Thermal currents - friendly guide
What's a thermal?
A thermal is a invisible column of hot air that rises. The sun heats up the ground, the ground heats up the glued air, and that hot air (less dense than the cold air around it) comes out and rises like a hot water bubble in a pot.
For a paragliding pilot or delta wing, a thermal is the free engine that takes you up. Without them, any free flight would be a continuous decrease of 10-20 minutes. With them, you can spend 5-8 hours in the air and cross hundreds of miles.
Why does the hot air go up?
The cold air is dense and heavy, the hot air is light. When the sun heats a dark field (asphalt, rock, ground), the air above is heats and wants to rise because it weighs less than the colder neighbouring air. That bubble goes up to 2,000, 3,000 or even 6,000 meters in the Andes.
Where are the thermals born?
Not anywhere. They are born in triggers (triggers): ground points that are more heated than the environment:
- Dark rocks and quarries: they absorb fast heat.
- Asfalto, villages, parking: concentrated urban heat.
- plowed or dry fields: much hotter than wet forests.
- Sun-oriented ladders: southern slopes in northern hemisphere, northern southern hemisphere.
- Changes in vegetation: the edge between forest and meadow usually fires.
The expert pilots know the "house" triggers at every flight site. They're fixed points where there's always heat on good days.
How does a thermal look?
Invisible most of the time. But when the thermal goes up enough to cool down to its dew point, the water vapor condenses and a * * cloud appears: the cumulus. A white cumulus with a flat and dark base is the visible indicator that just below is an active thermal.
Other visual tracks:
- Birds planning in circle without beating wings (vultures, condors, millans).
- * * Dust devils in dry areas.
- Other paragliders turning at the same point.
How does a pilot ride a thermal?
- Feel the ascent: the variometer sounds "beep beep" and the candle moves.
- It makes a 360 ° turn: it does not fly straight; it turns inside the thermal to stay in the rising area.
- Adjust: if the ascent increases when turning right, it stays right. If it goes down, it goes the other way. Focus on 2-3 laps.
- Up: as it turns, it goes up to 1, 2, 3 or more meters per second. In minutes you can earn 1,000 m.
Thermal hours
Thermal are not constant:
- Tomorrow (9-11h): weak, small. Day starts to start.
- Midday (11-14h): strong, well organized. Gold window.
- Late (14-17h): they remain strong, may become unstable.
- Stretch (17-19h): they go out. Last soft "glass-off" flight.
- Night: no. Just a windy hillside flight.
How do you make a long flight with thermal?
The secret of flight XC (cross country) is:
- You take off and you look for the first heat.
- You go up to the cloud base.
- From there you plan in a straight line towards another cloud cluster on the horizon.
- When you get there, you usually find another thermal under.
- You go up again, you plan again.
- So you cross tens or hundreds of miles.
The world record of free distance in paragliding is 564 km (Brazil, 2016). Eleven hours of flight with dozens of linked thermal.
What does it do to a "good" thermal?
- Strong: up to more than 2 m / s.
- Great: 100-300 m in diameter, easy to focus.
- High: it reaches up to 2,000 + m above the ground.
- Easy: without too much turbulence at the edges.
A "bad" thermal is small, weak, wind-broken, or with a lot of descending air around.
Thermal hazards
- Too strong: thermals of more than 8 m / s can fold a paraglider. They avoid edges and fly with appropriate wing.
- associated with storms: a thermal that grows to form a cumulonimbus (tower with anvil) is dangerous. Get away.
- cloud suction: near the base of an active cloud, the air continues to rise strong. You can be "aspirated" within the cloud (without visibility, with real danger). Get away from the base.
- Thermal landing: on the ground, entering a thermal can unbalance your candle. Watch out at the end of the flight.
Myths and realities
- Myth: "Thermal comes from heat." Reality: come from the temperature contrast (the ground heat the air more than the neighbouring air).
- Myth: "it flies better in summer." Reality: depends on the site. On the coast with breeze, spring is better. In mountain, late spring / summer.
- Myth: "a thermal is a continuous current." Reality: many are discreet bubbles that rise and separate.
Your first thermal
If you are a student, your instructor will give it to you: he will guide you to where he knows there is thermal, he will say "now turn," and you will feel the altimeter begins to add. It is one of the most magical moments of sport: to discover that the air * * holds you , that you don't fall, that in fact you go up* without engine.
That day you understand why someone said, "The pilots don't fly planes, the planes fly pilots. But a paraglider, the pilot dances with the wind."