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An area on the ground that is warmer than its surroundings heats the air above causing bubbles of rising air to break away from the surface, usually close together forming a column of rising air that is only vertical if there is no wind. Areas that heat up faster give off heat faster, so thermals can often be found above newly ploughed fields, areas of bare rock, factories producing heat and even built-up areas with large amounts of tarmac / concrete. As the air in a thermal rises it cools and if there is sufficient moisture it can condense to form clouds. So another good place to look for thermals is under small puffy cumulus clouds that appear to be growing. Such a thermal continues within the cloud - so in large clouds it is possible to keep climbing so long as the glider is suitably equipped and the pilot is experienced in cloud flying.
If you find a thermal, the idea is to locate the centre and circle tightly around it where the lift is greatest. There are a few techniques available to help you do this, but there is always an element of luck.
After a few early encounters with thermals I was asked to find and centre in one myself.
We were slowly sinking earthwards so the main challenge now was to find some lift. We spotted another glider circling near Loch Kinord and I turned us towards it to try and share the same lift. About halfway we felt the gentle kick of rising air and the variometer jerked a little into the positive climb half of the dial, from its normally downwards position. We were rising at about one knot, as we cruised along at 45 knots.
I circled the glider fairly tightly trying to stay in the lift but there was very little as this thermal was so weak. I was encouraged a couple of times to limit the bank angle as too much bank increases the stall speed and if you stall the glider, the inner wing can suddenly drop pulling you into a spin. Best avoided at this early stage of training. Meanwhile the lift had faded to a net zero, so all we could do was cling on to our current altitude. We popped out of the failing thermal and headed towards the loch again. Then we were suddenly in another weak thermal. This time there was a bit more lift and I managed to stay in the rising air until we had gained about 700 feet, taking us past our starting altitude of 2000 feet and up to 2500 feet.
By now the other glider had headed off west looking for better air. I wandered around the sky for a few minutes looking for another bit of lift but to no avail. So we too headed west up the valley above the winding river Dee towards the ridge on the south where we might find some hill soaring due to the wind blowing up the slope. As we drifted up the valley the ridge top loomed closer and closer but we were still losing height. No luck. A quick 180 degree turn and we were heading back towards the lower slopes and the airfield.
More Attempts |
Soon after releasing the tow rope the twin variometers were wobbling gently around zero lift. The audio variometer was changing back and forth between a steady tone indicating a descent, and a series of beeps indicating an ascent. Under the cloudbase there was some lift as we expected - but not much. So I turned us north west beyond Loch Kinord (thermals are rare over water) and the thermal strength soon increased, indicated by a definite shove under the seat followed a second or two later by a steadily rising tone in the audio vario beeps (indicating the instrument lag). I rolled us into a steep left bank and circled steadily as the variometer settled around 3 knots up, peaking occasionally around 7 knots, while the altimeter indicated a steady height gain. This was more like it... |
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After a couple of minutes I spotted another glider circling higher up about two miles to the south over the river Dee, due west of the airfield. I turned towards it and put us into a shallow dive to gain speed and at 60 knots I retrimmed to maintain the nose-down attitude. The idea is to move quickly between areas of lift: although the glider descends more quickly it gets to the destination faster, losing less net height than if you had flown slower. After a minute or so we could see the other glider more clearly now - it was the club's single seater Junior (the little brother of the Puchacz) which is the first solo aircraft for the club students after becoming established solo in the Puchacz. We popped in and out of a few wee thermals, drifting slowly downwind all the time towards the north-west beyond Loch Kinnord. I could see well into the Grampian mountains beyond, with a swathe of higher peaks in the distance. |
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The clouds above were now rather broken and there was less thermal activity and we were starting to descend steadily in an area of sink. So I turned us around 180 degrees to the south-east to make sure we didn't drift too far downwind (or else we might not be able to get back to the airfield).
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As we approached I became concerned that we were getting too close, especially when we were only about two hundred yards away from the Junior, about two hundred feet below it. But the instructor told me that on busy days gliders can get closer still, sometimes barely a couple of wingspans apart. This underlined the constant message I was receiving throughout my training - maintain a good lookout. The main cause of gliding accidents is a collision, usually because someone is failing to keep a good lookout. Usually in competitions when pilots fly closer to the edge - or conversely when you don't expect anyone else to be nearby. As our two gliders circled gently together in the thermal, my head felt like it was on a camera tripod swivelling up and down and left and right to keep it in sight while checking for anyone else joining us. |
The Junior was now vertically above us and gaining height a little faster, probably because it is lighter, or more probably because I was not flying perfect turns around the centre of the thermal.
During this flight a plentiful supply of thermals allowed me some decent practice at staying in the area of highest lift in a thermal. When you encounter a thermal there is often an initial region of sink due to colder air falling down around the outside of the warmer rising air towards the core. Then one wing may suddenly lift up - the wing nearest the centre of the thermal where there is most lift. That indicates the way to turn - you need to circle towards the centre of the thermal which is on the side of the rising wing. The next trick is to find the position of the centre and circle steadily around it. Usually the rate of turn you choose is not quite right and you spiral past or away from the centre. One trick is to straighten out for a couple of seconds as soon as you hit the area of strongest lift, then start circling in the same direction again. If you get it right you'll rise steadily with the thermal until it starts to fade out, or you reach the cloudbase when you have to leave the thermal to keep the ground in sight.
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The wind was pushing our bit of the sky containing the thermal and our glider steadily westwards, along the river Dee towards Ballater. I could see right along the valley to the west, up to Braemar and Glen Shee, and to the east I could see back to Banchory and far beyond that the distant North Sea coast and Aberdeen about 40 miles away. This was the best view I'd had since starting gliding: as mentioned before, the view alone is often worth a trip in a glider. |
Not quite this good a view of Aberdeen |
Once again the lift was fading and now we were sinking rapidly towards the hills on the south side of the river Dee. Time to turn around again. I headed back towards Loch Kinnord to try and find some more of the lift we'd picked up earlier, and once again we started climbing. Then there was a call on the radio - the folks on the ground asked if they should get the other Puchacz out of the hanger as there was now a queue of folks waiting to fly. I realised that we had been in the air for well over an hour. We held a quick cockpit consultation and decided that since we were starting to tire and it was a hassle for our colleagues to prepare the other Puchacz, we would head back to the club.
On another occasion I spotted the Junior once more in a nearby thermal:
I accelerated towards the thermal and slowed to join it. As I started my turn habitually to the left, the instructor reminded me that gliders joining a thermal should circle in the same direction as any other glider(s) already established there. One of the rules of the air. The instructor yanked us out of the thermal and accelerated quickly away, then turned sharply around and I flew us back to join the Junior in a clockwise turn. We gained a little more height but then lost the lift all together - and the Junior (now much higher) headed off northwards.