passes over cool water or land. The colder surface cools the air as it passes over, causing condensation and fog. This is a famous phenomenon in the San Francisco Bay; you've probably seen pictures of dense fog at the Golden Gate Bridge. This is also common when a warm front brings warmer air over snow cover in the winter time.