Atmospheric Inversions

Introduction

An atmospheric inversion, which is also called a thermal inversion, happens when temperature increases with altitude, instead of the normal decreasing temperature that occurs as altitude increases.

The inversion occurs in a limited region near Earth’s surface, where a zone of colder air is trapped between the ground and the warmer air above. This can trap air pollution close to the ground, rather than allowing the air to disperse, which can be unhealthy, particularly for people with respiratory ailments.

Although local geography is an important determinant of an inversion (for example, a coastal area that also has bordering mountains is prone to develop inversions, as are valleys), the presence of motor vehicles is also key, as they contribute the emissions that lead to the formation of photochemical smog.

Historical Background and Scientific Foundations

In an atmospheric inversion, a layer of colder air is trapped between the ground and overlying warmer air. Normally, the temperature in the troposphere—the atmospheric layer that is closest to Earth’s surface—decreases by an average of 3.57°F per 1,000 feet decrease in altitude, or 6.5°C for each 1,000 meter decrease in altitude.

For example, if the temperature at sea level is 59°F (15°C), the temperature at an altitude of 3,280 feet (1,000 m) would be approximately 47.3°F (8.5°C), and at 6,562 feet (2,000 m) high the temperature would be 35.6°F (2°C).

An example of an atmospheric inversion would be a surface temperature of 59°F (15°C), and temperatures at 1,640, 3,280, 4,920 feet (500, 1,000, and 1,500 m) of 53, 57, and 52°F (11.5, 14, and 11°C), respectively. The air at 1,640 feet (500 m) above the ground is relatively cooler than the air at sea level and immediately above.

The cooler zone in an inversion is stable, so pollutants that are emitted from vehicles or other sources on the surface will become trapped until the inversion disperses.

An atmospheric inversion can occur for three reasons. First, on a night when the sky is clear, the surface can rapidly radiate heat collected during the day back into the atmosphere. The air near the surface can become cooler than air at higher altitudes.

Atmospheric conversions along coastal regions can occur due to wind blowing onshore from over cold water. The colder incoming air is denser and so will settle closer to the ground than the warmer air found over the coastal land. This is called an advectional inversion. Cities such as Los Angeles can be affected in this way.

Finally, if an area is under the influence of a region of high atmospheric pressure, the air will be moving downward (subsiding). This air becomes compressed by the surrounding air, which warms it up. The effect creates a zone of cooler air underneath the warmer subsiding air. This effect tends to occur near coastlines and when mountains are present nearby. Los Angeles again provides an example.

However the inversion forms, the effect is to trap pollutants in the region close to the ground. This creates air conditions that can be less healthy than would normally be the case.

The problem is especially prevalent in urban centers, when vehicle emissions combined with sunlight give rise to ozone (ozone is normally present in another, higher layer of the atmosphere called the stratosphere). In the stratosphere, ozone is valuable as it blocks the harmful ultraviolet rays of the sun. Near the surface and when combined with hydrocarbons, however, the result of the presence of ozone is photochemical smog.

WORDS TO KNOW

ANTHROPOGENIC: Made by humans or resulting from human activities.

GLOBAL WARMING: Warming of Earth’s atmosphere that results from an increase in the concentration of gases that store heat, such as carbon dioxide (CO₂).

GREENHOUSE GASES: Gases whose accumulation in the atmosphere increase heat retention.

PRIMARY POLLUTANT: Any pollutant released directly from a source to the atmosphere.

Impacts and Issues

Atmospheric inversion can have economic consequences. For example, in agricultural regions of California and Florida, the cooler air near the ground can affect citrus crops. If the inversion is severe, the near-freezing temperatures can result in heavy crop loss. For example, in January 2007, a nearly week-long inversion that produced freezing overnight temperatures killed almost 75% of California’s citrus crop, producing nearly $1 billion in losses.

Built-up smog that can occur as a result of an atmospheric inversion can have health consequences. Photochemical smog is an eye irritant and can damage the lungs. More serious health effects can occur. An example occurred in London, England, from December 5-9, 1952, when a temperature inversion caused smog to linger at the surface. During the week before the inversion, about 2,062 people died, which was average for the time of year. But, during the week of the inversion, 4,703 people in London died. Up to 900 people died per day, many of these people had existing respiratory ailments. As the inversion ended and the smog cleared, the death rate dropped correspondingly.

Today, a number of major cities around the world frequently experience inversion-related smog. These cities include Mumbai (Bombay), India; Mexico City, Mexico; Sao Paulo, Brazil; Los Angeles, California; Cairo, Egypt; Hong Kong, China; and Salt Lake City, Utah.

See Also Air Pollution; Carbon Dioxide (CO₂) Emissions; Greenhouse Effect; Smog

BIBLIOGRAPHY

Books

Ho, Mun S., and Chris P. Nielsen. Clearing the Air: The Health and Economic Damages of Air Pollution in China. Boston: The MIT Press, 2007.

Schwartz, Joel. Air Quality in America: A Dose of Reality on Air Pollution Levels, Trends, and Health Risks. Washington: AEI Press, 2008.

Web Sites

Health Canada. “Health Effects of Air Pollution.” May 16, 2006. www.hc-sc.gc.ca/ewh-semt/air/out-ext/effe/health_effects_effets_sante_e.html (accessed March 25, 2008).