Atmospheric inversions
Atmospheric inversions are meteorological phenomena characterized by a reversal of the normal temperature gradient in the atmosphere, where air temperature increases with height instead of decreasing. This condition can significantly impact air quality, particularly in urban areas, by trapping pollutants close to the ground and contributing to the formation of smog. During temperature inversions, vertical air movement is suppressed, leading to stable atmospheric conditions that can exacerbate pollution issues.
Examples include radiation inversions, which often occur in winter under calm and cool conditions, and subsidence inversions, formed by high-pressure systems that cause air to sink and warm over larger areas. These inversions can persist for extended periods, particularly in regions with geographic features that limit air movement, such as mountains. Historical incidents, like the Donora smog in Pennsylvania and the 1952 London smog disaster, highlight the severe public health risks associated with prolonged exposure to trapped pollutants. Understanding atmospheric inversions is essential for addressing air quality challenges and mitigating their health impacts in affected communities.
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Atmospheric inversions
Definition: Vertical temperature profiles in which air temperature increases with height in the atmosphere
Temperature inversions play an important role in trapping anthropogenic (human-caused) pollutants near the earth’s surface, leading to the formation of smog and reduced air quality in many metropolitan areas. Temperature inversions also play an important role in the formation of severe thunderstorms and mixed precipitation (such as freezing rain and sleet).
An atmospheric inversion is defined in meteorology as a scenario in which air temperature increases with height. To understand its importance it is necessary to consider the concept in light of the broader structure of the atmosphere. The vertical structure of the atmosphere is characterized by four broad regions; in order of increasing altitude, these are the troposphere, stratosphere, mesosphere, and thermosphere. On average, temperatures decrease with height in the troposphere and mesosphere, and increase with height in the stratosphere and thermosphere. The atmosphere’s vertical temperature profile is important because it affects the ability of air to move vertically (that is, to rise and fall) in the atmosphere. Vertical motions are generally permitted in regions where air temperatures decrease with height, and they are suppressed in regions where air temperatures increase with height. The latter condition, known as a temperature inversion, creates a layer in the atmosphere that has the ability to limit, or cap, vertical mixing. The stratospheric inversion is a prime example; it caps vertical motions associated with storms in the troposphere.
Smaller-scale temperature inversions that occur within the troposphere may have significant impacts on air quality at urban and regional levels because they trap polluted air masses close to the ground. A common example of this is a radiation inversion, which often develops during winter months on days with limited wind and low humidity. These conditions allow the ground to cool quickly, thereby causing the air immediately above the ground to cool more than the air aloft. The warmer air aloft forms a stable layer, or cap, over the atmosphere below it. Radiation inversions are an air-quality concern because they often set up during the evening traffic rush hour and trap associated pollutants close to the surface. In general, radiation inversions affect small geographic regions and dissipate during the early morning, when solar radiation raises temperatures near the surface.
A larger-scale subsidence inversion forms when a high-pressure system causes air to subside and warm adiabatically (without gaining or losing heat) over a broad region. Different rates of subsidence between air near the surface and air aloft allow the air aloft to warm to a greater degree, setting up a large-scale temperature inversion. In contrast to radiation inversions, subsidence inversions have the potential to persist for long periods of time. This scenario is common in many metropolitan areas that are known for poor air quality, particularly Los Angeles, California. In Los Angeles, the impact of temperature inversions is magnified by the fact that the city is surrounded on three sides by mountains, which limit horizontal mixing and promote higher concentrations of air pollutants in the region.
The pollutants that build up in urban areas as the result of temperature inversions have important long-term health implications and, at times, can prove to be serious short-term threats. For example, in 1948 toxic air conditions that resulted from subsidence inversion in the western Pennsylvania town of Donora caused the deaths of twenty residents. A similar incident known as the London smog disaster killed nearly four thousand residents of London, England, in 1952.
Bibliography
Aguado, Edward, and James E. Burt. Understanding Weather and Climate. 5th ed. Upper Saddle River, N.J.: Pearson Education, 2010.
Ahrens, C. Donald. Meteorology Today: An Introduction to Weather, Climate, and the Environment. Belmont, Calif.: Brooks/Cole Cengage Learning, 2009.