Where Rising Air Comes From
Sunlight warms the ground unevenly. A dark ploughed field heats faster than the woodland beside it, and the air directly above the field warms with it. Warm air is less dense than the air around it, so it rises, and the column it leaves behind is a thermal.
A thermal is not a steady pipe. It is a bubble that forms, breaks away, and drifts with the wind as it climbs. Which means a glider circling in one is chasing something that is itself moving, and the climb rate changes as the bubble and the glider drift apart.
Where the ground is uniform there are no thermals worth having. The best soaring country is a patchwork of surfaces that heat at different rates, which is why glider pilots care about the colour of a field from several thousand feet.
The Two Ways to Use It
- Circle in it. Turn tightly in the strongest part of the column and climb until it stops working. Slow, reliable, and it costs you nothing but the time you spend going round in circles.
- Run along it. Follow a line of lift downwind, gaining a little from each column rather than a lot from one. Used when the thermals are weak or scattered and the day is better suited to distance than to height.
- Get the wrong one. Sink is not the absence of lift, it is air going down, and it is often stronger than the lift you were hoping for. Half of soaring is leaving a bad thermal early enough to find a good one.
The Other Kinds of Rising Air
A thermal is the kind of lift most people have heard of and it is not the only kind. The others are worth knowing about because they behave quite differently, and because each one suggests a different way to fly.
Ridge lift is the simplest. Wind blowing against a hillside has nowhere to go but up, and the air above the slope is going up for as long as the wind blows. It is steady in a way a thermal never is. A glider on a ridge can stay up all afternoon without circling once, which is why ridge sites are where distance records get set and why they only work when the wind is on the hill.
Wave lift is stranger. When a strong wind crosses a mountain range, the air downwind can settle into a standing pattern of enormous smooth waves, and the rising parts of those waves reach heights no thermal ever gets to. Gliders have climbed past thirty thousand feet in wave, wearing oxygen in a machine with no engine. It is smooth enough that the instruments say it is happening before the pilot can feel it.
Convergence is the third and the least obvious. Two masses of air meeting from different directions have to go somewhere, and the somewhere is up. The result is a line of lift rather than a column, sometimes tens of miles long, and a glider that finds it can run along it like a road.
A game has to pick one, because they do not play the same. Glide uses thermals with the ridge left out, and the reason is that a thermal is local and a ridge is not. Thermals make you choose a route. A ridge would make the route for you.
The Clouds a Thermal Leaves Behind
A thermal that has risen far enough cools as it climbs, and at some point the moisture in it condenses. That is a cumulus cloud, and it is the best evidence a thermal leaves of itself. The flat base of a cumulus sits at the height where the rising air reached its condensation point, which is why a sky full of clouds at the same level is a sky full of thermals all topping out together.
The shape tells you how the day is going. A crisp cloud with sharp edges and a flat base is a thermal that is still working, and the glider pilot under it is climbing. A cloud that has gone soft and fuzzy at the edges is one that is dying, because the droplets are falling out of it as rain or evaporating back into clear air. The lift under it has usually gone with it.
If the clouds keep growing instead, the day is turning. A cumulus that swells into a tower and flattens at the top has become a cumulonimbus, and that is no longer a thermal to be used. It is a thunderstorm with a violent updraft in it, hail, and descending cold air around the outside that will put a glider on the ground whether the pilot wants it there or not. Soaring clubs watch for that shape and everybody lands before it arrives.
The other sky to know is the flat one. A uniform grey layer with no detail in it means the air has stopped rising anywhere in particular, and the thermals have been capped before they got going. Nothing is wrong with the aeroplane on a day like that. There is simply no lift to find, and the honest thing to do is not launch.
Why an Invisible Thing Makes a Good Game
A thermal cannot be seen, only inferred. A bird circling, a dust patch, a change in the feel of the aeroplane. In a game the inference has to be made visible somehow, and how it is done decides what the game feels like.
Glide draws the columns as soft shapes you can see from a distance. That turns a game about reading subtle signs into a game about planning a route, which is a different and better fit for a two minute session. The trade is deliberate: it is less realistic and considerably more playable.
Balloon takes the opposite approach. You cannot see the wind at all, and the only way to learn which way it is blowing at a given height is to go there and watch the ground move. Same subject, opposite design, and both work because the underlying idea is genuinely interesting.