Fog is actually a stratus cloud that forms at or near the surface of the ground. Because it lies on or near the surface, fog can greatly restrict our visibility. While clouds aloft may be more of a nuisance than a threat to air travelers, fog can develop so thick near the ground that departures are often delayed and landings diverted. Fog also makes driving conditions hazardous, often reducing visibility down to less than 50 feet.
In the same way that clouds form above us, fog forms when the air near the ground becomes saturated with water vapor, and the vapor condenses onto aerosols in the air. Normally, this process is initiated in clouds as a result of the air being cooled by lifting. Fog can also develop as a result of a cooling process. The most common type of fog which forms in this manner is called radiation fog. Recall for a moment how temperature inversions form. At night, the ground cools faster than the air. The air just above the ground cools more rapidly than the air aloft. This differential cooling of the atmosphere near the surface produces a temperature inversion. If the air near the ground is moist, a fog will form. This is why we commonly see fog in the early morning. As the sun rises and starts to heat the ground, the fog evaporates.
Fog can also form by increasing the water vapor content in the air just above the ground without actually cooling the air. This often occurs over warm lakes and ponds when colder air moves over them. The water evaporates into the air, adding vapor to the air, which increases the relative humidity. If the air over the water becomes saturated, then fog will form. This type of fog is referred to as steam fog. In the winter, when we step outside, the steam or fog that forms from our breath is created in the same manner that a steam fog over a lake is formed. Our breath is warm and moist while the air outside is cold and dry. When we exhale, our breath quickly mixes with the air outside and saturates a small amount of it which becomes visible.
The thickness of fog varies based on the amount of moisture in the air, the degree cooling that occurs, and also, the "purity" of the air. City fogs are often much thicker than fogs that occur over the ocean. On average there are more aerosols in urban air than in the air over the oceans because of the numerous anthropogenic sources in the cities. The higher concentration of aerosols in the cities result in more numerous, but smaller, cloud droplets while the droplets in marine fog are larger and fewer in number. City fog, formed with harmful pollutants, can threaten our health, cause harm to plant life, and damage structures. Elevated concentrations of nitrogen oxides and sulfur compounds are commonly found in urban air. Fog that forms in their presence can be very acidic. For instance, sulfur dioxide (SO2) combines with the tiny water droplets in the fog to form sulfuric acid (H2SO4). This acid can be inhaled by humans and other animals causing respiratory illnesses or settle onto plants and structures causing excessive deterioration.
There are enough natural aerosols in the air for fog and clouds to form without the aid of humans. However, increased concentrations of aerosols, many of which are highly reactive chemically, can create more frequent and more threatening fog events. As mentioned above, clouds and fog normally form from saturated air. Sometimes, the same physics that produce clouds and fog in saturated air, can produce haze in unsaturated air. Haze is another phenomena which can greatly reduce visibility. We will discuss this next.
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