Fog

Fog is a meteorological phenomenon that occurs when water vapor in the air near the Earth’s surface condenses into suspended tiny water droplets or ice crystals due to the air layer reaching its dew point temperature or gaining additional moisture through evaporation. Lakra and Avishek define fog as an atmospheric boundary layer phenomenon. Fog forms when water vapor near the Earth’s surface condenses into suspended water droplets or ice crystals.

This phenomenon, which reduces horizontal visibility to below one kilometer, is also described as a stratus cloud that has descended to ground level. These microscopic droplets formed by condensation arise when the air in contact with the Earth’s surface becomes saturated and occasionally produces very light drizzle.

Fog is an important component of the atmospheric moisture cycle and holds strategic significance for both ecological balance and transportation safety. Local topography, marine influence, humidity levels, and wind conditions determine the frequency of fog formation. Meteorological analyses in Türkiye indicate that radiation fog is the most common type and develops during calm, humid, and cold nights. Therefore, accurate forecasting of fog is a crucial meteorological tool for aviation, transportation, agriculture, and public health planning.

Fog is gaining increasing importance due to the economic and social losses caused by reduced visibility. With growing air, land, and maritime traffic as well as solar energy production, fog is a critical factor for both safety and efficiency. However, due to the complexity of physical processes and the scarcity of observational data, it remains one of the most difficult meteorological phenomena to predict.

Formation

Fog arises through condensation or evaporation processes. It develops when a moist air layer reaches its dew point under various local or regional meteorological conditions. During this process, the air must be highly humid, close to saturation, and generally calm.

Fog formation occurs under conditions of high relative humidity, low wind speed, and air temperature near the dew point. This condition is completed microphysically by the accumulation of water vapor on condensation nuclei (aerosols).

Three essential elements are required for fog to form in the atmosphere:

1. Relative humidity above 80 percent,
2. Mild wind conditions,
3. Reach to saturation through cooling or increased moisture.

During clear and calm nights, rapid surface heat loss causes the ground to cool, and the air layer near the surface cools to the dew point, resulting in condensation (radiation fog). During daytime, rising temperatures increase evaporation and typically cause fog to dissipate.

Three fundamental processes are involved in the microphysical development of fog:

• Cooling: The temperature of the air layer near the surface drops to the dew point due to radiative heat loss.
• Moisture Supply: The air reaches saturation through evaporation or horizontal transport (advection).
• Condensation: When saturation is achieved, water vapor transforms into droplets.

The simultaneous or sequential occurrence of these three processes leads to the formation of different types of fog.

Classification

Fog has various types based on its formation mechanism, temperature conditions, and visibility range. These classifications can be grouped into three main categories: air mass (cooling) fogs, frontal fogs, and visibility-based types.

Air Mass (Cooling) Fogs

Radiation Fog

During clear and calm nights, the surface loses heat; the air near the surface cools to the dew point and condenses. It typically forms at night and dissipates as temperatures rise in the early morning.

Advection (Horizontal Air Movement) Fog

When a warm and moist air mass moves over a cooler surface, the water vapor within it condenses. This type of fog is especially common along coastlines.

Upslope Fog

Forms when air, carried by wind, rises along mountain or hill slopes and cools.

Upslope Fog (Pexels)

Orographic (Topographic) Fog

Forms when horizontally moving air is gently lifted by terrain features, causing it to cool.

Fog can transition between these categories depending on local conditions, and microscale factors such as land use, vegetation cover, and water bodies can complicate the process.

Frontal Fogs

Forms when a warmer air mass rises over a colder air mass moving in the opposite direction. Three subtypes are defined accordingly:

• Warm front fog,
• Cold front fog,
• Frontal zone fog.

Classification by Visibility Range

• Light fog: Visibility below 1.6 km,
• Dense fog: Visibility below 1 km,
• Very dense fog: Visibility below 400 m.

Classification by Temperature

• Warm fog: Forms above 0°C,
• Cold fog: Occurs between -30°C and 0°C,
• Ice fog: Forms in environments below -30°C.

Climate Change and Global Fog Trends

Some researchers have noted a declining trend in global fog frequency since the second half of the 20th century (global dimming effect). This decline has been linked to increasing aerosol concentrations, urbanization, and rising land surface temperatures. However, in certain coastal ecosystems, regional increases in fog frequency have been observed due to changes in sea surface temperatures.

Droplet Spectrum and Optical Properties

Fog droplets have radii ranging from approximately 1 to 10 micrometers. Droplet size is one of the primary factors determining fog optical thickness. Visibility range depends more on the total liquid water content within the droplets than on their number. Therefore, defining fog density solely by relative humidity is insufficient to fully capture its physical characteristics.

Fog Dissipation Phase and Turbulence Dynamics

The dissipation of fog is not solely dependent on solar radiation; atmospheric dynamics such as wind shear and vertical turbulent mixing also play a significant role. Transient instability events occurring at the upper boundary of the fog layer can cause the fog to dissipate from the bottom upward. This indicates that the dissipation mechanism of fog cannot be explained solely by radiative evaporation.

Effects of Fog

Fog exerts both positive and negative impacts on natural systems and human activities. Agriculturally, particularly in arid and semi-arid regions, fog droplets act as “fog precipitation,” directly delivering moisture to soil, plants, and organisms, thereby supporting the water balance. This process enhances air moisture content and moisture gradients, contributing to plant and soil hydration. In arid regions, fog contributes to plant and soil moisture and thus plays an indirect beneficial role in the continuity of ecosystems, flora, and fauna.

Conversely, fog’s limitation of visibility causes significant disruptions in land, sea, and air transportation. Events such as flight cancellations, maritime accidents, and traffic collisions lead to fog being classified as a meteorological natural hazard. Additionally, in areas with high air pollution, foggy days can concentrate airborne pollutants, increasing health risks for individuals with asthma, bronchitis, and other respiratory conditions.