How Does Fog Form?






By Jon Erdman


Published Oct 14 2013 01:12 PM EDT



While common in some locations, fog can simply amaze us.

According to the Glossary of Meteorology from the American Meteorological Society, fog is a collection of water droplets suspended in the atmosphere in the vicinity of the earth's surface that affects visibility. Specifically, fog reduces visibility below 1 kilometer (or 0.62 miles).

Some Common Types of Fog



The most common form of fog, known as radiation fog, typically occurs on clear nights as the earth's surface cools moist air immediately above it. If just enough light wind is present – a couple of mph, at most – this chilled air can be gently stirred through a deeper layer, forming a deeper radiation fog.

Often in the fall, you'll see morning fog hug lower valleys of the Appalachians. This valley fog, really just a type of radiation fog, results from cold, dense air draining down mountain slopes at night, collecting in the valley floors, then forming as any other radiation fog described above.

Incidentally, let's dispel a myth. Fog does not burn off, per se.

As solar energy heats the ground near the fog's edge, vertical mixing brings drier air into the fog's edge, evaporating it. A typical ground fog will dissipate first at its edges, where its depth is more shallow, working its way toward the thicker center of the fog.

Sometimes fog forms when warm air moves over a cold surface. Warm air moving over snow-covered ground in winter and sea fog drawn inland over a cool land surface along the West Coast are two prime examples of so-called advection fog. Unlike radiation fog, advection fog can sometimes be seen as moving laterally along or near the ground.

When surface temperatures are below freezing, water droplets in a fog are supercooled, waiting to freeze on contact with any subfreezing surface. These freezing fog events can be dangerous not only for a reduction in visibility but also for a light accumulation of ice on roads, particularly bridges and overpasses.

At even colder temperatures, fog made up solely of tiny ice crystals will form. This ice fog is common in the winter months in parts of Alaska's interior, among other locations closer to the poles.

You may also notice steam fog from some lakes in the fall or early winter. Cold air overlaying warm air near the warm lake surface is an unstable configuration, lending itself to rising air. The mixing of cool air chills the warmer, more moist air immediately above the lake to allow condensation and a cloud to form. You can typically see wispy, vertical currents of fog rising from the lake.

Some other, less common fog types include:
•Frontal fog: If warmer raindrops fall into colder, drier air, evaporation occurs. Eventually, the cold air moistens sufficiently to produce fog.
•Upslope fog: Air moving gently upward in elevation enough for the layer to reach saturation, such as behind a winter cold front in the High Plains and Front Range of the Rockies.
•Hail fog: On rare occasions, accumulated hail at the surface can chill the near-surface air enough to produce a shallow veil of fog. We have an example in the slideshow above.




Average days per year with dense fog (defined as reducing visibility to one-quarter mile or less) in the U.S. Areas with most frequent fog are shown in darker gray, red shading. (Image courtesy: NOAA)


Most Fog-Prone Areas

The Appalachians, parts of northern New England and the Pacific Northwest each typically see at least 40 days a year with dense fog (at least one-quarter mile visibility or lower). Of course, morning fog makes up the lion's share of these days, after which late-morning/afternoon sunshine is plentiful.

Parts of the northern Gulf Coast and California coast can also have frequent fog, if not always dense fog.

In winter, valley fog can hang stubbornly in lower elevations of the Great Basin, as well as California's Central Valley, as the combination of warmer air aloft moves over an area just soaked by the storm. Add an Arctic air mass spilling over the Continental Divide into the Great Basin, and this so-called "inversion" fog can hang in for days in places like Salt Lake City and Boise, trapping pollution near the ground, as well.

What about the least foggy location in the Lower 48 States? That would be the Desert Southwest, from southern Nevada and southwest Utah into Arizona and southwest New Mexico, averaging only a few days a year of dense fog.

Interesting, the way fog captures all pollutants.
Slightly off topic, could the mist be sucked out every morning so that we have a cleaner city? Some sort of low cost, wind tunnel? Not exactly a vacuum cleaner; just sucks the fog at lower pressure (larger volume) anid spits out the dirt.

In my area, the corporation workers use broom sticks & sweep up such a dust storm (without masks)!! oh yeah,in India the corporation rules.

The mist / sprinkler / spray concept is already in use to scrub emissions from some of the chemical plants; but OP's article reminds us that fogs are naturally available to us; use it.

It's probably cause morning fogs have been such an irritant (driving) that interesting properties escaped our notice.

Here is a bit from an article about fog catching for water.


http://education.nationalgeographic.com/encyclopedia/fog/

Fog Catchers

Many ancient cultures collected water from fog by placing large pots under trees and shrubs. As the water from fog collected on these objects, the pots collected the water. This method of water collection was effective, but not as effective as collecting rainwater or other liquid water.

Today, engineers are working on more sophisticated ways to collect water from fog. The most effective way has been the development of “fog catchers.” Fog catchers are very large screens constructed in arid areas. As fog glides in, water droplets form around the thin screens and drip to the collection pools below. In one day, a single screen can collect more than a hundred gallons of water.

The village of Bellavista, Peru, relies on fog catchers. Bellavista is an area that has little access to liquid water—no rivers, lakes, or glaciers are nearby. Wells dry up quickly. Water for irrigation and human consumption is threatened. Every year, however, huge fogs blow in from the Pacific Ocean. In 2006, the community invested in a series of fog catchers outside of town. Now, the residents of Bellavista have enough water to irrigate trees and gardens, as well as provide for their own drinking and hygiene needs.

Engineers warn that fog catchers will only work in small areas. Still, engineers and politicians are working on ways to make more powerful fog catchers that will perhaps reduce the need for people to rely so much on groundwater.