Fog is a meteorological phenomenon where a cloud forms at ground level, reducing visibility to less than 1 kilometer (0.62 miles). It consists of tiny water droplets or ice crystals suspended in the air, creating a thick, white or gray mist. Unlike higher clouds, fog forms at the surface and can have a significant impact on transportation, agriculture, and daily life.
Places That Frequently Experience Foggy Conditions
Some regions of the world are prone to fog due to specific climate and geographical factors. Notable foggy locations include:
- San Francisco, USA: Coastal fog results from the cool Pacific Ocean currents meeting warmer inland air.
- London, UK: Known historically for its fog, though pollution-related “pea soupers” have declined.
- Grand Banks, Newfoundland: A convergence zone of warm and cold ocean currents, creating frequent fog.
- Namib Desert, Namibia: Coastal fog provides a vital source of moisture in this arid region.
- Atacama Desert, Chile: One of the driest places on Earth, but experiences coastal fog (locally called “camanchaca”).
- Mount Washington, USA: Experiences dense fog frequently due to its elevation and position.
What Is the Difference Between a Cloud and Fog?
Simply put, a fog is a specific type of cloud that forms only near the ground. Clouds, in general, form at a variety of altitudes, from the ground all the way up to 12 miles above the Earth’s surface.
Mist, Virga, Haze, and Smog
- Fog vs. Mist: Both are ground-level phenomena, but fog is denser and reduces visibility to less than 1 km, while mist allows for visibility between 1 and 2 km. Mist contains fewer water droplets than fog.
- Fog vs. Virga: Virga refers to precipitation that evaporates before reaching the ground. Fog doesn’t involve precipitation but consists of moisture in the air.
- Fog vs. Haze: Haze results from fine particles like dust or pollutants in the air. Like fog, haze reduces visibility.
- Fog vs. Smog: Smog is a combination of fog and and pollutants.
How Fog Forms
Fog forms when the air near the ground cools and the moisture in it condenses.
- Cooling of air to its dew point: As the air temperature drops to the dew point, water vapor condenses into tiny droplets. This process occurs through radiative cooling; upslope flow where pressure decreases and temperature drops; advection of warm, moist air over a cooler surface; or mixing of two different air masses.
- Addition of moisture: Evaporation (from a body of water, wet surface, etc.) adds moisture and can saturate the air, leading to fog formation.
Factors that favor fog formation include:
- Calm, windless nights
- Clear skies (promote radiational cooling)
- High humidity
- Proximity to water bodies (lakes, oceans)
- Areas prone to temperature inversion
In contrast, some factors discourage fog formation by removing moisture, heating air, mixing moist air with drier air:
- Strong winds
- Precipitation or formation of heavy cloud cover
- Very dry conditions
- Warming during the day
Fog is more common at night than in the day, but it can occur at any time when the conditions are right. It lasts anywhere from a few minutes to several hours. Fog often dissipates after sunrise as the air warms.
How Fog Dissipates
Fog typically dissipates when:
- The sun heats the ground, warming the air and causing the fog droplets to evaporate.
- Winds increase, mixing the foggy layer with drier air from above.
- Dry air moves in, reducing humidity and preventing further condensation.
Types of Fog
There are several types of fog:
- Radiation Fog (Ground or Valley Fog):
Forms on clear nights with little wind, when the ground loses heat through radiation, cooling the air near the surface to its dew point. Common in valleys and low-lying areas.
Example: California’s Central Valley. - Advection Fog:
Occurs when warm, moist air moves over a cooler surface (land or water), causing condensation. It often forms along coastlines and is persistent.
Example: San Francisco Bay fog. - Upslope Fog:
Forms when moist air ascends a mountain slope, cooling as it rises and condensing into fog.
Example: Appalachian Mountains, USA. - Evaporation Fog (Steam Fog):
Created when cold air moves over a warmer water surface, causing evaporation and immediate condensation of moisture into fog.
Example: Over lakes in autumn when cold air sweeps over warm water. - Freezing Fog:
Occurs when supercooled water droplets remain in the fog and freeze upon contact with surfaces, forming rime ice.
Example: Occurs in cold regions like Alaska or Scandinavia. - Ice Fog:
Forms in extremely cold temperatures, where moisture in the air directly forms ice crystals.
Example: Arctic regions like Siberia and Alaska. - Valley Fog:
Forms in valleys, often as a result of cold air settling due to its higher density.
Example: The Shenandoah Valley, USA. - Frontal Fog:
Associated with weather fronts, particularly warm fronts, where warm, moist air overrides cooler air, leading to condensation.
Example: Along cold front boundaries in temperate regions.
Forecasting Fog: Key Considerations
When forecasting fog, meteorologists consider several factors:
- Temperature and Dew Point: A small gap between these values signals higher potential for fog formation.
- Wind Speed: Calm or light winds favor fog formation. Strong winds mix the atmosphere, preventing fog.
- Cloud Cover: Clear skies at night favor radiational cooling, which promotes fog formation.
- Terrain and Proximity to Water: Low-lying areas and proximity to bodies of water increase the chances of fog.
- Pre-existing moisture: Recent precipitation increases the chances of fog formation, as evaporation from damp ground increases moisture in air.
The Effect of Fog on Sound
Fog has a noticeable effect on sound transmission. The water droplets in fog scatter and absorb sound waves, so sound travels shorter distances and appears muffled. High humidity in fog bends sound waves (refraction), sometimes making distant sounds seem closer than they are.
The effect of fog on sound depends on its frequency. In foggy conditions, lower-frequency sounds (like those with lower pitch) travel farther and are less affected by scattering, while higher-frequency sounds (with higher pitch) scatter more easily and are absorbed by the water droplets in the fog.
This is why fog horns typically produce low-pitched sounds. These low-frequency tones cut through the fog more effectively, so they can be heard at greater distances.
Fogbow
Fog sometimes produces a rare optical phenomenon called a fogbow. A fogbow is like a rainbow, but it appears white or very pale due to the smaller size of the fog droplets.
Ecological Role of Fog
Fog plays a crucial role in many ecosystems, particularly in regions with limited rainfall. Fog acts as a water source for plants, animals, and even human communities.
- Fog as a Water Source: In coastal deserts like the Namib or Atacama, plants and animals depend on fog for survival. Certain plant species, like fog-harvesting lichens and fog-dependent trees (e.g., the redwood forests of California), rely heavily on fog to meet their water needs.
- Fog Nets: In regions where fog is abundant but rainfall is scarce, humans have developed fog nets to collect water. These large nets capture droplets from passing fog, which then condense and drip into collection troughs, providing potable water for communities. Fog nets are used in places like Chile, Peru, and Morocco.
References
- Gultepe, Ismail, ed. (2008). “Fog Visibility and Forecasting”. Fog and Boundary Layer Clouds. Springer. ISBN 978-3-7643-8418-0.
- Gultepe, I.; et al. (2007). “Fog Research: A Review of Past Achievements and Future Perspectives.” Pure and Applied Geophysics. 164: 1121-1159. doi:10.1007/s00024-007-0211-x
- Lu, Chunsong; Liu, Yangang; et al. (2014). “Examination of microphysical relationships and corresponding microphysical processes in warm fogs”. Acta Meteorologica Sinica. 27 (6): 832–848. doi:10.1007/s13351-013-0610-0
- Riddle, Laurence G.; Cayan, Daniel R.; Filonczuk, Maria K. (1995). “Variability of Marine Fog Along the California Coast“. Scripps Institution of Oceanography.