Inversions
Air pollution in a region can be intensified by a weather phenomenon known as a "temperature inversion".
Temperature inversions occur when cold air flows along the ground and becomes trapped below warmer air. Any pollutants contained in the cold air also become trapped and remain in the atmosphere until weather conditions change.
This is simulated and explained in more detail below.
Temperature inversion simulation
Usually the air closest to the ground is warmer, becoming cooler with increased distance from the surface of the ground. However, if cold air enters from outside an area, it may flow along the surface of the ground, below the warmer air. If the difference in the density and temperature of the two air parcels is sufficient they will form two distinct layers, with the cold air forming the lower layer and the warmer air becoming the upper layer.
The temperature inversion occurs when the colder air is not able to penetrate the warmer air. If the cooler air layer is polluted its pollution is also trapped within the temperature inversion until the layers begin to mix and the pollution can be dispersed.
A simulation of Brisbane city during a temperature inversion is shown below:
A real Brisbane temperature inversion
The cold air flows and calm conditions that are usually associated with temperature inversions occur most commonly on winter nights. As the day progresses and the air temperatures increase the temperature inversion usually begins to break up.
The picture below shows a temperature inversion across Brisbane as seen from Mt Gravatt on the morning of Saturday 23rd June, 2001.
Temperature inversion over Brisbane viewed from Mt Gravatt, Saturday 23rd June, 2001
Last updated: 21 October 2005