Radiation fog is a common type of fog that forms on clear nights, particularly during fall and winter in the United States. It occurs when the ground cools rapidly through radiative cooling, lowering the air temperature near the surface. This cooling causes the air to reach its dew point, leading to the condensation of water vapor into tiny water droplets, forming fog. This process is most effective under calm, clear skies with light winds.
How Radiation Fog Forms: A Step-by-Step Explanation
- Radiative Cooling: On a clear night, the Earth's surface loses heat through radiation, cooling down significantly.
- Ground-Level Cooling: This cooling effect extends to the air immediately above the ground.
- Dew Point Saturation: As the air cools, it reaches its dew point—the temperature at which the air becomes saturated with water vapor.
- Condensation and Fog Formation: At the dew point, excess water vapor condenses onto tiny particles in the air (aerosols), forming microscopic water droplets that create fog.
Characteristics of Radiation Fog
- Time of Occurrence: Typically forms overnight and dissipates as the sun warms the ground in the morning.
- Location: Most prevalent over land areas, especially in valleys or low-lying areas where cold air settles.
- Weather Conditions: Requires clear skies, light winds, and high humidity.
- Appearance: Often appears as a shallow layer of fog near the ground; sometimes called "ground fog" or "shallow fog". It may lift during the day as the ground warms.
Examples and Practical Insights
- Pacific Coast Fog: The combination of a cool, moist layer of air close to the surface, clear skies above and light winds can result in exceptionally thick radiation fog, as seen in certain coastal regions.
- Winter Fog in Valleys: Radiation fog frequently forms in valleys due to the accumulation of cold, dense air.
The Role of Aerosols
The presence of aerosols (tiny particles in the air) significantly impacts the formation of radiation fog. These particles act as nuclei for water vapor condensation, allowing fog to form more readily. Studies show that anthropogenic (human-caused) emissions contribute to the formation of this type of fog in coastal areas. Moreover, the interaction between aerosols and fog can affect the accuracy of fog forecasts.
