What is Evaporation Fog?
Evaporation fog, also known as cold advection fog, forms when cold, stable air moves over a warmer body of water or moist land. The warmer surface causes water to evaporate into the colder air. This adds enough water vapor to reach saturation, resulting in the formation of fog.
- Cold air mass: A mass of cold, relatively dry air is present.
- Warm water/land: This cold air moves over a surface significantly warmer than itself, such as a lake, ocean, or even moist land.
- Evaporation: The warmer surface causes rapid evaporation of water into the overlying cold air.
- Saturation: The addition of water vapor increases the air's humidity. When the air becomes saturated (it can't hold any more water vapor), condensation occurs.
- Fog formation: Tiny water droplets or ice crystals form, creating the visible fog.
Types of Evaporation Fog
There are two primary types of evaporation fog:
- Steam fog: This forms when very cold air moves over much warmer water. The difference in temperature causes significant evaporation, resulting in a visible "steam" effect as the water vapor quickly condenses in the cold air. Think of seeing your breath on a cold day—it's a similar principle on a larger scale. (National Snow and Ice Data Center)
- Frontal fog: This occurs when warm rain evaporates into a cooler, drier layer of air near the ground. This often happens along weather fronts where warm air is lifted over cold air. (Weather.gov)
Examples and Practical Insights
- Evaporation fog is common in coastal areas where cold air masses move over warmer ocean currents.
- It can also form over rivers, lakes, or even moist ground on cold mornings.
- Freezing fog, a type of evaporation fog, can occur when the temperature is below freezing, resulting in the formation of ice crystals. (Met Office)
- The fog in the valleys of New Zealand, pictured in the USGS gallery, exemplifies evaporation fog. (USGS)
Evaporation fog is a fascinating meteorological phenomenon that demonstrates the interplay between temperature, humidity, and air movement. Understanding its formation helps predict weather conditions and understand the processes that govern our atmosphere.
