Fog is a common weather event, but its relationship with rain is nuanced. Many people assume rain automatically clears a foggy sky, and while this can happen, the process depends on specific atmospheric conditions. Understanding how fog forms and how fog droplets interact with raindrops explains this relationship.
Understanding Fog Formation
Fog is essentially a cloud that forms near the ground surface. It consists of countless microscopic water droplets suspended in the air. Fog forms when the air cools to its dew point—the temperature at which it becomes saturated with water vapor. This cooling causes the invisible vapor to condense into visible liquid droplets. These fog droplets are typically less than 100 micrometers in diameter, making them light enough to be held aloft by slight air movements, unlike much larger raindrops that fall due to gravity.
The Washout Effect
The primary mechanism by which rain clears fog is the “washout effect,” also known as scavenging or coalescence. This process involves the collision and merging of suspended fog droplets with much larger, falling raindrops. A single raindrop, which can be several millimeters in diameter, sweeps through the air and collects numerous smaller fog droplets in its path. As the fog droplets stick to the raindrop’s surface, the raindrop’s mass and volume increase. This action accelerates the removal of moisture from the air, carrying the water to the ground and reducing the concentration of suspended particles. This scavenging action is highly efficient because the size difference prevents fog droplets from moving out of the raindrop’s way. Once the concentration of these microscopic droplets falls below the threshold for visibility reduction, the fog dissipates.
Variables Affecting the Clearing Process
Rain does not always succeed in clearing fog, as the outcome depends on the characteristics of both the rain and the fog layer. Primary is the rain’s intensity; light drizzle or very fine rain may not be sufficient to create a strong washout effect. Heavy, sustained rainfall is required for efficient scavenging, ensuring enough large raindrops fall to absorb the fog particles. The density and depth of the fog layer also play a role. An extremely thick or deep layer may persist if the rain clears only the lowest section, or if the rain formation process introduces new moisture. Temperature changes caused by the rain can also counteract clearing. If the rain causes the air temperature to drop further while the air remains saturated, it may prolong or thicken the fog by encouraging more condensation.
Non-Rain Related Dissipation
While rain is one way fog dissipates, the most common mechanisms do not involve precipitation.
Solar Heating
One frequent cause of dissipation is solar heating, or “burning off.” As the sun rises, the ground and nearby air warm, causing the tiny water droplets to evaporate back into invisible water vapor.
Wind and Mixing
Wind is another powerful factor, introducing air from adjacent areas. Moderate wind causes turbulent mixing, blending the saturated, foggy air with drier air masses. This mixing reduces relative humidity below the saturation point, causing the fog droplets to evaporate. Stronger winds may simply lift the fog layer off the ground, transforming it into a low-lying stratus cloud.
Advection
The movement of air masses, known as advection, can also cause dissipation. If a fog bank moves horizontally over a warmer surface, heat transfer warms the fog layer enough to cause evaporation. Similarly, a new, drier air mass moving into the region can replace the saturated air, ending the fog event.