Fog droplets are tiny floating ponds where bacteria grow and divide
Fog is a cloud that has come down to ground level, but recent research shows it is also a living habitat. Droplets in overland fog hold bacterial concentrations comparable to lakes or seawater, dominated by pink-pigmented Methylobacterium, and the microbes actively grow there. When the fog lifts, airborne bacteria nearby jump by about 45 percent.
By definition fog cuts visibility below one kilometre; anything thinner is mist. It forms when the air temperature comes within 2.5 degrees Celsius of the dew point and water vapour condenses onto specks of dust, ice or salt. The relative humidity is usually near 100 percent, though fog sometimes appears in drier air and occasionally fails to form in saturated air. A cool, stable layer trapped beneath warmer air holds it in place, and the height of that inversion sets the depth of the fog; high pressure squashes a coastal marine layer, while falling pressure lets it swell upward.
Meteorologists sort fogs by origin. Radiation fog develops on clear, calm nights as the ground loses heat and chills the air above it, pooling in valleys and swamps; a light breeze spreads it, a strong wind prevents it, and it is often densest just after sunrise. Advection fog forms when moist air slides over a colder surface, such as the upwelling waters off California, or warm air crossing snowpack, where it can keep forming in winds of 80 kilometres an hour. Frontal fog comes from rain evaporating into cool air near a front, and hail fog can appear suddenly over fresh hailstones.
Cold produces stranger varieties. Freezing fog coats exposed surfaces with rime ice and is common on mountain tops. In the western United States it goes by pogonip, from a Shoshone word for cloud; The Old Farmer's Almanac warns readers to beware it each December, and it can blanket the Columbia Plateau for up to three weeks. Ice fog, whose droplets freeze into crystals in midair, needs about minus 35 degrees Celsius and often forms over Arctic cities from vehicle exhaust, sometimes producing halos and light pillars.
Fog can also drizzle when its droplets merge, turning to freezing drizzle below zero. Sea fog condenses on airborne salt particles, which is why it clings to coastlines.
Source: Fog
