Have you ever been outside on a sunny day and suddenly noticed the wind become calm and the temperature rise several degrees? Then, after a few minutes, a strong gust of cool wind blows through, and the temperature and wind return to normal? Well, what you just witnessed was a thermal. A thermal is a current of warm, rising air. Thermals form because some surfaces, like ashphalt car parks and buildings, become very hot when subjected to the Sun's rays. These hot surfaces heat the air above them. The hot air rises when the conditions allow it to escape. As it rises, the hot air pushes the cold air above it out of the way. Below the thermal, more cold air blows in to replace the hot air that has just risen.

As the air rises, it cools. At some point the thermal will become colder than the surrounding air. This is the top of the thermal, and the height at which this happens is called the inversion layer. The colder air wants to go down, but it can't, because the air below it is pushing it up. So it spreads out. If there is enough moisture in the air, it condenses to form a puffy cumulus cloud.

Where and When do Thermals Form?
Thermals are more likely to form over areas that absorb heat but keep it on the surface. Many factors determine where this is most likely to be, including:

			The type of surface. Soil transmits heat downwards, whereas ashphalt retains heat.
			Colour. Dark coloured surfaces absorb heat, light coloured surfaces reflect it away.
			Vegetation. Trees use solar energy to evaporate moisture; crops keep heat on the surface.
			The moisture content in the ground.
			The time of day. Thermals are stronger when the Sun is almost directly overhead.

How fo I find a thermal?
Before you start searching for thermals, it's a good idea to learn how to get consistently safe, high launches using a bungee. You should also be comfortable with making smooth right and left turns at your model's minimum sink speed.

The Hunt Begins
Thermal activity varies considerably from day to day. The best days to start off with are calm and sunny with lots of puffy cumulus clouds. Mid-day is the best time since stronger thermals occur more often when the sun is directly overhead. Thermals are easier to find when there's lots of them!

To start off, launch your model and fly over an area you suspect may generate a thermal. At flying sites with no obvious thermal sources, fly in air you haven't tried yet. The more air you cover, the more chance you have of finding a thermal.

Most thermals are fairly small, so a sailplane won't often fly directly into one. If it does, the model may waggle its wings for a couple of seconds as it flies through the turbulence in the edge of the thermal, and then the model will start to climb. But more often than not, the model will only fly through the edge of the thermal, missing the core altogether. In this case, the wingtip closest to the thermal will be in stronger lift than the other, forcing the model to bank away from the thermal.

Staying in the Thermal
As soon as you think you've found a thermal, level the wings and fly straight ahead until you feel you're in the strongest lift, give it a couple more seconds and then make a smooth turn towards the thermal. Hopefully this will put your model in a circle centred in the core of the thermal.

Watch the sailplane carefully while it is circling. If the model isn't climbing on one side of the circle, move the circle towards the strongest lift by levelling off for a couple of seconds. Keep moving the circle in the direction of lift until the entire diameter of the turn is inside the thermal.

Once you're established in the thermal, try to maintain a constant bank and let the glider drift downwind with the lift, adjusting the centre of the circle as required. If the wind is light, the drift will be almost unnoticeable. But on windier days, be careful - don't let the model get too far downwind! If you lose the thermal, you will almost certainly end up in the sink that surrounds it, so always allow plenty of height to get back to the field.

If you lose the thermal, or if your initial turns showed no further sign of lift, don't worry. Open up your circle and probe the air around where you thought it was. It's probably still there, you just flew out of it. More often than not it's downwind from where you think it is!

Bailing Out of a Thermal
If you're new to thermal soaring, it may seem impossible that you could find a thermal that takes your plane so high that it flies out of sight and is lost. But it does happen! (Been there, done that...)

Plan to get out of the thermal at an altitude where you can still comfortably see the glider. Try flying at 90 degrees to the wind (this should take your model out of the lift) or spinning the model. Diving the model is not a good idea - sailplanes are very efficient, so they can build up speed very quickly and might break up in the rough air.

If you do lose sight of your model, don't panic. Yell to everyone in the area to look up and smoothly apply controls for a spin (full up elevator and full rudder). The spinning motion will make the sailplane easier to see as the sun reflects off the covering.

Tips for finding more thermals
Pay attention to other sailplanes and, better still, birds. Soaring birds will not only show you where the thermal is but where the centre of the thermal is.

Flying under a cumulus cloud can often lead to a good thermal, but be aware that the life cycle of a fair weather cumulus cloud is about twenty minutes. So it's a good idea to watch the clouds and see which ones are growing and which ones are dying. Growing clouds have a well-defined base and hard, well-formed edges, whereas dying clouds are much more ragged in appearance.

Keep track of what the wind is doing. If a cool breeze suddenly blows through, it's probably because a thermal has just gone off nearby. The breeze will be blowing towards the thermal.

If you notice the temperature rise by a few degrees, you're standing in a thermal. You may also notice small bugs being sucked up by the thermal. Sometimes small birds flock together in the thermal to eat the bugs, so a bunch of swallows flying around in one area may indicate lift.

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