Advection Fog

Advection fog is a type of fog formed when warm, moist air moves horizontally (advects) over a cooler surface, causing the air to cool to its dew point and condense into tiny water droplets suspended near the ground. It is a common meteorological phenomenon in coastal regions, over oceans, and in areas where warm air masses encounter colder surfaces.

Formation Process

Advection fog develops through horizontal movement of air, unlike radiation fog, which forms due to ground cooling by terrestrial radiation. The key steps in the formation of advection fog are as follows:

1. Warm, Moist Air Movement: A mass of warm, humid air flows over a cooler surface such as cold ocean water, snow, or a cool landmass.
2. Cooling to Dew Point: As the lower layer of the warm air comes into contact with the cooler surface, it loses heat by conduction. When its temperature drops to the dew point, the air becomes saturated.
3. Condensation: Excess water vapour condenses into minute droplets, forming a layer of fog close to the surface.
4. Continuous Advection: The process is sustained as long as warm, moist air continues to move over the cooler surface, maintaining a constant supply of saturated air.

Conditions Favouring Formation

Advection fog typically forms under the following conditions:

• Presence of warm, moist air: Often from maritime or tropical sources.
• Cool underlying surface: Such as cold ocean currents, snowfields, or cool land during winter.
• Light to moderate wind: Required to transport the warm air; strong winds may mix the air too much and disperse the fog.
• Small temperature difference: Between the warm air and the cool surface, sufficient to reach saturation without triggering instability or cloud formation aloft.

Geographic Occurrence

Advection fog is commonly associated with coastal and oceanic regions where temperature contrasts between air and surface water are frequent. Major regions include:

• California Coast (United States): Warm Pacific air flowing over the cold California Current produces dense sea fog.
• Newfoundland (Canada): One of the foggiest places in the world, where warm Gulf Stream air meets the cold Labrador Current.
• British Isles: Frequent advection fogs form when moist maritime air flows over cooler land surfaces.
• Norwegian Coast and North Sea: Caused by warm Atlantic air moving over cold northern waters.
• Japan and Kuril Islands: Fog formation is common where the warm Kuroshio Current meets the cold Oyashio Current.

Types of Advection Fog

1. Sea Fog: Occurs when warm, moist air passes over a colder sea surface. It is widespread over coastal waters and can drift inland with the wind.
2. Coastal Advection Fog: Forms along coastlines when moist air from the sea moves over cooler land during night-time or seasonal temperature contrasts.
3. Land Advection Fog: Appears when warm, moist air flows over cold ground, snow, or frozen lakes — common in winter and early spring.

Characteristics of Advection Fog

• Covers extensive areas, sometimes hundreds of kilometres.
• Usually thick and persistent, lasting for several hours or even days.
• Common in all seasons but especially frequent in spring and early summer, when air–surface temperature contrasts are greatest.
• Unlike radiation fog, it can occur both day and night.
• The fog layer is generally shallow but can reduce visibility to near zero.

Meteorological Explanation

The cooling process in advection fog occurs due to horizontal heat exchange. When the air mass passes over a colder surface, heat is transferred from the air to the surface through conduction and turbulent mixing. As the air cools, relative humidity increases until it reaches 100%, leading to condensation. The fog layer remains maintained as long as moist air continues to flow over the cold surface, creating a dynamic equilibrium.

Differences Between Advection Fog and Radiation Fog

Dissipation

Advection fog can dissipate when:

• The air mass warms up, raising the temperature above the dew point.
• Wind speed increases, causing turbulent mixing that dries the air.
• Solar radiation heats the surface, evaporating the fog from below.

In some cases, however, particularly over cold ocean currents, advection fog can persist for days or even weeks, posing significant hazards to navigation and aviation.

Importance and Impacts

1. Transportation Hazards: Advection fog significantly reduces visibility over land and sea, affecting aviation, marine navigation, and road transport.
2. Climatic Indicator: The presence of frequent advection fogs indicates strong oceanic influence, stable atmospheric conditions, and the interaction of warm and cold air masses.
3. Ecological Role: In coastal deserts such as the Atacama (Chile) and Namib (Namibia), fog provides an essential source of moisture, sustaining unique ecosystems where rainfall is minimal.

Examples of Notable Advection Fog Regions

• Grand Banks of Newfoundland: Formed by the convergence of the warm Gulf Stream and cold Labrador Current.
• San Francisco Bay: Frequent during summer due to warm inland air meeting the cold Pacific.
• Namib Desert Coast: Fog from the cold Benguela Current supports desert vegetation and wildlife.