Fog and regulation of the global climate

Definition

Fog is a cloud near the ground. Fog forms as the temperature of the air falls below its dew point, and condensation occurs. Fog also forms and dissolves rapidly, depending on whether the temperature is higher or lower than the dew point. Fogs are named based on the specific process through which the humidity has reached its saturation point, causing the fog to form.

Radiation fog forms as air temperature falls below the dew point through rapid radiation cooling on a clear, calm night. The cool and heavy air containing this type of fog often settles in low-lying areas, such as mountain valleys. Most inland fogs are radiation fogs. Advection fog usually forms in coastal regions as warm air blows over a cold surface and loses heat to the underlying surface. Upslope fog forms as air blows over mountain slopes, becomes cool, and reaches saturation.

Evaporation fog occurs as cold, dry air blows over warm, moist surfaces. Thermal and humidity gradients between the two entities facilitate rapid evaporation from the surface. As the warmer, moist air from the ground mixes with cold, dry air above, it reaches saturation and forms fog. Steam fog over lakes during late fall and early winter is a typical form of evaporation fog.

Significance for Climate Change

Fog, as a surface-level cloud, has roughly the same climatic effects as low-level clouds. Fog dictates air and surface temperature patterns by controlling the amount of radiation that enters and leaves the surface. High moisture content imparts fog with great thermal inertia. Thus, once a fog has formed, it will prevent the air temperature from changing. In the morning, fog’s high albedo allows less incoming solar radiation to reach the surface. Thus, morning fogs keep air temperatures low, allowing only gradual increases until the fog evaporates. As a result, fogs decrease daytime high temperatures.

In the night, fog traps outgoing longwave radiation and prevents air temperature from dropping as it would on a clear night. Fog also potentially warms the air near the surface, because condensation is a warming process. Overall, however, fog has a greater cooling effect than a warming effect on Earth. This increased fog may affect humans by reducing visibility and by trapping air pollutants to form smog. Fog can also produce precipitation in the form of rain, drizzle, and light snow. As global climate change advanced throughout the 2010s and 2020s, coastal fog levels declined. Into the mid-2020s, observed trends and model projections pointed to decreases or complex, region-specific changes in fog occurrence.

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Keim-Vera, K., et al. "Fog Types Frequency and Their Collectable Water Potential in the Atacama Desert." Atmospheric Research, vol. 312, 2024, p. 107747. ScienceDirect, doi.org/10.1016/j.atmosres.2024.107747. Accessed 25 Sept. 2025.

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