Fog is essentially a cloud that forms at ground level, consisting of countless microscopic water droplets suspended in the air. The interaction between falling rain and low-lying fog is surprisingly complex. Sometimes the air clears, but other times the fog seems to persist or even thicken. The outcome depends on a subtle interplay of droplet size, precipitation intensity, and the overall stability of the atmosphere.
The Physical Mechanism of Fog Clearing
The primary way rain helps to clear fog is through a process known as coalescence, also referred to as “washout” or “scavenging.” Fog droplets are incredibly small, typically about 20 micrometers in diameter, allowing them to remain suspended easily in the air. By contrast, a typical rain droplet is significantly larger, often possessing a diameter 100 times greater than that of a fog droplet.
As the larger raindrops fall, they descend through the layer of suspended fog droplets. The descending raindrops collide with the smaller fog droplets, absorbing them into their mass. This collision and merging process effectively removes the liquid water that was causing the visibility reduction. The rain carries this accumulated water to the ground, scavenging the fog from the atmosphere.
The effectiveness of this removal is directly related to the size and number of falling raindrops. Heavier rain showers, with their greater volume and velocity, are more efficient at sweeping the tiny droplets out of the air. This droplet-to-droplet interaction represents the most direct mechanism by which precipitation can clear a foggy environment.
When Rain Maintains or Intensifies Fog
While coalescence works to clear the air, not all rain is heavy enough for effective scavenging, leading to scenarios where fog persists or even worsens. This counter-intuitive effect is most often associated with very light precipitation, like drizzle or mist. If the precipitation is too fine, the droplets lack the mass and velocity needed to collide and absorb the fog particles efficiently.
Instead, as these light droplets fall through drier air layers below the cloud base, they can begin to evaporate. This process adds water vapor to the air near the ground, rapidly increasing its humidity. Since fog requires the air to be nearly 100% saturated, this added vapor can sustain the existing fog or cause more water vapor to condense, intensifying the fog’s density.
This phenomenon is sometimes referred to as “precipitation fog” or “frontal fog.” This occurs particularly when warm rain falls through a layer of cold, nearly saturated air near the surface. The evaporation raises the dew point of the cold air, pushing it past the saturation point and causing new fog to form or existing fog to thicken.
Atmospheric Conditions Required for Complete Clearing
The most complete and lasting fog dissipation rarely relies solely on the scavenging of raindrops; instead, it is driven by larger-scale atmospheric dynamics. Fog often forms in calm, stable air under a temperature inversion, such as radiation fog, which forms on clear nights when the ground cools rapidly. The key to breaking this stability is the introduction of wind and mixing.
An increase in wind speed introduces turbulence that mixes the shallow, saturated layer of fog near the surface with the drier air above it. This mixing drastically reduces the local humidity. This causes the fog droplets to evaporate back into water vapor, leading to a rapid clearing of the air.
The Role of Wind and Frontal Systems
Too little wind allows for dew formation, while very light wind can help the fog persist by mixing moisture through a shallow layer. Heavier, more organized rain is frequently associated with the passage of a weather system, such as a cold or warm front. This frontal passage brings an entirely new air mass that is often warmer or drier than the fog-laden air it replaces. When this drier air moves in, the fog droplets evaporate quickly, causing the widespread and complete dissipation often observed after prolonged rain.