Temperature Inversions.

Temperature Inversions

Normally, the air near the ground is warmer than the air at higher altitude. An inversion is the reverse situation--colder air at ground level than higher up.

Cold air is heavier than warm air. Thus once an inversion forms, the air is very stable. Mixing that would normally occur by the rising of warm air is inhibited. Consequently pollutants such as carbon monoxide, sulphur compounds and ice fog remain trapped near the ground when there is an inversion.

Types of Inversions

The stability and mixing of air near the surface is controlled to a great degree by the presence of temperature inversions in the lower atmosphere. These inversions typically are very extensive, blanketing entire cities or regions. In each case, warm air is produced over a layer of cooler air in contact with the surface.

There are basically four types of temperature inversions, each are explained below.

Marine Inversions

If you live near a coastline, you have probably experienced a marine inversion layer. Near cool oceans, such as in the western U.S. and Western Europe, a marine inversion can turn a hot summer day at the beach into a cool, foggy, overcast shiver. The ocean is typically quite cool compared with the land, particularly at middle latitudes where the heating by sunlight is not strong. Air sitting over cold water also tends to be cool. Moreover, a great deal of water evaporates from the oceans. This moisture readily condenses in the marine mixed layer (or marine boundary layer) into stratus clouds and fog. Particularly on the western coastlines of continents, the marine air is blown inland by prevailing winds. In addition, the relatively intense heating of the coastal plain generates strong onshore sea breezes, which also draw marine air inland.

Sea and land breezes are created by the contrasting temperature between the land and the oceans (or other large bodies of water, such as lakes). During the day, sunlight heats the land more than the adjacent ocean. Warm air rises over the land and is replaced by cool marine air flowing in near the surface. The warm air can circulate out toward the ocean above the marine boundary layer, setting up a temperature inversion in the coastal region. As a result, pollutants emitted into the marine air layer overland are trapped near the ground. In the evening, the land cools ...

This is a preview of the whole essay

High Pressure Inversions

A large-scale subsidence inversion can form when a stationary high-pressure system settles over a region. For example, during the summer season, the southern California coastal zone lies under the eastern edge of the Pacific high. This high-pressure system is a downward branch of the tropical Hadley circulation. Within the dome of high pressure, very dry air slowly subsides as it circulates clockwise about a centre located off the coast of California. The air parcels brought to the Los Angeles basin thus arrive from the north and northeast. Subsidence compresses and warms the dry air as it descends into the lower troposphere. The dome of warm air then effectively traps the layer of cool marine air in the coastal region. A relatively shallow marine layer of several hundred meters to one kilometre in depth is formed. Separating the two air masses is a strong temperature inversion layer, which persists almost continuously during the spring and summer half of the year and appears occasionally at other times of the year. The temperature inversion suppresses convection and mixing, allowing pollutants to build up in the lower layer.

Large weather systems that create conditions leading to high-pressure subsidence are fairly common. In some regions, these weather patterns regularly generate unusually intense hot winds. When the subsiding air encounters a cool stable layer near the surface, a strong temperature inversion can take place, as in the Los Angeles area.

Radiation Inversions

A radiation inversion is created when heat is rapidly lost from the surface by thermal radiation. At night, the ground and the air just above the ground can cool off by radiating their heat energy while the air higher up remains warm. This is because air, particularly dry air, is a poor heat radiator, whereas land is an excellent radiator. The air just above land can lose heat by coming into direct contact with the surface. However, the air must be mixed downward to the surface. Recall that at night, convection dies away and the turbulent mixing of the lower atmosphere is shut off. The result is a very shallow layer of cold air that forms at the surface.

Since desert air is extremely dry, we can witness the formation of a radiation inversion on almost any summer night in the desert. During the day, the sun heats the soil and the atmosphere over it. That heat is carried from the surface into the atmosphere by intense mixing in the lower air layers. When the sun sets in the evening, the source of heat disappears. The desert sands quickly radiate their heat and cool down. Since sand is an excellent heat insulator, and the warmth deeper in the sand cannot reach the surface fast enough to maintain its temperature. The air just above the surface is immediately cooled, and loses its heat to the ground. The air temperature in this area has been known to drop 60 degrees F in a few hours.

The cold surface and warm upper air layers constitute a temperature inversion. Inversions like these are typically close to the ground and strong. The inversion is usually found to be the strongest at sunrise, after a long night of surface cooling and deepening of the warm inversion layer. Under dry, clear weather conditions, such an inversion can occasionally be seen in the Los Angeles basin at first light in the morning. The pollution from the automobiles from early-bird commuters is seen as a dense layer of smog close to the ground beneath the shallow inversion. As the sun rises and warms the surface, the inversion is broken down because of increased mixing.

Radiation inversions are most likely to form on clear nights with low winds. Clear skies are necessary for the surface radiation to escape; overlying clouds or fog absorb thermal radiation and effectively radiate it back to the surface. Calm winds also prevent the mixing of warm air at higher altitudes downward to the surface.

Regional Subsidence Inversions

When air flows over an obstacle such as a mountain range or blows from a high plateau and descends into a lower basin, a regional subsidence inversion can be created. Air that descends heats by adiabatic compression. This descending air therefore creates a hot wind, like the Santa Ana winds that often blow through Los Angeles. The air is typically very dry. It might be dry because of the source over land, or it might have happened as it rose over a mountain barrier, cooled, and lost water vapour by condensation and precipitation. As it is compressed, the dry air heats up along a dry adiabat. This produces the greatest warming for a given altitude change, which explains the unusually high temperatures of these winds.

If the descending air encounters colder air at the surface, it may be unable to push aside this denser air. The warm winds may then spread out above the surface layer, producing a temperature inversion. In other cases, strong subsidence winds fill a region with dry warm air that later cools at the surface by radiation, creating a radiation inversion. In southern California, weak Santa Ana winds often blow from the northeast across the mountain ranges to the north of the Los Angeles basin. These warm winds can trap colder denser marine air that is often present in the basin. These warm winds are usually associated with a high-pressure system.

Peer Reviews

Here's what a star student thought of this essay

thumbsup

20th of Apr 2012

Quality of writing

Grammar, spelling and punctuation are all excellent. There is an exceptional amount of geographic terminology, which is vital at A level, even without the inclusion of case studies. The introduction could have been made stronger with more powerful geographic terminology, as it sounded similar to that of GCSE standard. A clear definition of temperature inversions to initiate the essay would improve the mark of the essay.

Level of analysis

The main analysis is also satisfactory, the depth of research and relevant information is excellent. However, at A level a more analytical approach would be to include case studies with more information than just the mentioning of LA. Short and long term effects could be applied to a case study, including solutions to the problems of photochemical smog produced by temperature inversions, for example pollution reducing policies in Britain. By including this aspect of human geography, a synoptic element is brought into the essay, which is essential at A level. The diagrams are good, especially for marine temperature inversions, but the annotations could be in more detail. Although it may not be absolutely necessary, there is no conclusion or final paragraph to give an overall opinion on temperature inversions, which I feel is so important at A level to show a good level of understanding about the topic.

Response to question

A slightly weak introductory response to the 'temperature inversion' title, this is due to the lack of clear and insightful knowledge necessary at A level. The marine, high pressure and radiation temperature inversions are responded to in high detail, conveying plenty of depth in research. However, they could be compared to show further knowledge and understanding of temperature inversion theories, and to gain more marks. Although it is hard to judge to extent to which the response has fulfilled its criteria due to the lack of depth in the title, I personally feel the overall response was satisfactory

Did you find this review helpful? Join our team of reviewers and help other students learn