Fog doesn't "burn off" as the common saying suggests; instead, several processes contribute to its dissipation. The primary mechanism is heating from the ground, which causes the fog to lift and become low-level clouds. This is often accelerated by the sun's energy, as solar heating can lift advection fog, eventually transforming it into a layer of stratus clouds.
Mechanisms of Fog Dissipation:
-
Solar Heating: The sun's energy warms the ground, which in turn warms the air immediately above it. This warmer air becomes less dense and rises, lifting the fog droplets with it. As the fog lifts, it mixes with drier air, reducing its density and visibility. [Source: Atmospheric layers with high amounts of water vapour can help to dissipate fog by long wave radiation. Heat from the ground can lift the fog into low cloud.]
-
Wind Shear: Increasing wind speeds at higher altitudes create shear, mixing drier air into the fog. This drier air reduces the water vapor saturation, causing the fog droplets to evaporate. [Source: Increasing shear at fog top caused by increasing wind above the Fog entrains drier air into the cloud and accelerates the dissipation.]
-
Evaporation: As the air warms and mixes, the water droplets within the fog evaporate, reducing the density of the fog and leading to its dissipation. This process is enhanced by the drier air introduced through wind shear and the lifting action of solar heating.
-
Radiation: While not the primary driver, atmospheric layers with high amounts of water vapor can contribute to fog dissipation through longwave radiation.
Factors Affecting Fog Lifting Speed:
The rate at which fog dissipates varies greatly depending on several factors, including:
- Intensity of solar radiation: Stronger sunlight leads to faster warming and more rapid fog lift.
- Wind speed and direction: Higher wind speeds enhance mixing and evaporation, while wind direction influences the transport of drier air into the foggy area.
- Type of fog: Different fog types (radiation, advection, etc.) respond differently to these factors.
- Humidity: Higher humidity slows down evaporation.
The fog often dissipates gradually over a period ranging from less than an hour to several hours. [Source: Over the course of anywhere from under a hour to a few hours, the fog should gradually be eroded.]
