Autumn is well and truly with us here in the UK and with the new season comes new challenges for paragliding pilots wanting to keep flying. Richard Chambers, a member of the SkySurfing club, after talking to Steve Purdie, safety officer, posted a very insightful Telegram post about the risks related to flying in ground inversion. Below, we are expanding a little about what inversion is and re-iterating Richard and Steve’s message on what to look for and stay safe.

Understanding the “Inversion” meteorological phenomenon

A quick Wikipedia search brings the following definition:

Understanding the inversion in thermal lapse

In meteorology, an inversion is a deviation from the normal change of an atmospheric property with altitude. It almost always refers to an inversion of the thermal lapse rate. Normally, air temperature decreases with an increase in altitude. During an inversion, warmer air is held above cooler air; the normal temperature profile with altitude is inverted.

Essentially, it means that directly over the ground, a ‘layer’ of air cools quickly and gets to a temperature much lower than the one of the layers above it. Cool air being heavier, it becomes trapped under the warmer layers and stagnates until it gets a chance to heat up again and dissipate.

The reverse phenomenon of what causes thermals

As pilots, we are well familiar with the phenomenon that produces thermals: the ground gets heated up by the sun. In turn, it heats up the layer of air directly in contact with it. Ultimately, bubbles of hot air are released as thermals. As the sun goes down, the reverse happens and can cause an inversion if the temperature is low enough:

  • The once-hot ground radiates its heat energy. If there’s no blanket of cloud to bounce it back, the energy is lost into space and the ground gets cold,
  • The air touching the cold ground gets cold and dense,
  • As evening draws into night, the layer of cold air gets thicker and colder. Anything between a few feet and a up to several hundred feet, perhaps even up to a thousand  feet marks the boundary of the inversion layer.

What are the characteristics of an inversion?

The inversion layer traps a cooler airmass under it and directly above the ground: the air there is much heavier than the layers above. As a result:

  1. This mass of air is far less affected by the prevailing meteorologic conditions. The inertia of this airmass means that while it is a windy day, there is hardly any wind in it!
  2. The mass of air is much cooler and denser and therefore your glide ratio is negatively impacted: you will be dropping down faster and on a longer distance.
  3. The ground layer is “trapped” under the air mass above and going from one to another will expose you to different flying conditions that can be dangerous if not anticipated.

This is typically the source of severe potential risks for the unwary pilot

  • It’s been windy all day and close to sunset the wind drops to flyable. If you fly and climb a bit, you might get up to the windy layer and find a turbulent surprise in the shear layer between the two. Above the shear layer it will still be as windy as it was all day.
  • The wind early morning is light and flyable (possibly with plenty of wind from the north as the cold layer flows out to sea, just like a river). With some sun on the ground, thermals start and climbing begins. If you get a decent early climb, you might get to the shear layer and receive a battering from the turbulence. If you get through that in one piece there might be a lot more wind above.
  • At about the point in the day when the thermals are strong enough to get our intrepid shear-layer researcher aloft, all those thermals belting through the cold layer stir up the whole system and mix the cold air in with the normal air above. That usually takes 10 minutes to half an hour and is not a nice time to fly. The thermals race up through the cold air, lumps of windy air from above are brought down and sensible pilots will be on the ground having coffee.
  • Once the mixing is complete and the ground inversion is gone, all the cold air having mixed up with the air above, thermal activity will slow right down. That’s because the air over the fields is now warmer and the fields and their thermals need to get hotter to make the required temperature difference. After another coffee it should get going again and this time the climbs will go all the way up.

Sometimes, the effects of inversion are visible and can be felt on the ground

smoke trapped in the inversion layer
Smoke rising in Lochcarron, Scotland, is stopped by an overlying layer of warmer air – Source Wikipedia

In some cases, it is possible to see the cool air mass trapped under the warmer layers above. Typically, it is the case in the morning.

In the photo here, you can clearly spot the smoke trapped by the inversion layer under the warmer air mass above. In some cases, it may be less obvious, but it is possible to see a shimmering line at the boundary between both air masses.

Towards the end of the day and close to sunset, you get that distinctive autumnal feel – it’s cold and humid (even perhaps forming dew or mist) and markably less windy than it has been during the day. Well, that’s because you are standing in the cold air mass of the Inversion layer that is forming.

What are the other signs to spot an inversion?

Observe the weather pattern and look at what your glider and other pilots are doing:

  • It has been a warm, breezy day and in the evening the wind slows as it cools off,
  • The skies are fairly clear allowing the energy to escape,
  • It’s not windy on the ground, but the clouds are moving quickly above your head,
  • The forecast is for wind, but it’s not windy on take off,
  • The isobars and other higher level wind forecasts show wind but the simple forecasts for ground level wind show much less,
  • It’s clearly shown on the forecast soundings – on RASP of Meteo Parapente, looks for the inverted “S” shape of the temperature line,
  • You’re soaring around take-off where there’s plenty of wind but not much lift. You get a bit low and slope land only to discover there’s no wind at all. You’re in the cold lake and only the top bit of the hill was sticking out in the breeze,
  • You’re bottom landing into wind and at 10 feet the glider dives as it enters the cold, still layer and you land long (and fast). Hopefully you followed procedure and came in with excess airspeed and legs down so you didn’t stall.
  • You’ve watched someone take off half-way up, climb 100 feet above the hill, take a series of collapses, then start going backwards.

Do you have further tips or thoughts to add?

Or perhaps an experience flying in ground inversion so share? Then email us at info@parawaiting.eu and we will add it here