Temperature Inversions
A temperature inversion is a meteorological phenomenon where air temperature increases with height. Above the top of an inversion, is a windshear zone created at the confluence of light and relatively strong winds. The eddies generated in the shear zone cause air speed fluctuations as an aircraft climbs or descends.
Radiation Inversion
The infrared radiation emitted by the ground at night represents a loss of heat and a subsequent loss of temperature (Nocturnal Inversion). The air in contact with the ground then loses heat by conduction to the ground, and ends up being cooler than the air above. The air above is not losing heat as fast due to the fact that air is a poor conductor, which then leads to the formation of warm air overlying cold air, a temperature inversion (the air temperature increases with increasing altitude).
Because the effect of heat conduction that cools the air acts only in the layer of air next to the ground, this inversion is shallow and is often seen as a ground fog or a thin layer of haze next to the ground.
After the sun heats the ground during the daytime, the air next to the ground heats up and the radiation inversion is destroyed.
Advection Inversion
Cold marine air flows inland due to the sea breeze. The higher density cold air flows underneath the warmer inland air, forming the inversion configuration (warm air over cold air).
Note how the cold air forms a wedge as it pushes inland. Pollution emitted at the ground is advection inland into the wedge by the sea breeze (thus ventilating the coastal area) and the shallowness of the wedge concentrates the pollutants (low mixing height). Inland receptor areas are then inundated by pollution.
Frontal Inversion
A temperature inversion may occur between two air masses, often during the passage of a cold or warm front.
Subsidence Inversion
A more long-lived temperature inversion accompanies the dynamics of the large high-pressure systems depicted on weather maps. Descending currents of air near the centre of the high-pressure system produce a warming (by adiabatic compression), causing air at middle altitudes to become warmer than the surface air. Currents of warm air lose their buoyancy and are thereby inhibited from rising when they reach the warmer, less dense air in the upper layers of a temperature inversion. During a temperature inversion, air pollution released into the atmosphere’s lowest layer is trapped there and can be removed only by strong horizontal winds. Because high-pressure systems often combine temperature inversion conditions and low wind speeds, their long residency over an industrial area usually results in episodes of severe smog.
Low Level Jet Streams
A fast-moving river of air known as a "Low Level Jet Stream” sometimes forms at about 1,000 feet above the ground after sunset.
On a clear evening the atmosphere cools down. And if conditions are calm, a stable temperature "inversion" sets up where cool air aloft, which is heavier than warm air, sinks to the ground and any leftover warm air sits on top of it. The stable air in an inversion acts like a nearly solid object and allows the air above it to flow rapidly past the inversion like wind blowing over water.
Differences in air pressure on either side of the developing low-level jet help to concentrate the flow of air into a corridor or stream less than several hundred nautical miles wide. Winds in the stream can flow at speeds of 60 kt or more. Mountain ranges can further enhance low-level jet stream winds.
Night-time, low-level jet streams are marked by a rapid change in wind speed with height. The sudden shift in wind can catch late-night fliers by surprise and be hazardous to landing aircraft.
Once the sun begins to heat the land, the lower atmosphere begins to mix as the warm air rises, breaking the inversion. As this happens, the jet rises in some places and sinks in others like a giant roller coaster. Without a smooth surface to glide over, the jet encounters friction and slows down. But the same conditions the next night could allow the low-level jet to reform with equal strength and similar consequences.
Fog
Fog is a cloud in contact with the ground. It occurs when moisture from the surface of the Earth evaporates; as this evaporated moisture moves upward, it cools and condenses into the familiar phenomenon of fog. Fog differs from cloud only in that fog touches the surface of the Earth, while clouds do not. All types of fog form when the relative humidity reaches 100% and the air temperature drops below the dewpoint, pushing it lower by forcing the water vapour to condense.
Fog reduces visibility to less than 1000m which is particularly hazardous at airports. Visibility greater than 1000m is considered mist. Some attempts have been made to develop methods (such as using heating or spraying salt particles) to aid fog dispersal. These methods enjoy some success at temperatures below freezing.
It can form in a number of ways, depending on how the cooling that caused the condensation occurred:
- Radiation Fog: is formed by the cooling of land after sunset by thermal (infrared) radiation in calm conditions with clear sky. The cool ground then produces condensation in the nearby air by heat conduction. In perfect calm air the fog layer can be less than a metre deep but a 6kt wind can promote a deeper weaker layer. Radiation fog is common in autumn and usually does not last long past sunrise.
- Advection Fog: occurs when moist air passes over cool ground by advection (6kt wind) and is cooled. This form is most common at sea when tropical air encounters cooler higher-latitude waters. It is also extremely common as a warm front passes over an area with significant snow pack.
- Steam Fog: is the most localized form and is created by cold air passing over much warmer water. Water vapour quickly enters the atmosphere by evaporation and condensation occurs once the dewpoint has been reached, thus creating a wispy steam. Steam fog is most common in polar regions, and around deeper and larger lakes in late autumn and early winter. It is closely related to lake-effect snow and lake-effect rain, and often causes freezing fog, or sometimes hoar frost.
- Precipitation Fog: (or frontal fog) forms as precipitation falls into drier air below the cloud, the liquid droplets evaporate into water vapour. The water vapour cools and at the dewpoint it condenses and fog forms.
- Upslope Fog: forms when winds blow air up a slope (orographic lift), adiabatic cooling it as it rises, and causing the moisture in it to condense. This often causes freezing fog on mountaintops, where the cloud ceiling would not otherwise be low enough.
- Valley Fog: forms in mountain valleys, often during winter. It is the result of a temperature inversion caused by heavier cold air settling into the valley, with warmer air passing over the mountains above. It is essentially radiation fog confined by local topography, and can last for several days in calm conditions.
- Ice Fog: is any kind of fog where the droplets have frozen into extremely tiny crystals of ice in midair. Generally this requires temperatures well below the freezing point, making it common only in and near the Arctic and Antarctic regions. Extremely small amounts of this falling from the sky form a type of precipitation called ice crystals.
- Freezing Fog: occurs when liquid fog droplets freeze to surfaces, forming white rime ice. This is very common on mountaintops which are exposed to low cloud. It is equivalent to freezing rain, and essentially the same as the ice that forms inside a freezer which is not of the "frostless" or "frost-free" type.