Mist, Fog and Haze: Eleven Ways the Air Hides the View

Mist is a suspension of small water droplets in cool air, formed when warm, moist air is suddenly cooled, as when breath meets cold winter air. It is separated from fog mainly by visibility, since mist stays thinner and more transparent.

Fog is in effect a cloud formed at ground level, and it appears when air comes within about 2.5 degrees Celsius of its dew point. Radiation fog, advection fog, sea fog and freezing fog are told apart by how they form.

A fog bow is the white cousin of the rainbow. Its droplets are far smaller than raindrops, typically under 0.05 millimeters, so diffraction washes out the colors and the bow looks mostly white.

Haar is a cold sea fog most frequent along Scotland's east coast from April to September. It forms when warmer, moist air crosses the cooler North Sea, and easterly winds carry it several miles inland.

Fog is a common summer feature of the San Francisco Bay Area and the California coast south to Baja California. Some studies suggest Bay Area fog has become less frequent since 1951, though other researchers say the data are insufficient.

June Gloom is the Southern California name for cool, overcast late spring and early summer weather, caused by low stratus clouds forming over the cool California Current. Related nicknames such as May Gray and Fogust cover the neighboring months.

In a fog desert, fog drip rather than rainfall supplies most of the moisture for plants and animals. The Atacama, Baja California, the Namib and the coast of the Arabian Peninsula are examples.

Haze is dust, smoke and other dry particulates that obscure the sky, which the World Meteorological Organization distinguishes from fog and mist. Farming fires, vehicle and industrial emissions, wind-blown dust, volcanoes and wildfires all feed it.

Pea soup fog was a thick, often greenish-yellow fog loaded with soot and sulphur dioxide from burning soft coal. It could be lethal to the elderly, infants and people with respiratory conditions, and in Britain it was called a London particular.

The word smog joins smoke and fog and was coined in the early twentieth century. Winter smog builds when a temperature inversion traps heating emissions near the ground, while summer smog forms as vehicle and industrial emissions react in sunlight.

The Southeast Asian haze is a recurring transboundary pollution problem driven mainly by agricultural burning, worst between about July and October and in El Nino years. Fires on peatland make the smoke considerably denser and more toxic.