In meteorology, an inversion is a layer of warmer air held above colder air. Normally, air temperature gradually decreases as altitude increases, but in an inversion this relationship is reversed.
Temperature inversion in the Lake District, England, forms clouds at a low level beneath clear skies.
Smoke rising in Lochcarron, Scotland, is stopped by an overlying layer of warmer air (2006).
Smog trapped over the city of Almaty, Kazakhstan during a temperature inversion.
Smoke-filled canyons in northern Arizona, 2019. During morning and evening hours, dense smoke often settles in low-lying areas and becomes trapped due to temperature inversions—when a layer within the lower atmosphere acts as a lid and prevents vertical mixing of the air. Steep canyon walls act as a horizontal barrier, concentrating the smoke within the deepest parts of the canyon and increasing the strength of the inversion.
Smog, or smoke fog, is a type of intense air pollution. The word "smog" was coined in the early 20th century, and is a portmanteau of the words smoke and fog to refer to smoky fog due to its opacity, and odor. The word was then intended to refer to what was sometimes known as pea soup fog, a familiar and serious problem in London from the 19th century to the mid-20th century, where it was commonly known as a London particular or London fog. This kind of visible air pollution is composed of nitrogen oxides, sulfur oxide, ozone, smoke and other particulates. Man-made smog is derived from coal combustion emissions, vehicular emissions, industrial emissions, forest and agricultural fires and photochemical reactions of these emissions.
Airplane used to collect airborne hydrocarbons, May 1972
The presence of smog in California is shown near the Golden Gate Bridge. The brown coloration is due to the NO2 formed from photochemical smog reactions.
Highland Park Optimist Club wearing smog-gas masks at banquet, Los Angeles, circa 1954
During the autumn and spring months, some 500 million tons of rice and wheat crop residues are burnt, and winds blow from India's north and northwest towards east. This aerial view shows India's annual crop burning, resulting in smoke and air pollution over Delhi and adjoining areas.
