What is Fog

Fog consists of tiny water droplets suspended in the air, closely resembling clouds. Essentially, both fog and clouds are comprised of minute water droplets suspended in the atmosphere. Typically, when the air cools to at least the dew point or below, fog forms near the Earth’s surface. The majority of fog formations occur due to relatively cold surfaces or when the surface temperature drops below the dew point of the air aloft, resulting in fog formation.

The Definition of Fog

Fog is defined as the suspension of very small water droplets or ice crystals in the air, leading to horizontal visibility of less than 1000 meters. It forms through the cooling of air by contact mixing or, at times, through the saturation of air by an increase in water content. When the air cools to the extent that the relative humidity exceeds 90%, mist thickens into fog.

Formation of Fog

The formation of fog primarily arises from the cooling of air that adjusts to the Earth’s surface conditions. Occasionally, fog may form due to water evaporation into air warmed by the surface. These two types of fog formation, namely cooling-induced and evaporation-induced, account for most fog occurrences. During calm and clear nights, the ground releases heat through outgoing radiation, causing the air in contact with it to cool. Under sufficiently humid conditions, this cooling brings the air to its saturation point, resulting in fog formation. Similarly, when air flows towards colder regions, it relinquishes heat to the underlying surface, potentially leading to fog development.

Types of Fog

Various types of fog may form under different conditions:

a. Radiation Fog

Radiation fog develops primarily from ground cooling during the night. In some instances, radiation fog may occur even before saturation is reached, with a relative humidity of 80%. Favorable conditions for radiation fog formation include high relative humidity, clear nights facilitating surface heat loss through radiation, and light winds confining cooling to the surface layer.

b. Advection Fog

Advection types of fog occurs when warm air passes over a colder surface, such as land or sea, with the surface temperature below the dew point of the moving air. This phenomenon is common in winter after cold spells, particularly when milder air from the sea encounters cooler land, leading to further cooling and fog formation.

c. Sea or Steam Fog

Sea or steam fog typically occurs when very cold air traverses over a relatively warm sea surface, often observed in polar regions. As the cold air passes over the warmer sea, evaporation occurs, saturating the air mass. When the air cannot contain all the water vapor, condensation transpires, forming thick fog.

d. Frontal Fog

Frontal fog arises either from cold air descending to the surface during a front passage, common over hills, or from saturation due to continuous rainfall. It may manifest as a broad belt spanning hundreds of miles along the front’s path.

Diurnal and Seasonal Variation of Radiation Fog

While the lowest night temperature typically occurs around dawn, maximum radiation fog tends to occur about an hour after sunrise due to light winds facilitating turbulence, which spreads cooling through a deeper layer. Some turbulence is essential for fog formation. Additionally, the winter half of the year is more prone to radiation fog, although autumn experiences significant fog occurrences due to the seasonal cooling of the land.

Dispersal of Fog

Fog dispersal can occur through various means:

a. Dispersal of Radiation Fog

In winter, radiation fog often dissipates late in the night or early morning due to increased insulation during the day. Higher wind speeds aid in dispersing fog by lifting it into low stratus clouds. However, the presence of early morning cloud cover may impede fog clearance by reducing insulation.

b. Dispersal of Advection Fog

Advection fog over the sea experiences minimal diurnal variation and typically disperses with changes in air mass or increased wind speeds. Fog formation poses challenges in aviation, with historical methods of fog dispersal involving the use of heat-producing burners to clear airfields. This method, while effective, has become rare due to its high cost.

Haze (HZ)

Haze refers to the suspension of extremely small dry particles in the air, invisible to the naked eye but numerous enough to give the air an opalescent appearance, reducing horizontal visibility to 5000 meters or less.