Fog is essentially a cloud that forms at or near the Earth’s surface, consisting of tiny suspended liquid water droplets that reduce visibility. This stable atmospheric condition forms when air cools to its dew point and reaches 100% relative humidity, causing water vapor to condense. Fog persists with calm conditions and high moisture, but nature dissipates it through processes that evaporate the liquid water, physically move the fog mass, or cause the droplets to fall out of the air column.
Evaporation Through Solar Heating
The most common way fog disappears is driven by the sun’s energy warming the surface below the fog layer. As the sun rises, solar radiation penetrates the fog and is absorbed by the ground. This absorbed energy warms the Earth’s surface, which then heats the air layer closest to it.
This warming disrupts the fog’s stability. As the air temperature rises, its capacity to hold water vapor increases, causing the relative humidity to drop below 100%. The liquid water droplets then undergo a phase change, turning back into invisible water vapor. This process effectively makes the droplets disappear, clearing the air from the ground upward.
This upward heating creates convection, where rising pockets of warmer, drier air mix with the saturated fog layer. The fog dissipates first at the bottom, sometimes leaving a layer of stratus cloud above the surface. This thermal action must raise the air temperature above the dew point throughout the entire depth of the fog to completely clear the sky.
Dispersion via Wind and Air Mixing
Wind removes fog by physically moving it and mixing it with drier air from adjacent areas or higher altitudes. Fog forms in a stable, shallow layer near the ground with little vertical movement. Even a light breeze introduces turbulence, which breaks this stability.
This turbulence causes the fog layer to mix vigorously with drier air, often found just above the fog or in nearby clear areas. Since this surrounding air has a relative humidity less than 100%, its introduction causes the fog droplets to evaporate quickly, a process known as “mixing out.” Stronger winds are effective because they generate more turbulence and entrain a greater volume of dry air.
Horizontal movement, known as advection, also dissipates fog by moving the entire mass out of an area. If the wind carries the fog over a significantly warmer or drier surface, the suspended droplets gradually evaporate. The combination of horizontal movement and vertical mixing ensures the fog is actively broken down, not simply relocated.
Removal by Droplet Coalescence
Fog removal can occur through the physical process of droplet coalescence and sedimentation. Fog droplets are incredibly small, typically measuring between 1 and 10 micrometers in diameter, allowing them to remain suspended in the air. Coalescence occurs when these tiny droplets collide and merge, forming increasingly larger and heavier drops.
When these merged droplets grow large enough, often reaching the size of drizzle or small raindrops, gravity overcomes the forces keeping them suspended. They then fall to the ground or onto vegetation, sometimes called “fog drip.” This process removes the liquid water content from the air column. This natural precipitation clears the air by physically depositing the water content.