Fog is a meteorological phenomenon consisting of tiny water droplets or ice crystals suspended in the air at or near the Earth’s surface. It is essentially a cloud that reduces horizontal visibility to less than 1,000 meters (about 0.62 miles). This low-lying obstruction forms when the air’s temperature and moisture content reach a specific balance point, causing invisible water vapor to condense into a dense, observable veil.
The Science of Formation
The transformation of clear air into fog requires three conditions in the lower atmosphere, all revolving around condensation. First, the air must reach saturation, meaning the relative humidity must be at or near 100%. This occurs when the air temperature cools to the dew point, which is the temperature at which the air can no longer hold all of its water vapor in a gaseous form.
Second, the air must cool below this saturation point, forcing the excess water vapor to change phase. This cooling causes the vapor to condense into liquid droplets.
The final ingredient is the presence of microscopic airborne particles known as condensation nuclei. These tiny specks, which can be dust, smoke, salt from sea spray, or pollution, provide a surface for the water vapor to condense upon. Water molecules readily attach to these nuclei, forming the billions of minuscule droplets that collectively become visible as fog.
Primary Categories of Fog
One common type is Radiation Fog, which forms on clear, calm nights. The ground cools rapidly by radiating heat into space, which in turn cools the layer of air immediately above it to its dew point. This fog typically dissipates shortly after sunrise as the sun warms the ground and evaporates the droplets.
A second type is Advection Fog, which forms when warm, moist air moves horizontally over a cooler surface, such as a cold body of water or land. The lower layer of the moving air mass chills to its dew point. This type is common along coastlines where warm, humid air drifts over cooler coastal waters or land, and it tends to be more persistent than radiation fog.
Upslope Fog occurs when moist air is forced to rise along the side of elevated terrain, such as a hill or mountain. As the air climbs, the pressure decreases, causing the air to cool adiabatically. This cooling process eventually leads the air to its saturation point, forming fog that can extend hundreds of feet above the ground.
The final major category is Evaporation Fog, which forms when cold air moves over much warmer water. The warmer water evaporates rapidly into the cold air above it, increasing the air’s moisture content until it quickly saturates. The resulting fog is often shallow and wispy, appearing like steam rising off the water’s surface in cold conditions.
Distinguishing Fog from Similar Atmospheric Events
While fog is a suspension of water droplets near the surface, its density sets it apart from similar atmospheric phenomena like mist. The difference between fog and mist is based on visibility. If the water droplets reduce visibility to less than 1,000 meters (one kilometer), it is classified as fog.
If visibility remains at or exceeds 1,000 meters, the air is termed mist. Mist is less dense, contains fewer water droplets, and typically dissipates more quickly than fog.
Haze is a distinct condition characterized by a suspension of extremely small, dry particles, such as dust, smoke, or pollution, rather than water droplets. These solid particles scatter light, giving the air an opalescent or reddish appearance but not the damp, thick feeling associated with fog.