Visualising thermals
Sadly, air is invisible, cold as well as warm. So to get the most of the conditions we need to be able to build a mental picture of the thermal we’re circling in, to visualise it in our minds, see picture 1.4. Some thermals are very large and may even be kilometres long, for example under long cloud streets. Others are small, narrow, or made up of several cores, each showing significantly different climb rates. In the next pages we’ll be showing a number of different thermal structures, to help you in your own visualisation process.
The vortex structure of thermals
Smoke rings blown by cigarette aficionados give a good visual clue as to the structure of thermal vortices. We know from experience that the thermal is several times stronger in its core than closer to the edges. Why is that so? To explain it, the vortex-structure theory was developed. The accuracy of the theory can be affirmed often enough through observations: when the thermal rises the friction against the surrounding air slows it down along the edges. This sets off a rotating movement from the inside out, like when we turn out a sock. These vortices may be observed both in thermal bubbles and in thermal columns.
Let’s look at the significance of this vortex ring structure for the pilots A to E
Pilot A is in the core and climbing twice as fast as pilot B, who is at the apex of the climb. Once pilot A catches up with B they will continue with identical climb rates. Pilot B is flying against the head wind caused by the vortex ring. If he’s carrying a GPS he may notice that his ground speed is lower than it was shortly before. If he flies over the centre of the core he’ll suddenly have a tail wind combined with lower climb rates. It is possible to sense the acceleration when moving over the core and into the tail wind, and the pilot should turn immediately to stay in the core and maximise his climb. The core of the vortex ring is rather turbulent and the pilot must continuously respond to the pitching of his wing and carry out adjustments to remain in the strongest lift.
Pilot C is still above the thermal. Only when he has sunk down to the level, and it has climbed up to meet him, may he commence thermalling and gaining altitude. Pilot D has fallen out of the side of the thermal and is heading away. Provided he’s carrying a GPS he may notice his ground speed picking up whilst his sink rate is also increasing. Pilot E is approaching the thermal low down. He has a tail wind and already should see reduced sinking - he’s practically being sucked into the thermal. The example above shows how important it is for each of us to always try to build up a mental picture of the thermal we’re in; to visualise it. Only by doing so may we understand what is going on all the time in relation to which part of the thermal we’re in, and make the most of it. This also allows us to re-enter the core quicker in case we loose itTip: If I’m flying straight and suddenly feel an increased drift towards one side, maybe even combined with reduced sinking, I always follow the drift immediately. Chances are I’ll fly right into a thermal, just like pilot E is about to do.
Experience
I had the most amazing experience surfing a vortex ring in a very strong thermal once. I was going up at 9+ m/s on a no-wind day, and suddenly realised by looking at my GPS that my forward speed had gone to zero! At the time I was thinking hard about where that very strong headwind had suddenly come from, but today I realise that I was simply at the apex of the vortex ring, pointing right into the outflowing air (see illustration 1.29, pilot B). I eventually made 1,500 m vertically in very little time, flying straight and remaining practically on the spot! Impressive!
Let’s imagine two pilots trying to exploit the lower end of a thermal bubble rising as a vortex structure; one is
A thermal bubble rising. The lower pilot loses it early on, and searches in vain for the now much higher thermal. The upper pilot just makes it into the centre of the vortex ring, where he manages to remain. The lower pilot can only watch as his buddy becomes smaller and smaller.
Within the thermal are several ’hotspots’ where the lift may be even better
TIP: Vortex rings are commomplace among isolated thermals with narrow cores. When the thermals grow large, as they do under cloud streets etc. the structure is rare.
A cloud showing classic vortex shape. By looking at such clouds we may learn a lot about the structure of the invisible thermals.
Thermal Flying is a 272 page colour book brimming with all the latest techniques and tips from one of
February 14th, 2008 at 11:26 pm
GOOD ARTICLE ! Nasty Nasty ‘Thermals’. Those ‘ol rising air currents caused by heating from the underlying surface, especially such a current when nature is not producing clouds. If we could only see them? Clues are there… These wretched wisps of nature are described to me by my neighbor, Doug Foster of Newport Beach, California in a quick story. Doug is a ‘Old Pilot Pioneer’ from the early 1970’s school of flight. School of hard nocks…and crashes!..Doug flew with the ‘Wills’ Brother’s Bob and Chris Wills throughout the early 1970’s. A quick “Thermal” story Doug told me: He described ‘A GIGANTIC THERMAL’ knows as an “Inversion Layer”, that came outta nowhere during a scenic Silmar flight, that began to pitch Doug, Bob, & Chris up… and forward at a rising speeds…. This “Inversion Layer” propelled Bob and Chris from Silmar where the launched, to San Bernadino! Doug landed back in Silmar, however the other sky kings (Bob& Chris) thought it’d seem fit to go-with-the flow and fly home. 40 miles + away… They landed at home in thier neighborhood. It’d like to have sen that !- Don Fuschetti, Newport Beach. Calif. CDM Class of ‘88.
August 21st, 2008 at 3:41 am
Hello…this is Doug Foster here: All I can say, is those guys had way more balls than i did. All I just wanted to do was to get down!!! BuT, it was a ‘hanglider cometition’ and I did manage to land right on the bullseye. -Doug Foster, Class of 1970, Newport Harbor High, Newport Beach Calif. USA.