My son is a student at a university located atop a mountain in rural Tennessee. The fog there is so heavy and frequent during parts of the year that it’s jokingly treated like a part of campus, no different than the dining hall or dorms. In fact, the student-run radio station is called “The Sewanee Fog” in honor of the natural phenomenon. Situated near the lower Appalachians, the fog rising from the valleys in that area gave rise to the nickname the “Smoky Mountains,” as many people refer to the range.
It was equally foggy here in Pensacola this week, and it got me thinking about why. Fog is composed, of course, from particles of water vapor, similar in structure to a cloud. But why do these clouds dwell so close to the ground sometimes? And why more frequently in some places than others?
First, you’ll need humid conditions. The air must be so saturated with water that water vapor (a gas) condenses into visible water droplets. The droplets condense around particles of some sort, including salt (in coastal areas), dust, pollution, or natural organic compounds from trees. Fog falls into several categories, including radiation fog, advection fog, valley fog, and freezing fog. Radiation fog is the most common type, forming this time of year overnight as temperatures cool down. Fog forms near the ground, and as air continues cooling it will rise and form a thicker layer higher off the surface. Eventually, it will “burn off” after the sun rises and temperatures heat up. Advection fog is more common on the west coast, forming over the water and pushed by wind over land. This type of fog can last for several days.
I wrote about freezing fog a year ago on this blog, after experiencing it in North Carolina. You can read more about rime ice here, but that one is extremely rare in our neck of the woods. The Appalachian Mountains are typically known for their valley fog, a type of radiation fog in which the dense air is trapped in the deeper parts of the mountain ranges. The water vapor mixes with volatile organic compounds transpiring from trees that reflect a bluish tint, creating the hazy blue smoke-like fog in the Smokies.
While fog can reduce visibility and may contribute to accidents, it is not inherently dangerous. However, there have been incidents of dangerous fog when water droplets attached to particles of severe air pollution. In 1930, sixty people died from a steel-mill polluted valley fog in Belgium. Due to a combination of weather factors and smog from burning large amounts of coal, an estimated 4,000 people died in London from lung ailments related to the fog in 1952. Disasters like this helped bring momentum to enact environmental regulations reducing air pollution.
On the other hand, sometimes fog can be a lifesaver. In drought-prone desert areas, fog collectors are being used to literally squeeze droplets of water from thin air to harvest drinking water. Fog is collected by hanging specialized mesh nets upright from posts. When the fog moves through, water droplets attach to the mesh, running down to gutters and containers below. A highly effective fog harvesting project completed recently in Morocco provides enough water for 400 individuals a day, and has reduced the amount of time women (who traditionally handle water resources for the families) spend collecting water by 3 hours a day. This innovative process uses solar power to operate a filtration system, insuring a reliable and safe source of water.