Bureau of Meteorology > Weather Services > Aviation

REFERENCE MATERIAL
Temperature Inversions 

Temperature inversions are of interest because: 

•	they generally limit vertical development of clouds; 
•	they trap pollutants that reduce visibility; 
•	noticeable changes in aircraft performance sometimes occur when flying through them; 
•	turbulence is frequently encountered when flying through and/or near inversions. 
•	fog and low cloud often form in light winds, within low-level inversions, when moisture levels are high. 

Three common causes of temperature inversions in the lower atmosphere are:

•	radiation from the earth's surface; 
•	subsidence (sinking air) associated with high pressure systems; 
•	frontal systems. 

Subsidence, radiation and frontal inversions are evident in the Perth soundings. For the latest sounding (red line):

•	a radiation inversion exist from the surface to about 980 hPa; 
•	a frontal inversion lies just above 600 hPa. 

For the previous sounding (blue line): 

•	a subsidence inversion existed near 800 hPa. 

Other features to note in the figure above are: 

•	the tropopause for the latest (red) sounding is near 360 hPa; 
•	the tropopause for the previous sounding (blue) was near 410 hPa. 

Surface radiation inversions Surface inversions form on clear, light-wind nights and are strongest around sunrise. They form as air in contact with the ground cools by conduction. Because air is a poor conductor, strong surface inversions tend to be shallow, typically a few hundred feet thick, and can occasionally have a temperature increase with height of up to 10°C from the surface to the top of the inversion. Deeper radiation inversions may extend many hundreds of feet if light turbulent winds mix the very cold surface air into the warmer air above. Figure below shows the structure of a radiation inversion with a shallow layer of cold air under warmer air.

(above) A temperature altitude graph showing the lapse rate at the time of surface minimum temperature, as a thick black line, BCD. The line ACD represents the lapse rate at the time of surface maximum temperature. Layer BC is the inversion layer.

The rapid decrease in density associated with strong surface radiation inversions can have a noticeable affect on climb performance, especially for light aircraft. 

Surface inversions can also be associated with strong wind shear when the less dense air at the top of the inversion tends to flow over the colder air, rather than mixing with it. An aircraft climbing through the top of an inversion may experience a sudden increase in wind speed and/or direction and turbulence.

Within the inversion layer visibility may be reduced by trapped particles of smoke and dust. Fogs are prone to form within radiation inversion layers.

(above) A temperature altitude graph typical of a subsidence inversion. The lapse rate (shown as a thick black line) shows high-level warm air overlying low-level cooler air. The demarcation zone between the warm and cool air is referred to as the subsidence inversion. 

Subsidence Inversions 
Subsidence inversions (depicted in Figure above) frequently occur with high-pressure systems that cause air in the upper levels to sink and thus warm. When the upper layer warms at a greater rate, an inversion is formed. A typical subsidence inversion in the Australian region occurs at an altitude of 4000 to 6000 feet. Subsidence inversions are strongest and lowest on the eastern flank of high pressure systems. 

Subsidence inversions can be of significance to aviation because: 

•	thick haze and smoke can be trapped beneath the inversion so that descent through the inversion is coincident with markedly reduced visibility; 
•	convective currents are inhibited so that low-level convective clouds are limited in their vertical development. Cloud tops therefore spread out below the inversion layer to form a solid layer of stratus or stratocumulus cloud. With slow moving high-pressure systems this cloud layer may persist for several days especially during winter (sometimes referred to as anti-cyclonic gloom); 
•	persistent low stratus cloud may form, if surface conditions are moist and light winds mix the air to saturation below the inversion; 
•	they provide ideal environments for fog because: 
- the atmosphere is generally clear above the inversion; 
- winds below the inversion are light; 
- pollutants are trapped within the inversion layer providing an abundance of condensation nuclei; 
- humidity is often high; 
- conditions are favourable for radiation inversions to form below a subsidence inversion. 

(above) A temperature altitude graph showing the lapse rate as a thick black line depicting a frontal inversion at upper levels.

Frontal Inversions 
Frontal inversions (depicted in Figure above) form at the frontal surface in the upper atmosphere between the cool and warm air separating two air masses. Similar inversions occur on a more local scale at the sea breeze front. The inversion itself is not as significant to aviation as the weather that accompanies it. However turbulence and wind shear typically occur at the interface zone between cooler and warmer air that may upset smooth flight.

Airservices Australia is the official distributor of aviation forecasts, warnings and observations issued by the Bureau of Meteorology. Flight briefing services are available at: www.airservicesaustralia.com. Telephone contact details for elaborative briefings are contained in Airservices’ Aeronautical Information Publication Australia (AIP), which is available online through their website.

The Australian Bureau of Meteorology provides a wide range of observations, synoptic charts, satellite and radar images, forecast and warnings via the internet at www.bom.gov.au. 

©Commonwealth of Australia, 14 February 2011