To start with, a thermal is formed rather like the shape of a
balloon when the sun heats the ground and the ground heats
the overlying air in turn. If the heat source is strong, the
thermal bubble transforms into a narrow pillar shape or column
which grows upwards in continuous fashion, provided the warm
air supply is large enough. Some days can exhibit both balloon
and column-type thermals, though usually one or the other exists
in a certain time period.
A thermal actually comprises the internal upward draft and a cooler descending draft surrounding. In flight, a pilot should expect to encounter both sets of drafts, one shortly after the other. This phenomenon establishes an essential equilibrium in the atmosphere.
Here is a guide to estimating the thermal strength you will encounter based on the wind speed variation at launch. Assuming: Thermal vertical flow speed = 9/10 of the variation in wind speed at launch. Your average sink rate is 1.2 m/s (240 FPM) and knowing that 1 km/h = 0.278 m/s (1 mph = 88 FPM), we can establish that 4.3 km/h (2.7 mph) of vertical flow is needed to give you sustained flight which is equivalent to 4.8 km/h (3 mph) of wind speed variation felt on launch (4.3/.9). Every 4.0 km/h (2.5 mph) beyond that will add another 1 m/s (200 FPM) of lift.
Thermaling, or soaring in thermals, consists of remaining as much as possible in rising parcels of air and avoiding descending ones. As long as you maintain a low speed inside the thermal you'll be able to stay in it longer and then pass quickly through the descending currents by speeding up. Remember, turbulence usually exists around thermals, so maintain good control and inflation pressure.
Working a thermal basically involves
flying where the lift is either by circling within
it or by flying a figure 8. A combination of techniques is also
a good idea. Your climbing motion can be either upward circles
or by hovering vertically in rare cases where the wind is right. It
is all a matter of experience and instinct, which is
why women pilots tend to excel at thermaling!
In more detail, this is what happens: When you encounter a thermal you may feel the sensation of lift on one side of the wing. You should turn toward this lifted wing to enter the thermal. A thermal is stronger at its center and weaker on its rim or edge. You should find the area with the strongest stable lift, which is near the core or center of the thermal and circle to remain within it. The stronger the lift, the more you may be able to widen your turn radius and vice-versa. The air within a thermal is constantly and randomly changing. While flying circles in the thermal, apply sufficient pressure on the brakes, shift your weight towards the inside of the turn and control the brake on the opposite side to produce a smooth, coordinated turn. You must apply only the appropriate amount of pressure on the brakes, which is a matter of experience. That's what makes the difference between pilots in the same thermal.
A basic asset of thermaling skill is the ability to concentrate well. Listen to the indications of your variometer and adapt your maneuvering accordingly. Many pilots tend to alter their turn radius improperly and as a result drop out of the thermal. Those pilots who can detect the thermal drift or core variations best will climb highest. When the degree of lift is steady it means that you are located close to the center of the thermal.
Thermals vary in size. Tighter turns must be made for small thermals, and indeed there are occasions when you will not be able to complete a turn within the thermal diameter. Very large thermals are wide enough to maintain a straight flight or figure 8s, and thus you will not to have to make turns in them.
Thermals vary in their degree of lift. Strong, large thermals can be best utilized with a shallow bank angle. In weak thermals you can climb better by applying weight shift opposite to the turn. Thermal cores may merge into one and as a result it is not uncommon to have two pilots in separate thermals converging at a higher altitude. After landing a typical exchange of words between them would ensue along the line of:
"I was there first",
"I don't think so. It was mine"
"You followed me" and so forth. In fact, reality shows that they had each climbed in two different thermal cores that went on to merge into one.
A number of pilots claim they can smell the thermal's whereabouts. Indeed, a waft of soil, traces of dust and leaves can be seen, so it is not idle talk.
My suggestion, after some years of flying, is to concentrate on your wing and feel its center of pressure. When you enter a thermal the tip is lifted, thus the center of pressure is to the side of the wing. You must bring it to the center by turning to the lifted side. It is easy to feel the pressure on your canopy if you concentrate a bit. In order to locate the thermal's shape just keep the center of pressure in the center. In a weak lift this is an essential technique.
Spotting a thermal straight after launching is important,
especially when hillsides and slopes are not very high. Sometimes thermals tend
to stand still against a ridge before assuming their long slender cylindrical
formation at a higher altitude.
If a thermal has not assumed its shape because it is expanding by the slope or hillside we get the impression that we are experiencing a general wind such as in ridge flying. This is not the case, but if we steer away from the slope it is likely that we will enter sink and have to make a quick landing. It is better to fly in this area and wait for a stronger thermal.
After launching try to achieve as much altitude as possible,
either by gradually ridge soaring upwards or performing figure
8s in areas of lift. Then find a stronger lift, fly circles once you
have ample clearance and finally climb above the slope. The first
circle in close proximity to the launching area will be the trickiest
to execute, as it needs to be a tight turn to avoid getting too close
to the hill.
Thermals can often be found further out from the slope so you will have to track them down after reaching adequate altitude. Should you lose altitude, return to the lift at the slope and try again. Be patient. It's the end result that counts. There will be times when flights are terminated solely due to one minor oversight or error, so do not despair or feel sorry for yourself. As an example, in Spain once there was a group of pilots stuck that was low on a hillside. We had to wait for an hour for a decent thermal, but it was well worth the wait. We got over 2000 m (6,500 ft) of altitude out of it. Many smaller thermals had passed before, but none were able to lift us away from the hill. Sometimes clouds cover the sky and thermal activity is stopped for a while. Try to keep your height by ridge soaring until thermal activity returns.