Fog is scientifically classified as a cloud, specifically a stratus cloud that forms directly at the Earth’s surface. This visible atmospheric phenomenon is a mass of condensed water vapor suspended in the air, creating a dense layer that impacts ground-level visibility. While the physical makeup is identical to high-altitude clouds, fog is distinguished by its location and formation processes near the ground.
Composition: The Shared Identity
The physical makeup of fog and clouds is the same, consisting of countless tiny liquid water droplets suspended in the atmosphere. When the air temperature cools to the dew point, water vapor reaches 100% relative humidity and begins the process of condensation.
This transformation requires microscopic airborne particles known as Cloud Condensation Nuclei (CCNs) to act as a surface for the water vapor. These particles, typically dust, salt from ocean spray, or soot, are essential for both fog and cloud formation. Without these nuclei, water vapor would need to reach extreme supersaturation, which rarely occurs naturally. The resulting water droplets are small enough (around 0.02 millimeters in diameter) that they remain suspended by air currents rather than falling as precipitation.
Altitude: The Defining Difference
The primary difference between fog and a cloud lies in the height of the condensed water mass relative to the ground. Fog is simply a cloud layer whose base is in contact with the Earth’s surface. A cloud is an elevated mass of these same water droplets with its base located above the ground.
For meteorological and practical purposes, the distinction is based on visibility standards. An atmospheric layer is officially termed fog only when horizontal visibility is reduced to less than 1 kilometer (0.62 miles). If the visible water droplets reduce visibility but remain above this threshold, the phenomenon is classified as mist or haze. This definition is important for ground transportation and aviation safety, where clear visual range is paramount.
Specific Mechanisms of Fog Creation
The formation of fog near the ground is typically driven by cooling or by adding moisture, both of which cause the air to reach saturation. Unlike high-altitude clouds that often form from rising air that cools adiabatically, fog usually results from processes that cool the air horizontally or vertically across a shallow layer. These distinct mechanisms give rise to several types of fog.
Radiation Fog
Radiation fog forms on clear nights with light winds when the ground rapidly loses heat. This cooling ground then chills the air layer immediately above it through conduction, causing the temperature to drop to the dew point. Since this process requires heat loss from the ground, radiation fog is common inland and rarely forms over large bodies of water. This type of fog is often shallow and usually dissipates quickly after sunrise as the ground begins to warm.
Advection Fog
Advection fog occurs when warm, moist air moves horizontally across a colder surface. As the air travels over the cold surface (such as a cool ocean current or snow-covered ground), the air is chilled from below. This cooling lowers the air temperature to the saturation point, causing widespread condensation. Advection fog can cover vast areas, persist for long periods, and is most commonly observed along coastlines where warm air meets cold land.
Steam Fog
Steam fog, also called evaporation fog, forms when cold, dry air moves across a much warmer water surface. The warm water evaporates rapidly into the overlying cold air, quickly adding moisture and raising the dew point. The moist layer immediately cools and condenses to form a shallow layer of fog that often appears as wisps rising from the water. This phenomenon is frequently observed over lakes and rivers during colder months.