Fog is essentially a cloud that forms at or near the Earth’s surface, composed of millions of tiny water droplets suspended in the air. The formation of fog requires the air to reach saturation, which occurs when the air temperature cools to its dew point, or when enough water vapor is added to the air to raise its dew point to the air temperature. Meteorologists classify the different types of fog based on the specific physical process or mechanism that causes the air to reach this state of saturation. The four primary categories reflect whether the air is cooled or moisture is added to trigger condensation.
Radiation Fog
Radiation fog forms exclusively over land, typically during the night when the sky is clear and winds are calm. During these conditions, the ground rapidly loses heat by radiating it upward into space, a process known as radiational cooling. The air layer directly in contact with the cooled ground then loses heat via conduction, causing its temperature to drop.
This cooling process happens in situ, meaning the air is cooled where it is located, without significant horizontal movement. As the temperature of this lowest air layer falls, it eventually reaches the dew point, leading to the condensation of water vapor into fog droplets. The lack of wind allows the cold, dense air to settle and remain undisturbed, resulting in a shallow, localized fog layer often seen in valleys and low-lying areas. This type of fog is commonly known to dissipate shortly after sunrise, as the sun warms the ground and evaporates the suspended water droplets.
Advection Fog
Advection fog is created by the horizontal transport of a warm, moist air mass over a surface with a significantly lower temperature, such as a cold landmass or ocean current. The air mass cools from the bottom up as it loses heat to the cold surface through conduction, eventually reaching its saturation point.
Unlike radiation fog, advection fog requires wind to transport the air and can form under cloudy skies at any time. It is frequently observed along coastlines where warm, humid air moves inland over cooler terrain. This fog tends to be thicker and can persist for longer periods than radiation fog because the continuous movement of warm air sustains the cooling and condensation process.
Upslope Fog
Upslope fog forms when moist, stable air is forced to move upward along a gradually sloping terrain, such as a hill, mountain, or plateau. As the air mass rises due to the terrain, it expands because the atmospheric pressure decreases at higher altitudes. This expansion causes the air to cool adiabatically, meaning the cooling happens without any exchange of heat with the surrounding environment.
If the air is forced high enough, this steady cooling causes the air temperature to meet its dew point, and water vapor condenses into fog. The presence of topography is the defining factor for this type of fog, and its depth and density are directly related to the moisture content of the air and the height of the terrain.
Evaporation Fog
Evaporation fog, sometimes called “steam fog” or “sea smoke,” is unique because its formation is primarily driven by the addition of water vapor into the air, rather than just cooling the air mass. This occurs when cold, dry air moves across a much warmer body of water or moist land surface.
The temperature difference causes rapid evaporation from the warm surface into the cold air above it. The newly added water vapor instantly raises the dew point of the cold air layer. Since the cold air has a low capacity to hold moisture, the air quickly becomes saturated when the highly localized mixing of warm, moist air with the cold air takes place. This process causes the excess water vapor to condense, forming wispy plumes of fog that resemble smoke rising off the water’s surface.