Weatherwatch with Irv Lee

Inversion

At altitudes of interest to the GA pilot, air temperature usually decreases the higher you fly.

But an inversion occurs when the normal rule of temperature reduction with altitude is temporarily reversed, and, for a band of a few thousand feet, the temperature actually rises slightly with altitude, before decaying again as normal.

This type of inversion is produced over a matter of days by air descending in a stable, very slow moving, high pressure system. These anticyclones are independent phenomena, often known as 'blocking highs', as they divert the run of low pressure weather systems approaching from the west, leading to days without frontal systems running through.

Within an anticyclone system, the air descends from the upper levels causing a higher pressure, and also warms itself up. The air near the ground cannot descend, and as its temperature is regulated from below by the ground, eventually an inversion is formed as the descending air reaches a slightly higher temperature than the air underneath.

Air in the lower level, beneath the inversion, is trapped by the descending air above, and convection currents caused by ground heating during the day do not easily break through the inversion layer in a powerful stable high pressure system. Over time, pollution and dust, unable to escape upwards due to the descending air, is trapped in the layer a few thousand feet thick between the ground and inversion level.

Due to the higher air temperatures experienced, the general term is a 'heat haze'. Horizontal visibility worsens day by day. By looking vertically through air, visibility is reasonable, as there is only about 3000 to 5000ft of poor quality air to see through. Looking horizontally, there is miles of it. When viewed from the ground, if the blueness of the sky above reduces to nearly white by the horizon, expect poor visibility when flying - becoming atrocious looking into a low sun.

Once flying, visibility above the inversion level is very good but it can appear that you are flying above a grey sea, which is the polluted layer underneath. You'll only spot navigation features when almost above them. Winds are much stronger above the inversion than below. Differences in groundpeeds achieved by flying above the inversion can be significantly different to those achieved by flying below it.

Metform 214 will indicate the level of the inversion through its temperature with altitude table. It will also give the wind vectors at different altitudes and alert pilots to the pros and cons of using the higher winds.

As ground warms in the sun, convection currents can develop during the day in the layer under the inversion. Later in the day they can cause sufficient mixing with higher winds to increase the wind speed at ground level. In weaker systems, the mixing can destroy the inversion by early afternoon, evident by higher and higher cumulus clouds forming. Whether clouds form or not, the circulation trapped below the inversion can produce turbulence. Rough air can also be trapped into the inversion on the lee side of low hills.

The descending air in the high pressure system warms up to produce high temperatures. The effect of this on aircraft performance is significant on take-off distances, Be careful. VHF air bands can be badly affected by continental interference during very stable high pressure days. The inversion can 'duct' euro transmissions and make them sound local.