What is an inversion?
Under normal atmospheric conditions, the temperature decreases continuously with increasing altitude. An inversion is a reversal of this principle: In a certain layer of air, the temperature rises with altitude. It is therefore also referred to as a temperature inversion. Inversions can occur at different altitudes. If the temperature increase begins directly above the ground, this is known as a ground inversion.
Inversions are characterised by their pronounced stability. The layers of air above and below the inversion are strongly separated from each other and there is hardly any vertical exchange. This lack of mixing has far-reaching consequences: Moisture, water droplets and aerosols accumulate in the lower, cold layer of air, while the air above the inversion is often exceptionally clear. This results in a typical picture with excellent visibility above the inversion and significantly reduced visibility below, often caused by damp haze, fog, high fog or dry haze.
In regions with pronounced topography, such as the Swiss Plateau, an inversion layer with stratus clouds can be lifted by regional wind systems. The result is the well-known high fog, which can extend over large areas and persist for days or even weeks.
Typical weather conditions for inversions
Inversions occur preferentially in stable high-pressure areas, less frequently also in flat pressure distributions. In a high-pressure area, the air sinks over a large area and heats up adiabatically. This process, known as subsidence, leads to cold air being „trapped“ in the lower layers, especially if it cannot flow sideways due to the terrain structure.
In Switzerland, the Jura and Alps favour the formation of such cold air lakes on the Central Plateau. Similar effects occur in valleys and basin locations, where cold air collects and is covered by a warmer layer above it. An inversion can be clearly recognised in temperature and dew point profiles, as the temperature rises with altitude or at least does not decrease any further.
Effects on flight performance
An often underestimated consequence of inversions affects aircraft performance. Near the ground, the cold air is denser than the warmer air above the inversion. If an aircraft climbs through this layer, it changes from high to low air density within a short time. The result is a change in density altitude, which can lead to a noticeable loss of performance.
This effect is particularly critical in the climb after take-off. Depending on the strength of the inversion, the climb gradient can decrease significantly, which is particularly relevant for heavily loaded aircraft. For flight planning, this means that performance calculations should not only be based on the ground temperature, but must also take into account the temperature curve in the lower thousand feet.
Turbulence and wind shear in the inversion area
Inversion layers not only separate air masses of different temperatures, but often also different wind regimes. Different wind directions and speeds can prevail below the inversion than above it. When flying through this layer, abrupt changes in the airflow can therefore occur.
Such wind shear can have a short-term effect on speed and lift. The combination of a drop in performance due to decreasing air density and a simultaneous headwind or crosswind during climb is particularly unfavourable. This requires increased attention and a conservative power reserve, especially at low altitudes.
Visibility when flying through the inversion
Visibility can also change abruptly when an inversion passes through. While clear conditions often prevail above, visibility below can be severely restricted. During prolonged inversions, not only moisture but also pollutants and aerosols accumulate in the lower layer of cold air.
This becomes particularly problematic when the sun is low in the sky. The sunlight is strongly scattered by the particles, which can lead to pronounced glare and contrast effects. For VFR pilots, this means significantly reduced visual perception, even if the formal visibility is still maintained.
Technical support for recognising traffic can significantly improve situational awareness, especially under such conditions. Electronic visibility systems make an important contribution here to recognising other aircraft at an early stage.
Risk of icing in inversion situations
Inversion weather conditions are often associated with fog or high fog. For IFR flights, this means an increased risk of icing within the stratus layer. Supercooled water droplets can freeze immediately on contact with the airframe and cause relevant ice accumulations within a short time.
It is therefore advisable to fly quickly through these layers if this is possible in terms of performance and traffic. Special care must be taken during VFR flights in rising terrain: The moisture trapped below the inversion cannot escape, which means that the available vertical distance between the terrain and cloud base decreases continuously.
Freezing rain is a particularly dangerous scenario. During warm fronts, warm air glides over a cold base layer at high altitude. If snow melts into rain in the warm layer and then falls through sub-zero temperatures close to the ground, it freezes abruptly when it hits the aircraft. This form of icing is considered extremely dangerous and should be avoided wherever possible.
How do you recognise inversions during flight preparation?
Indications of inversion weather conditions can already be found in classic aviation weather information. TAFs and METARs from aerodromes on the Central Plateau often indicate poor visibility and low cloud bases, while aerodromes at higher altitudes report good conditions. A comparison of several stations at different altitudes often provides a clear picture.
Automatic information systems at aerodromes also provide valuable information. In the case of pronounced inversions, these are sometimes reported explicitly with altitude information and wind shear warnings. Satellite images often show inversions impressively when lower-lying regions are covered by fog or high fog, while alpine areas remain cloud-free.
Additional information is provided by vertical temperature profiles from radiosoundings. If the temperature rises with altitude, an inversion can be clearly identified. If you are unsure, you can also call the aviation weather service for advice.
Conclusion: An often underestimated winter topic
Inversions are a classic winter phenomenon that deserves much more attention in flight planning. They not only affect visibility and weather, but also performance, turbulence behaviour and icing risks. Anyone who recognises their typical characteristics and realistically assesses the associated risks can also carry out safe and well-prepared flights in the cold season.
Source references:
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