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DECATUR, Ill. (WAND) - Dictionary.com defines fog as: a cloudlike mass or layer of minute water droplets or ice crystals near the surface of the earth, appreciably reducing visibility.

Not all fog is created equal, though. Did you know there are several different types of fog? 

There's radiational fog, which occurs on clear, calm nights when the ground cools rapidly after sunset, cooling the air to its dew point temperature. This causes water vapor to condense into tiny droplets. 

There is also advection fog, which happens when warm, moist air moves over a cooler surface like cold ocean water or snow-covered ground. The cooling of the air to its dew point results in fog. 

How about upslope fog? This type of fog forms when moist, stable air is pushed up a hill or mountain. The air expands and cools as it gains elevation, reaching its saturation point.   

Valley fog develops when cooler, dense air sinks to the valley floor under calm nighttime conditions, cooling the air to saturation.

Don't forget steam fog. This fog forms when cold, dry air passes over a warm body of water. Moisture evaporates from the water into the air and condenses, looking like rising smoke.  This is common in fall or winter over lakes and rivers.

Precipitation fog comes about when warm rain falls through cooler, drier air. The rain evaporates and saturates the cooler air near the ground. 

Frontal fog develops when warm, moist air is lifted over a cooler air mass at a warm front, causing the moisture to condense into a cloud, or fog.

Freezing fog forms like typical fog but in sub-freezing temperatures. The water droplets remain liquid (supercooled just barely below 32 degrees) until they touch a surface, freezing instantly on contact. 

And, finally there is ice fog. This fancy fog occurs in extremely cold weather, typically below -25°F, where water vapor turns directly into tiny ice crystals suspended in the air. 

Goodness, that's a lot of fog! Tonight's version is the most common type: radiation fog.  

Stay safe, stay sweaty and stay weather-aware. 

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