Introduction • What is fog? Fog is defined as a cloud with its base at or very near the ground, yet fogs and clouds form by different processes. • Physically, there are no differences between the appearance and structure of clouds and fog. • However Clouds result when air rises and cools adiabatically. Fog on the other hand results from cooling or when air becomes saturated through the addition of water vapor.
Fog can create many hazardous on roads. Drivers should exercise caution when driving in inclement weather.
Lighthouses were erected to help boats find their way through less than visible condition such as darkness and fog.
Introduction Cont’d • There are 5different types of fog. Radiation, Advection, Upscale, Steam and Frontal fog.
Radiation Fog results from radiation cooling of the ground and adjacent air.It’s a nighttime phenomenon requiring clear skies and a high relative humidity. Under clears skies, the ground and the air immediately above cools rapidly.
Normally radiation fog dissipates within one to three hours after sunrise. The sun warms the ground, the lowest layer of air is heated first, and fog evaporates from the bottom up.
Advection fog is the fog that is produced when damp air is moved across a surface that is cooler than the air.
When the temperature of the air is lowered to its dew point, changes start to happen. First, saturation occurs. Then, fog
It is most commonly seen over seas or other bodies of water, but it is possible over snow-covered or frosty land masses seen here.
A classic example of frequent advection fog is around San Francisco’ Golden Gate Bridge.
For Advection Fog, turbulence of 6-18 mph is required.Wind facilitates cooling through dense layers of air, therefore it elevates the fog. Advection Fog extends 1000-2000 ft. above the surface, and persists longer than radiation Fogs.
Cape Disappointment, Washington. The Foggiest Location in the United States. 106 Days of Fog a year.
Frontal (Precipitation) Fog • Frontal fog occurs when raindrops falling from relatively warm air above a frontal surface evaporate into the cooler air below and cause it to become saturated.
Frontal Fog is most common on cool days during extended periods of light rainfall.
UPSLOPE FOG - Created when relative humidity air moves up a gradually sloping landforms or up the steep slopes of a mountain.
Because of upward movement:, air expandsand cools adiabatically.If dew point reached, an extensive layer of fog will form. UP-SLOPE FOG Fog Forms on Slope As air masses with the terrain, it cools to condensation temperature
STEAM FOG - When cool air moves over warm water, enough moister may evaporate form the water surface to saturate the air immediately above.
Rising water vapor meets cold air, condenses and rises with the air that is being warm from below. • The rising foggy air looks like “steam that forms above a hot cup of coffee. • Common over lakes and rivers on clear crisp mornings in the autumn when water is relatively warm & air is rather cold. • Water droplets mix with unsaturated air above and evaporates • dense, especially during winter • Cold arctic air pours off the contents • Ice shelves toward the comparatively warm open ocean • Steam fog produces as the rising water vapor saturates a large volume of air.
Review Questions • Fog is defined as a cloud with its base at or very near the ground, yet fogs and clouds form by different processes. Describe the similarities and differences in the formation of fogs and clouds. • Why does radiation fog form mainly on clear nights as opposed to cloudy nights?