Radiation fog is a type of ground-level cloud that forms under specific atmospheric conditions. It manifests when the Earth’s surface cools rapidly, which in turn chills the air directly above it to its dew point. This process leads to the formation of a visible fog layer close to the ground, significantly reducing visibility.
Key Atmospheric Conditions
Clear skies are crucial, as they allow heat to escape from the Earth’s surface into space without obstruction. Calm or very light winds, typically less than 5 knots, are also necessary because stronger winds can mix the cooling air near the surface with warmer, drier air from higher altitudes, preventing fog formation. A high relative humidity ensures that only a small temperature drop is needed for the air to become saturated. Long nights, especially during autumn and winter, provide sufficient time for the ground to cool extensively, creating the sustained conditions needed for this type of fog.
Ground Cooling and Air Saturation
The formation of radiation fog begins with the cooling of the Earth’s surface. On clear nights, the ground loses heat by emitting long-wave radiation into space, a process known as radiative cooling. This cooled ground then chills the lowest layer of air through conduction, which is the direct transfer of heat by contact. As this air cools, its temperature drops until it reaches the dew point, which is the temperature at which the air becomes fully saturated with water vapor.
Visible Fog Formation
Once the air temperature reaches its dew point, water vapor undergoes a phase change. The water vapor condenses into tiny liquid water droplets. These droplets form around microscopic airborne particles, known as condensation nuclei, which can include dust, pollen, or salt. Without these nuclei, water vapor would struggle to condense into droplets, even in saturated air. The accumulation of these tiny water droplets scatters light, leading to a reduction in visibility and creating the characteristic opaque appearance of fog.
Common Occurrence Patterns
Radiation fog is often observed in low-lying areas, valleys, and depressions where cold, dense air can settle and accumulate after radiating heat. The formation typically occurs in the late evening, overnight, and early morning hours, as this period allows for maximum radiative cooling of the ground. This type of fog commonly dissipates shortly after sunrise, as the warming ground begins to heat the air above it, causing the water droplets to evaporate. While it usually clears quickly, thick instances of radiation fog, especially in winter, can persist throughout the entire day. Radiation fog is most prevalent during the fall and winter seasons due to the longer nights and cooler temperatures.