Do you ever wonder why on still cold winter evenings the smoke from chimneys only rises a short distance before spreading out? Or why on some sunny summer days the clouds start to build up and flatten out to fill the whole sky, or indeed sometimes disappear altogether? Well the answer to all those questions is because there is a temperature inversion.
Normally as you move from the ground up through the atmosphere the temperature decreases but a temperature inversion is where there is warmer air above so slowing down or indeed stopping convection currents from rising.
The winds above the surface can, and often do, blow in different directions. In fact if you look at the clouds at different heights in the sky you can often see them being blown along in different directions, so that the air directly above us could well have come from different parts of the hemisphere and consequently have different temperatures.
Many times a day from eight or so sites in the United Kingdom The Met Office fly large balloons filled with helium gas to measure the temperature, humidity and wind velocity thereby getting a three dimensional structure of the atmosphere.
One of the reasons this is done is to be able to calculate whether air, if heated or lifted by some other means, would carry on rising enough to form cloud or would sink back down again probably then giving sunshine.
As the air rises it cools down at a rate of 3 degrees Celsius for every 1000 feet of ascent and if it is still then warmer than the air already there brought in by the upper level winds, it would continue to rise. If not it would stop rising. So a temperature inversion acts as a lid to stop the air continuing to rise.
Inversions form in many different ways. The most common, I suppose, forms on still winter evenings when the ground, and the air near the ground, cools quickly, but, since air is a poor heat conductor, the air say 20 feet above the ground doesn't cool down very much. An inversion is formed as the air at 20 feet is much warmer than ground level so if you light a bonfire, the smoke can only rise a little way before spreading out under the temperature inversion. This inversion will only disappear when, on the following morning, the sun heats up the ground again above the temperature of the inversion.
Another inversion forms as pressure rises. I wrote, in Weather and High Pressure, that above an anticyclone the air is descending and warming up. This process itself can cause a temperature inversion and are frequently seen in the winter months. When this happens all the pollution from cars, factory chimneys, power stations and domestic fires, get trapped under the inversion and, because of the low light levels from the sun at this time of the year, make for very gloomy weather. It looks as though there is likely to be an absolute downpour any minute, but it never rains. In fact, forecasters call it anticyclonic gloom.
I suppose one of the most famous, but often forgotten, temperature inversions occurs at the top of the troposphere (which is the part of the atmosphere in which we live). That is where it stops getting colder the higher you go to where it suddenly starts to get warmer into the stratosphere. The demarcation line is called the tropopause, and is, in fact a temperature inversion.
So when you plan a bonfire in your garden this coming autumn don't just think where the wind will take the smoke but whether there is likely to be an inversion to keep the smoke trapped in a shallow layer or no inversion when the smoke can disappear into the upper atmosphere.
