Negative lapse, or a negative lapse rate, describes a meteorological condition where the temperature increases with height in the atmosphere, contrary to the typical decrease.
In atmospheric science, the term "lapse rate" refers to the rate at which temperature changes with altitude. While typically temperature decreases as one ascends in the troposphere (a positive lapse rate), a negative lapse indicates an unusual atmospheric state where the air gets warmer, not colder, as you go higher. This phenomenon is more commonly known as a temperature inversion, and it has significant implications for local weather and air quality.
Understanding Lapse Rate
The atmosphere's temperature profile is crucial for understanding weather and climate. For a detailed definition of lapse rate, you can refer to the American Meteorological Society's Glossary.
- Positive Lapse Rate: This is the most common scenario, particularly in the lower atmosphere (troposphere). It means that as you ascend, the temperature decreases. An average environmental lapse rate is about 6.5°C per kilometer (3.5°F per 1,000 feet). This decrease is due to the earth's surface being the primary heat source for the lower atmosphere.
- Negative Lapse Rate (Temperature Inversion): This occurs when the temperature increases with altitude. It signifies a layer of warmer air situated above cooler air, leading to a stable atmospheric condition.
Temperature Inversion: The Essence of Negative Lapse
A negative lapse rate is synonymous with a temperature inversion. This condition is called an "inversion" because it reverses the normal atmospheric temperature profile. Inversions are critical atmospheric phenomena due to their profound impact on vertical air movement.
How Temperature Inversions Form:
Temperature inversions can arise through several mechanisms:
- Radiation Inversion: Often occurs on clear, calm nights when the ground rapidly cools by radiating heat into space. The air directly above the surface becomes cooler than the air higher up, creating an inversion layer.
- Advection Inversion: Forms when warm, moist air moves horizontally over a colder surface (like snow or a cold body of water), cooling the lower layer of the air mass from below.
- Frontal Inversion: Associated with weather fronts, where a warmer air mass overruns a colder, denser air mass. The boundary between the two air masses often creates an inversion.
- Subsidence Inversion: Develops when a large mass of air slowly descends and spreads out (subsides) over a broad area, typically under a high-pressure system. This compression causes the air to warm, creating an inversion layer at the top of the descending air mass.
Impact and Significance of Negative Lapse (Temperature Inversions)
The high stability caused by a negative lapse rate has several important consequences for the environment and human activities:
- Air Quality Degradation: Perhaps the most well-known impact. Inversions act like a lid, trapping pollutants (smog, smoke, industrial emissions) close to the ground. This prevents vertical mixing and can lead to dangerous levels of air pollution, especially in urban areas or valleys, posing significant health risks.
- Fog and Frost Formation: Clear, calm nights that promote radiation inversions are often ideal for the formation of fog (if sufficient moisture is present) and frost (if temperatures drop below freezing). The cold, heavy air trapped at the surface facilitates condensation and freezing.
- Altered Sound Propagation: Sound waves can be refracted downwards by temperature inversions, causing sounds to travel much farther than usual and sometimes creating distinct "echoes" or clear propagation over long distances. Conversely, they can also create "shadow zones" where sound is muffled.
- Aviation Considerations: While generally indicating stable air, the transition zones above and below inversions can sometimes be associated with wind shear or turbulence, which can be a concern for aircraft. Additionally, visibility can be severely reduced by trapped fog or haze layers.
Comparing Positive and Negative Lapse Rates
To further clarify, here's a comparison of the key characteristics:
| Characteristic | Positive Lapse Rate (Normal) | Negative Lapse Rate (Temperature Inversion) |
|---|---|---|
| Temperature Change | Decreases with increasing height | Increases with increasing height |
| Atmospheric Stability | Generally less stable; promotes vertical mixing | Highly stable; suppresses vertical mixing |
| Common Name | Environmental Lapse Rate | Temperature Inversion |
| Typical Conditions | Daytime, cloudy conditions, general troposphere | Clear, calm nights; valleys; under high pressure |
| Impact on Pollutants | Disperses pollutants | Traps pollutants close to the surface |
Example of Impact: The notorious Great Smog of London in 1952 was severely exacerbated by a persistent temperature inversion that trapped industrial and domestic smoke, leading to thousands of deaths and prompting significant changes in environmental policy. Similarly, many cities located in valleys, such as Los Angeles or Salt Lake City, frequently experience poor air quality during strong inversion events because pollutants are unable to escape the basin.
Understanding negative lapse rates is crucial for meteorologists, environmental scientists, and urban planners to predict air quality issues, manage public health risks, and forecast specific weather phenomena.
