Temperature inversion layers, also called thermal inversions or just inversion layers, are areas where the normal decrease in air temperature with increasing altitude is reversed and the air above the ground is warmer than the air below it. Inversion layers can occur anywhere from close to ground level up to thousands of feet into the ATMOSPHERE
Inversion layers are significant to meteorology because they block atmospheric flow which causes the air over an area experiencing an inversion to become stable. This can then result in various types of weather patterns.
More importantly, though, areas with heavy pollution are prone to unhealthy air and an increase in smog when an inversion is present because they trap pollutants at ground level instead of circulating them away.
Causes
Normally, air temperature decreases at a rate of 3.5°F for every 1,000 feet (or roughly 6.4°C for every kilometer) you climb into the atmosphere. When this normal cycle is present, it is considered an unstable air mass, and air constantly flows between the warm and cool areas. The air is better able to mix and spread around pollutants.
During an inversion episode, temperatures increase with increasing altitude. The warm inversion layer then acts as a cap and stops atmospheric mixing. This is why inversion layers are called stable air masses.
Temperature inversions are a result of other weather conditions in an area. They occur most often when a warm, less dense air mass moves over a dense, cold air mass.
This can happen, for example, when the air near the ground rapidly loses its heat on a clear night. The ground becomes cooled quickly while the air above it retains the heat the ground was holding during the day.
Temperature inversions also occur in some coastal areas because upwelling of cold water can decrease surface air temperature and the cold air mass stays under warmer ones.
Topography can also play a role in creating a temperature inversion since it can sometimes cause cold air to flow from mountain peaks down into valleys. This cold air then pushes under the warmer air rising from the valley, creating the inversion.
In addition, inversions can also form in areas with significant snow cover because the snow at ground level is cold and its white color reflects almost all heat coming in. Thus, the air above the snow is often warmer because it holds the reflected energy.
Consequences
Some of the most significant consequences of temperature inversions are the extreme weather conditions they can sometimes create. One example is freezing rain.
This phenomenon develops with a temperature inversion in a cold area because snow melts as it moves through the warm inversion layer. The precipitation then continues to fall and passes through the cold layer of air near the ground.
When it moves through this final cold air mass it becomes “super-cooled” (cooled below freezing without becoming solid.) The supercooled drops then become ice when they land on items like cars and trees and the result freezing rain or an ice storm.
Intense thunderstorms and tornadoes are also associated with inversions because of the intense energy that is released after an inversion blocks an area’s normal convection patterns.
Smog
Although freezing rain, thunderstorms, and tornadoes are significant weather events, one of the most important things impacted by an inversion layer is smog. This is the brownish-gray haze that covers many of the world’s largest cities and is a result of dust, auto exhaust, and industrial manufacturing.
Smog is impacted by the inversion layer because it is, in essence, capped when the warm air mass moves over an area. This happens because the warmer air layer sits over a city and prevents the normal mixing of cooler, denser air.
The air instead becomes still and, over time, the lack of mixing causes pollutants to become trapped under the inversion, developing significant amounts of smog.
During severe inversions that last over long periods, smog can cover entire metropolitan areas and cause respiratory problems for the inhabitants.
In December 1952 such an inversion occurred in London. Because of the cold December weather, Londoners began to burn more coal, which increased air pollution in the city. Since the inversion was present over the city, these pollutants became trapped and increased London’s air pollution. The result was the great smog of 1952 that was blamed for thousands of deaths.
Like London, Mexico City has also experienced problems with smog that have been exacerbated by the presence of an inversion layer. This city is infamous for its poor air quality, but these conditions are worsened when warm subtropical high-pressure systems move over the city and trap air in the Valley of Mexico.
When these pressure systems trap the valley’s air, pollutants are also trapped and intense smog develops. Since 2000, Mexico’s government has developed a plan aimed at reducing ozone and particulates released into the air over the city.
London’s Great Smog and Mexico’s similar problems are extreme examples of smog being impacted by the presence of an inversion layer. This is a problem all over the world, though, and cities like Los Angeles, Mumbai, Santiago, and Tehran frequently experience intense smog when an inversion layer develops over them.
Because of this, many of these cities and others are working to reduce their air pollution. To make the most of these changes and to reduce smog in the presence of a temperature inversion, it’s important to first understand all aspects of this phenomenon, making it an important component of the study of meteorology, a significant subfield within geography.
Most herbicide labels warn farmers not to apply herbicides during a temperature inversion. However, there is little research on inversion with respect to agriculture.
When University of Missouri weed scientist Mandy Bish went searching for temperature inversion research, she found data based on 1960s pollution work. So over the last two years, she’s studied and collected information about temperature inversion in Missouri for farmers, companies and government agencies.
Here are eight things about temperature inversion based on University of Missouri research you should know now.
1. What it is a temperature inversion?
On a typical day, cumulus clouds fill the sky as the sun radiates energy to the earth and warms the air. Warm air rises, because it is less dense. With warmer air rising, cooler air drops, is warmed by the earth and then rises. “It is a constant shuffling of air,” Bish explains. “We feel this, because wind occurs.”
An inversion happens when the sun sets and is not warming the earth’s surface. The cool air sinks, the air shuffle stops, there is no wind and the cumulus clouds dissipate. Now, the cool air is trapped on the bottom
2. Why is inversion a problem?
Warmer air temperatures near the earth’s surface allow volatile compounds to dissipate into the upper air levels. The problem with an inversion scenario — where cold air is trapped near the earth’s surface — is that it is very stable. “So when the air is not mixing, any particles suspended in that air stay suspended in the air,” Bish says. “If you have particles suspended in the air and you have a horizontal wind gust come through, it is going to push those particles somewhere else.” Think smog in a city.
3. When is the most likely time for a temperature inversion?
Last year, in June and July, inversions started at 6 p.m. and continued until 7 p.m. “That was a little bit alarming, because it is still daylight out,” Bish says. “The wind has died; it seems like a pretty good time to apply herbicides.” But it is not.
This year is showing that inversions last longer. For example, in June, there were 12 temperature inversions reported in Columbia, Mo. Eight of those lasted from 6 p.m. to 6 a.m., Bish says. Most are lasting more than eight hours.
4. Does wind speed indicate inversion?
For the past two years, Missouri weather data recorded wind speeds every 3 seconds in March, April, May, June and July. Combining all of the data, Bish looked at where inversions occurred, and how often, at that inversion, the wind was speed less than 3 mph. “Over 90% of the time in June and July, when you have an inversion, wind speeds were less than 3 mph,” she says. “That is why we think wind speeds are an indicator of a temperature inversion setting in.”
5. Where do most temperature inversions form?
Some farmers believe inversions happen only in valleys. There is a type of inversion that can happen specifically over a valley, Bish says, but really, anyplace where the sun can hit the earth’s surface can have an inversion.
Temperature inversions can also form, break up and reform in the same location in a matter of hours. “We had a crazy event on June 20 where an inversion formed, a rainstorm came through, it broke it up, then it came right back,” Bish says. “So we are still learning a lot about these.”
6. How locally restricted are these inversion?
Temperatures vary around the state, causing inversions to occur on different dates. However, in Missouri the entire state experienced inversion on the same day last month. “There were a couple of nights [June 24-25] with clear skies and calm winds across the entire state,” Bish says. “In every area we were monitoring, there was a temperature inversion those nights.”
There are localized inversion events. One location had temperature inversions three nights in one week. Then the next week, the same location saw cloudy skies and rain and not many inversions. “So, sometimes they come in chunks,” Bish warns.
7. Can you track or predict a temperature inversion?
Predicting temperature inversion is tricky. There are tools for farmers that record soil temperature and relative humidity, but not many for inversion potential, according to Bish. The University of Missouri has developed a RTTIM WEBSITE for real-time temperature inversion monitoring.
The site reports air temperatures from eight Missouri locations. On the home page, a state map displays locations, along with a blue arrow. A downward arrow means there is likely an inversion, an upward arrow means there is likely not an inversion. Along the left-hand side of the page, there is a links to graphs and charts that include inversion, temperature and wind speed.
Another tool that may help predict temperature inversion is Agrible’s Spray Smart phone app. The tool provides real-time data for inversion risk, atmospheric conditions, wind direction and wind speed.
No access to internet or website? Bish says to step outside. If it is a clear sky with no wind or cumulus clouds, it is the right atmosphere for an inversion.
8. Do smoke bombs work as a test for inversion?
Some herbicide labels say use a smoke bomb to see if there is an inversion. The University of Missouri tested the theory. Researchers lit off a smoke bomb at 4 p.m. when there was no inversion. The smoke dissipated within 50 seconds. Then they set one off at 7 p.m. The red smoke lingered well beyond a minute, indicating inversion. Bish says smoke grenades can validate an inversion, but more work still needs to be completed.