Fog is scientifically a type of cloud, specifically one whose base is at or very near the Earth’s surface. This visible atmospheric phenomenon is a mass of tiny water droplets or ice crystals suspended in the air, exactly like any other cloud. The distinction between fog and a cloud is based solely on location, not composition or formation process. Fog represents a state where the lowest layer of the atmosphere has become saturated, causing invisible water vapor to condense into a visible form.
How Meteorological Science Classifies Fog
Meteorologists classify clouds based primarily on their altitude and appearance. Clouds are divided into high, middle, and low categories, with the low-level clouds generally existing below 6,500 feet. Fog fits directly into the low cloud classification, as it is structurally similar to a stratus cloud layer. A stratus cloud is characterized by its flat, featureless, and uniform gray appearance, and fog is essentially a stratus cloud that is resting on the ground.
The primary factor distinguishing fog from a low cloud is the visibility it creates. By international meteorological convention, the phenomenon is classified as fog only when the horizontal visibility is reduced to less than 1 kilometer (0.62 miles). If the visible moisture reduces visibility to between 1 and 2 kilometers, it is typically called mist. The altitude of the cloud base also plays a role, as a low stratus cloud is considered fog when its base descends to ground level.
If you are standing on a mountain and a cloud passes over you, you are technically experiencing fog. The official classification is tied to whether the visible vapor is touching the ground where the observer is located. Fog is defined by its proximity to the surface, regardless of the elevation of that surface above sea level. As the day warms, fog may lift off the ground, transitioning into a low-lying stratus cloud.
The Physics of Ground-Level Condensation
The formation of fog is driven by the general principle of condensation, where air becomes saturated and cools to its dew point. The dew point is the temperature at which the air must be cooled to achieve 100% relative humidity, forcing water vapor to change from an invisible gas into liquid water droplets. This phase change requires the air temperature and the dew point temperature to become nearly equal.
Simply reaching the dew point is not enough; the condensation process also requires Cloud Condensation Nuclei (CCN). These microscopic airborne particles, such as sea salt, dust, or pollution, act as surfaces upon which water vapor can condense. Without these nuclei, the air would need to reach a state of supersaturation far exceeding 100% relative humidity for droplets to form spontaneously. The presence of CCN allows water vapor to condense efficiently, forming the visible aerosol we recognize as fog.
Fog formation typically involves non-adiabatic cooling mechanisms, distinguishing it from high cloud formation. High clouds often form through adiabatic cooling, which occurs as air rises and expands. In contrast, fog usually forms near the surface through processes like radiational cooling or advection. These processes cool the air without significant vertical movement, allowing the water droplets to remain suspended at or near the ground within the boundary layer of the atmosphere.
Primary Categories of Fog
Fog types are categorized based on the specific mechanism that causes the air to cool to its dew point. Radiation Fog is one of the most common types, forming on clear, calm nights when the ground rapidly loses heat by radiating it into space. The cooling ground then chills the air immediately above it through conduction, causing moisture to condense. This type of fog is often shallow and dissipates quickly after sunrise as the ground warms.
Advection Fog forms when warm, moist air moves horizontally over a cooler surface. This cooling from below causes the air mass to reach saturation. This frequently occurs along coastlines where humid air from the ocean moves over colder waters or land. Advection fog tends to be thicker and more persistent than radiation fog, as it is not dependent on overnight cooling.
Upslope Fog occurs when moist air is forced to rise up the side of a hill or mountain. As the air gains altitude, it expands and cools adiabatically, lowering the temperature until the water vapor condenses. This process creates a persistent layer of fog that clings to the terrain.
Steam Fog, also called evaporation fog, forms when cold, dry air passes over a much warmer water surface. Water evaporates from the warm surface into the cold air, quickly saturating the air mass. This causes the vapor to condense immediately into wisps of fog.