Lee Parsons

Recently in one of the message board, someone asked about the formation of fog and the conditions needed to produce fog. Drawing on my knowledge of aviation weather, I emailed a brief explanation and promised to send more detailed explanations. After giving it some thought, I decided to post this information as an article for all to benefit . Information in this article is based on the ASA "Aviation Weather" book and information obtained from Encyclopedia Britannica.

Fog;

Fog is a surfaced based cloud composed of either water droplets or ice crystals. Fog is the most frequent cause of surface visibility below 3 miles and is one of the most common and persistent weather hazards.

Small temperature dew point spread is essential for fog to form. Typically, a temperature dew point spread of 5 degrees or less will indicate the presence or likelihood of fog. Fog is prevalent in coastal areas where moisture is abundant. However, fog can occur anywhere. Abundant condensation nuclei enhance the formation of fog. Thus, fog is prevalent in industrial areas where by-products from combustion provide a high concentration of these nuclei. Fog occurs mostly in the colder months, but the season and frequency of occurrence vary from one area to another.

Fog may form (1) by cooling air to its dew point, or (2) by adding moisture to the air near the ground. Fog is classified by the way it forms and formation may involve more than one process.

Radiation fog is relatively shallow fog. It may be dense enough to hide the entire sky or may conceal only part of the sky. "Ground Fog" is a form of radiation fog.

Conditions favorable for the formation of radiation fog are clear sky, little or no wind, and a small temperature-dew point spread (high relative humidity). The fog forms almost exclusively at night or near daybreak. Terrestrial radiation cools the ground; in turn, the cool ground cools the air in contact with it. When the air is cooled to its dew point, fog forms. When rain soaks the ground, followed by clearing skies, radiation fog is not uncommon the following morning.

Radiation fog is restricted to land because water surfaces cool little from nighttime radiation. Winds up to about 5 knots mix the air and tend to deepen the layers. Stronger winds disperse the fog no mix the air through still deeper layer, with stratus clouds forming at the top. Ground fog usually "burns off" rather rapidly after sunrise.

Advection fog forms when moist air moves over colder ground or water. It is most common along coastal areas but often develops deep in the continental areas. At sea it is called "Sea Fog". Advection fog deepens as wind speed increases, up to about 15 knots. Wind much stronger lifts the layer into a layer of low stratus or stratocumulus clouds.

This fog frequently forms offshore as a result of cold water and then is carried inland by the wind. During winter, advection for over the central and eastern United States results when moist air from the Gulf of Mexico spreads northward over colder ground. Water areas in northern latitudes have frequent dense sea fog in summer as a result of warm, moist, tropical air flowing northward over colder Artic waters.

Upslope fog forms as moist, stable air being cooled adiabatically moves up sloping terrain. Once the upslope wind ceases, the fog dissipates. Unlike radiation for, it can form under cloudy skies. Upslope fog is common along the eastern slopes of the Rockies and somewhat less frequent east of the Appalachians. Upslope fog is often quite dense and extends to high altitudes.

When relatively warm rain or drizzle falls through cool air, evaporation from the precipitation saturates the cool air and forms fog. Precipitation induced fog can become quite dense and continue for an extended period of time. This fog may extend over large areas. It is most commonly associated with warm fronts, but can occur with slow moving cold fronts and with stationary fronts as well.

Fog induced by precipitation is in itself hazardous as in any fog. It is especially critical, however because it occurs in the proximity of precipitation and other possible hazards such as thunderstorms.