How Do Thermals Work? A Paraglider Pilot's Visual Guide

Short answer: a thermal is a column of warm air rising because it's less dense than the cool air around it. The sun heats a surface, the air above that surface warms up, and once it's warm enough, it detaches and climbs — carrying anything soaring inside it. Paragliders circle within these invisible columns to gain altitude, sometimes for hours. Understanding thermals — how they form, where they trigger, when they peak — is what separates a pilot who does 15-minute sled runs from one who stays up for the whole day. This is the plain-language explainer, no jargon, no how-to-core instructions — those belong in a school's SIV or thermal course.

What a thermal actually is

Air behaves a lot like water: warm water floats, cold water sinks. When the sun heats a surface — a bare rock face, a ploughed field, a dark road, a big roof — the layer of air just above it warms up. Warmer air is less dense than cooler air, so at some point it detaches from the ground as a bubble or column and starts rising.

That column keeps climbing until its temperature matches the surrounding air. In summer, that can be 1,000 metres above launch or 3,500 metres above sea level. When there's enough moisture in the air, the top of the column condenses into a puffy cumulus cloud — the same cloud you see on postcards of summer afternoons. That cumulus is the visible flag of an invisible engine underneath.

A paraglider that flies into a thermal starts climbing. To stay in it, the pilot turns in a tight circle so the wing stays inside the rising air column. That's the sport in one sentence.

What triggers a thermal

Not all surfaces heat equally. Sun-facing rock, dry sand, ploughed brown fields, tarmac and buildings absorb heat fast. Wet grass, forest, snow and water lag behind. So on any given day, thermals concentrate over specific "triggers" — the exact patches of terrain where warm air detaches first.

The classic beginner-friendly triggers:

• South-facing rock outcrops in the mountains (in the northern hemisphere)
• Bare ploughed fields on the plains
• Roads and parking lots on flatland flights
• Dark roofs and buildings in urban-adjacent flying
• Ridge lines where sun-warmed slopes push air up mechanically as well as thermally

Local pilots know their site's triggers by heart. It's the single biggest piece of knowledge to acquire when you fly a new area — better than any weather app.

The daily rhythm

Thermals are seasonal AND daily.

Season. Strong thermals require sun heating and a certain instability in the air (see reading the forecast for the details). In the northern hemisphere Alps, thermals become useful around April, peak from June to August, and fade in October. In coastal or desert regions, the season stretches longer.

Daily cycle. Almost every good thermal day follows the same pattern:

• Morning (10:00–12:00): thermals wake up. Climbs are gentle, the air is smooth. This is the safe learning window.
• Midday to early afternoon (12:00–15:00): peak thermal activity. Biggest climbs, strongest, roughest air. Advanced pilots love these hours; beginners often land at the peak and re-launch later.
• Late afternoon (15:00–17:00): thermals soften but stay useful.
• Evening (17:00 to sunset): the 'glass-off'. As the sun sets, remaining warm air releases as a wide, gentle band of lift over the terrain. Beautiful, forgiving, one of the best-loved times to fly.

What thermals look like from launch and in the air

Two experienced pilot habits worth borrowing:

From launch: watch the sky more than the wind. If cumulus clouds are forming with flat bases at a consistent altitude, thermals are working. If those clouds are growing tall (congestus) and turning dark at the base, the day may be overdeveloping — plan to be down before storms build. Also watch the birds: eagles, buzzards and vultures find thermals by profession, and they mark them for you.

In the air: three signs a thermal is nearby:

1. A birds circling higher than you
2. A wing suddenly bumped upward as you cross an invisible edge
3. A puff of dust or hay rising off a bare field below

The rhythm of thermals matters. Over a good trigger, you'll often see thermals come every 5 to 20 minutes. If nothing is working over a spot that "should" work, sometimes patience wins over hunting — wait, drift with the wind, come back.

What a thermal feels like

The first time is memorable. There's a lift under one side of the wing, sometimes rougher on one edge and smoother on the other. A vario (or your phone) chirps. The wing wants to turn away from the strongest lift, and the pilot's job is to bring it back into the core.

We're deliberately not writing "how to core a thermal" — that's a school topic, ideally taught with radio guidance in mild conditions and reinforced with a proper thermal course. Anyone reading this article who wants to learn to thermal actually should book a thermal-focused progression clinic with a school after their initial licence. Reading about it changes nothing; the wing has to teach you.

When thermals stop being fun

Thermals themselves aren't dangerous. What's dangerous is the transition zone at the edges of a thermal — where rising air meets sinking air, producing turbulence that can collapse a wing near the ground. Two situations to respect:

• Take-off and landing during peak thermal hours (13:00–15:00). The safest window for beginners is the morning or the evening glass-off. Between the two, the field can have surprise gusts and rotor from convection.
• Overdevelopment days, when cumuli grow into congestus and eventually thunderstorms. The gust front of a growing storm strikes many kilometres ahead of the rain, and it can arrive without warning. If the forecast flags overdevelopment or a storm risk, plan to be packed up early.

If in doubt, don't launch during the peak — that's the wind limits rule applied to convection.

From understanding to flying

Recognising thermals is a huge unlock — suddenly a "boring" glide becomes a scan for lift, and 15-minute sled runs stretch into hour-long climbs. But recognising them and flying them safely are two different skills. The path most pilots take:

1. Get the initial licence in gentle conditions
2. Fly the morning and evening windows for a full first season
3. Enrol in a thermal-progression course with a school for the peak hours
4. Add a SIV course over water to learn what a collapse looks like before you meet one over rock
5. Then start venturing into midday flying, one calm-looking day at a time

There's no shortcut, and there's no shame in a slow build. The pilots who fly for 30 years are almost always the ones who spent their first two seasons flying morning air and refusing to force it.

Where the app comes in

The reason most pilots miss the good days isn't that they can't thermal — it's that they can't tell in advance which days will be thermally rich for their local site. That's a forecasting problem, not a skills one. We built OutDare, the free flight app, around exactly this: a 7-day forecast per flying site using local high-resolution models, with a thermal indicator so you can see which day is worth taking off work for. It doesn't teach thermalling — no app can. It just makes sure you're at launch on the days that matter.

Understand the sky. Fly the mild edges of the day. Enrol in the courses. And save the peak thermal window for when a school is teaching you to core.