Both clouds and fog are visible atmospheric phenomena formed by the same fundamental physical process: the condensation of water vapor into tiny liquid water droplets or ice crystals suspended in the air. This transformation happens when the air cools to its dew point, the temperature at which it becomes saturated with water vapor. Water molecules require microscopic particles, known as condensation nuclei (such as dust, pollen, or salt), to cluster around and form these visible masses. The difference between them is not in their composition, which is identical, but in their location relative to the ground and the cooling process that creates them.
The Critical Difference: Altitude Relative to the Ground
The most significant distinction between a cloud and fog is altitude. A cloud is a mass of condensed water vapor suspended high above the Earth’s surface, while fog is simply a cloud that is touching the ground. This difference is formalized by a specific visibility threshold used by meteorologists. For a low-lying condensation event to be classified as fog, it must reduce the horizontal visibility at the surface to less than one kilometer (0.62 miles). If visibility is greater than one kilometer, the phenomenon is classified as mist.
Distinct Cooling Processes for Formation
The difference in altitude relates directly to the distinct physical processes that cause the air to cool and trigger condensation.
Cloud Formation: Adiabatic Cooling
Clouds, suspended above the surface, primarily form through adiabatic cooling. This occurs when a parcel of air rises, expands due to lower atmospheric pressure, and cools in the process, leading to saturation and condensation high in the sky.
Fog Formation: Surface Cooling
Fog relies on cooling mechanisms that operate near the ground. A common method is radiative cooling, where the ground loses heat to space overnight, cooling the air immediately above it and causing condensation. Another mechanism is advection cooling, which happens when warm, moist air moves horizontally across a much cooler surface, such as a cold body of water, chilling the air from below.
Impact on Visibility and Meteorological Classification
The practical impact of fog is its severe reduction in visibility, which is the fundamental criterion for its definition. Because a cloud is an elevated mass, it does not directly impair ground-level visibility. The presence of fog necessitates specific meteorological warnings due to the hazard it poses to transportation. Meteorologically, fog is considered a type of Stratus cloud that has reached the surface. Stratus clouds are characterized by their flat, featureless, layered appearance and low altitude.