Thermals

Where thermals come from, what they are like inside, how the wind bends them, and why your first flights happen in the gentle air of morning and evening.

In this lesson you will

On a sunny day at the training hill you notice thermals before you understand them. The windsock perks up, then droops. A wing laid out on the grass twitches. A buzzard circles without a single wingbeat.

Later in your training, thermals will carry you up. For now they matter for a different reason: they are what makes the air rough. In Swiss accident reviews, most in-flight incidents have happened while thermalling or soaring for years¹. Knowing where thermals come from, and when, is how you choose smooth air while you learn.

Where thermals come from

The sun heats the ground unevenly. Ploughed fields, rock, sand and bare, dry ground warm quickly; water, trees and other vegetation warm slowly. So rising air tends to form over bare, dry ground and sinking air over water and woods, and thermal strength can change a lot over short distances².

Good sources include sun-facing slopes, dark or dry ground, towns surrounded by fields, harvested fields next to green ones, and rocky outcrops. Wet ground spends the sun’s energy evaporating water instead of heating, and cool air that pooled in valley bottoms overnight takes longer to warm in the morning³.

Warm air collects over a good source until something sets it free. Pilots call that a trigger, and often find triggers at edges such as a hedge, a field boundary or the top of a slope. After a spot has released a thermal, the ground needs time to reheat before the next one⁴.

There are two simple pictures of what rises. A small heated patch tends to release a single bubble, a rising ring of air with the fastest rise in its middle and sinking air around its sides. A large warm area triggered at one spot keeps feeding a column. Real thermals often behave like columns with bubbles inside them⁵.

Figure: How a thermal is born. Notice where the warm air collects, where it breaks away, the stronger cycle at the windsock as air rushes in, and the sinking air around the rising bubble. Switch to the stronger-wind variant to see the same climb carried much further downwind.

How a thermal is born. Notice where the warm air collects, where it breaks away, the stronger cycle at the windsock as air rushes in, and the sinking air around the rising bubble. Switch to the stronger-wind variant to see the same climb carried much further downwind.

Inside a thermal

A thermal is not a neat tube. The core rises faster than the edges, and real thermals are lopsided⁶. The glider flying handbook gives a typical diameter of about 150–300 m490–985 ft, narrow near the ground and widening with height like a cone⁶.

Sources disagree on size because they measure different things. A model used in soaring research gives an updraught radius of about 90 m295 ft at 375 m1,250 ft up in a 1,500 m4,900 ft deep layer, while a university text says thermals are about as wide as the convective layer is deep, counting the sinking air around them⁷. The safe summary: thermals come in many sizes, and grow with the depth of the layer they rise through.

Every thermal has sink around it. The sinking air covers a wider area than the rising air, so it sinks more slowly, and over a whole area the two balance. Stronger sink just before a climb often means a thermal is near⁸.

How strong? Meteorologists describe a day’s thermals with an average “convective velocity”, typically around 1 m/s200 ft/min; over a year of desert measurements in Nevada it averaged 2.6 m/s520 ft/min⁹. These are averages across the whole layer: cores rise faster. Your wing’s own sink, about 1 m/s200 ft/min at best and more when circling, comes off whatever the air does¹⁰.

Individual thermals last roughly 5–30 min. Some spots release them again and again (“house thermals”), but between releases the ground must reheat, so lift comes and goes in cycles¹¹. On launch you feel those cycles as pulses of stronger wind with lulls between⁴.

Thermals don’t need clouds. In dry air they stop below their cloud base, and these “blue” thermals can be just as strong; often the only warning is the turbulence itself¹².

Wind: lean and drift

Thermals drift with the wind and lean in it. On cumulus days the lift is usually upwind of the cloud it feeds, and further upwind the lower you are and the stronger the wind. The wind can also change direction with height¹³, and strong wind shear can tear thermals apart⁶. Cool, stable air drifts too: avoid areas downwind of big lakes or irrigated land¹³.

You drift as well. While you circle, the whole air mass carries you downwind, so a climb can take you a long way from where you started. Leaving a thermal that is drifting you away is part of the skill¹⁴.

Figure: Drawn to true scale. Most thermals break away from their source and drift with the wind, and you drift with them. Notice how far one climb carries you, especially in a weak thermal. The climb readout turns amber above USHPA’s P2 limit of 1 m/s (200 ft/min).

Drawn to true scale. Most thermals break away from their source and drift with the wind, and you drift with them. Notice how far one climb carries you, especially in a weak thermal. The climb readout turns amber above USHPA’s P2 limit of 1 m/s200 ft/min.

The daily cycle

Thermals follow the sun. Convection is zero around sunrise, builds through the day and dies away towards sunset; sources put the peak anywhere from around midday to mid or late afternoon. Convective turbulence is most active on warm summer afternoons with light winds, and dust devils are seen most often between about 11 a.m. and 2 p.m. Early mornings and evenings have the smoothest air¹⁵.

That is why many schools train early and late in the day. SafePro keeps first-stage students to stable conditions with light, smooth headwinds¹⁶, and warns that trying to work any lift is especially dangerous so close to the ground¹⁷.

Reading thermal activity before you fly

Note the time of day and the sky.
Cue Midday sun, light wind, growing cumulus or a hard blue sky.
Why Convection is strongest from midday into the afternoon¹⁵.
Watch the launch wind for several cycles.
Cue Pulses of stronger wind with lulls between; streamers lifting and swinging.
Why Thermals passing through launch come in cycles⁴.
Scan for markers.
Cue Circling birds, other wings or sailplanes, swirling dust, smoke looping up and down.
Why They show where air is rising¹⁸¹⁹.
Tell your instructor what you see before you clip in.
Cue You can say where the cycles come from and how strong they are.
Why Early students keep out of thermals; your instructor decides whether the air suits you today¹⁷.

Common mistake Launching into a strong cycle

Symptom: The windsock stands up and swings, the wing shoots overhead and surges, and the launch feels rushed and bumpy.
Cause: A thermal cycle is passing through launch, often in the middle of the day.
Fix: Watch several cycles before you clip in, and let your instructor pick a gentle moment, or call the session off⁴.

Common mistake Trying to climb in your first lift

Symptom: The variometer (vario) beeps, a wing lifts, and you start turning to stay in it, low over the slope.
Cause: Lift feels like a gift, and turning in it feels natural.
Fix: As a student, fly straight through lift or turbulence to calmer air and land normally¹⁷. Thermalling comes later, with your instructor.

Instructor's note Thermic air on the landing approach

Uneven heating affects landings too. Rising air over bare, dry ground can balloon you past your spot, while sinking air over water or woods can put you down short²⁰. Swiss accident reviews list strong sink just before a thermal released among landing-phase causes¹. On a thermic afternoon, fly your approach with extra margin, keeping your speed up and flying actively, as your instructor briefs it²¹.

Dust devils

A dust devil is a small, vigorous whirlwind that forms under clear skies, in light wind, over strongly heated ground. It marks a thermal leaving the ground. Typical ones are 3–90 m10–295 ft across and 150–300 m490–985 ft high and last a few minutes; larger ones can have winds of 100 km/h62 mph or more, strong enough to flip a parked sailplane onto its back²².

They are not a curiosity. Of 48 German paragliding deaths from 2018 to 2022, three were caused by dust devils, one of them during ground handling, and the Swiss federation’s 2025 review lists a violent dust devil during launch preparation among launch accidents²³.

Safety-critical Dust devil near launch or the landing field

Watch for spinning dust, leaves or grass, or a sudden swirling gust. Don’t inflate, kite or launch until it has gone well clear, and don’t leave a wing lying open where one could catch it. Your instructor makes the call; when in doubt, wait.

Thermals and your limits

Standards differ Thermal limits for students and new pilots

Common ground: Early students keep out of thermals. SafePro, the international reference, warns Stage 1 students that working any lift close to the ground is especially dangerous, and tells Stage 2 students to avoid lift or turbulence, or fly straight through it to calmer air and land normally¹⁷.

USHPA (USA): P2 pilots should not fly in thermal lift where peak climb rates exceed 1 m/s200 ft/min; P3 pilots not above 2.5 m/s500 ft/min, or where there is significant vertical cloud development²⁴.
BHPA (UK): No numeric thermal limit is published²⁴. Club Pilot training includes soaring and flying with others, under your school’s limits²⁵.
APPI (international): Thermal flying is introduced at APPI Pilot level, with attention to bank angle and outside collapses¹⁷. The Thermalling certification needs at least 10 flights in thermal conditions²⁶.

Summarised from each body’s published material, which changes over time. Check the current official documents and follow your school.

Instructor's note Peak, not average

USHPA’s limit is worded as the peak climb rate, not the average²⁷. A thermal that averages a gentle climb can still have cores well above the limit, which is one more reason to stay out of thermals until your instructor says otherwise.

Questions students ask

How will I know I’m in a thermal?

You feel pushed up into the seat, the vario beeps faster, and the wing gets busier, sometimes lifting one side. First thermals covers what to do about it, later in your training.

Why do birds and other wings circle?

They are climbing in thermals. Circling birds, other wings and sailplanes often mark one, and in dry climates dust devils show thermals leaving the ground¹⁸.

What is a house thermal?

A spot that releases thermals again and again, often a sunny slope or a dry field. Even there, nobody can promise one is working right now¹¹.

If evenings are smooth, why not fly every evening?

Many training sessions do run in the evening for that reason. But evening air brings its own changes, such as downslope winds as slopes cool, so your instructor still reads the conditions every time (see Local winds).

Key points

Thermals form where the ground heats fastest: bare, dry, dark or sun-facing ground. Water, woods and wet ground warm slowly.
Warm air breaks away at a trigger as bubbles or columns. The core rises fastest, and sinking air surrounds every thermal.
Thermals are typically 150–300 m490–985 ft across; size and strength vary a lot, and they don’t need a cloud on top.
Thermals lean and drift downwind, and you drift with them while you climb.
Thermals follow the sun: smoothest early and late, strongest from midday into the afternoon. Many schools fly students early and late.
Dust devils are violent and have killed pilots, one of them during ground handling. Wait until they are well clear.
Stay out of thermals as a student; USHPA’s P2 limit is 1 m/s200 ft/min peak climb.

Ground to Solo is a study aid. Paragliding is learned in person with a qualified instructor at a recognised school; nothing here certifies you to fly.