There are actually five recognized types of fog, named according to how they form, not four: radiation fog, advection fog, evaporation fog, valley fog, and upslope fog.
Types of Fog Formation
Fogs are classified based on the processes that lead to their formation. These processes involve cooling air to its dew point or adding moisture to the air near the surface. Here's a detailed look at each type:
1. Radiation Fog
- Formation: This type of fog forms overnight when the ground cools through radiation, chilling the air directly above it to its dew point.
- Common Conditions: Occurs on clear nights with calm winds and moist air.
- Example: Commonly seen in valleys and low-lying areas.
2. Advection Fog
- Formation: Forms when warm, moist air moves horizontally over a colder surface. The cooling of the warm air to its dew point results in fog.
- Common Conditions: Often occurs near coastlines or large bodies of water where temperature differences are significant.
- Example: Coastal fog along the Pacific coast.
3. Evaporation Fog
- Formation: This happens when water evaporates into cooler air, increasing the humidity and causing saturation and the formation of fog.
- Common Conditions: Frequently occurs over warm water bodies on cold days when the air above the water is significantly colder.
- Example: Steam fog rising from lakes and rivers in the winter.
4. Valley Fog
- Formation: A type of radiation fog that develops in valleys. Cold, denser air settles into the valley bottom, allowing fog to form.
- Common Conditions: Occurs during clear, calm nights.
- Example: Fog seen in mountain valleys during the early morning.
5. Upslope Fog
- Formation: Forms when moist air is forced to rise over sloping terrain, such as a mountain range. As the air rises, it cools, leading to condensation and fog formation.
- Common Conditions: Often observed on the windward side of mountain slopes.
- Example: Fog on the eastern side of the Rocky Mountains.
| Fog Type | Formation Process | Common Conditions |
|---|---|---|
| Radiation Fog | Ground cooling through radiation chills adjacent air. | Clear nights, calm winds, moist air. |
| Advection Fog | Warm, moist air moves over a cold surface. | Near coastlines or large bodies of water. |
| Evaporation Fog | Water evaporates into cooler air, increasing humidity. | Warm water bodies on cold days, air above is much colder. |
| Valley Fog | Cold air settles in valleys, a type of radiation fog. | Clear, calm nights in valleys. |
| Upslope Fog | Moist air is forced up a slope, causing cooling. | Windward side of mountain slopes. |
