ATOC 5600 Physics and Chemistry of Clouds & Aerosols

Fog characteristics

Fogs are composed of fine droplets of water suspended in the air near the Earth's surface. The presence of these droplets act to scatter the light and thus reduce the visibility near the ground.  The formation of a fog layer occurs when a moist air mass is cooled to its saturation point (dew point). This cooling can be the result of radiative processes (radiation fog), advection of warm air over cold surfaces (advection fog), evaporation of precipitation (precipitation or frontal fog), or air being adiabatically cooled while being forced up a mountain (upslope fog).

Another type of fog is the so-called valley fog. This fog forms as a result of air being radiatively cooled, during the evening, on the slopes of topographical features . This air becoming denser than its surroundings, starts going down the slope. This results in the creation of a pool of cold air at the valley floor. If the air is cold enough to reach its dew point, fog formation occurs. 

 

A fog layer is reported whenever the horizontal visibility at the surface is less than 1 km. A typical evolution of visibility during a radiation fog episode is shown in Figure 4. After sunset, a strong cooling occurs near the surface through the effect of longwave radiation divergence. As the cooling proceeds, the relative humidity increases until fog droplets are activated. The visibility drops rapidly toward its minimum value. Then the fog layer grows in the vertical through the interaction of radiative and turbulent processes. Dissipation occurs after sunrise as the solar energy warms the surface. 

Figure 4. Observed temporal evolution of horizontal visibility and fog depth. Lille, France, November 6-7 1988. Adapted from Bergot (1993). 

 

In reduced visibility situations, but with visibilities higher than 1km, haze is reported since visibility reduction is mainly associated with aerosols that have "swelled" up by taking up water as the relative humidity increases. 

Typical fogs are composed of unactivated cloud droplets (haze particles) and activated Cloud Condensation Nuclei (CCN). The unactivated droplets are generally smaller than 2.5 microns in diameter, while activated droplets are larger than 2.5 microns. So, compared to other cloud types, fog droplets are generally smaller. Although great variability in fog droplets size distributions have been observed. In some instances, larger than expected fog droplets were observed (Gerber, 1991). Also in contrast with other cloud types, fogs have small liquid water contents (LWC). Most fogs have LWC ranging from 0.05 to 0.5 gm^-3. 

 

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