Understanding thermals: how paragliders gain height
A thermal is a rising column or bubble of warm air. The sun heats the ground, the ground heats the air above it, and when that air is warm enough compared with its surroundings, it rises. Paraglider pilots use thermals to climb and fly cross-country.
How a thermal forms
- Sun heats the ground unevenly. Dark fields, rocks, roofs and south-facing slopes (in the northern hemisphere) heat faster.
- A bubble of warm air detaches and starts to rise.
- As the air rises it cools. If the air is humid enough, it condenses into a cumulus cloud at the cloudbase.
See our cloudbase estimator for a quick way to estimate where that cloud will form.
Finding thermals
- Terrain: slopes facing the sun, ridges and convergence points.
- Clouds: young, growing cumulus clouds often sit on top of active thermals.
- Birds: raptors circling are a sign of lift. They find it quickly.
- Other pilots: if someone is climbing, move to that area, keeping safe separation.
- Dust and smoke: wind and thermal indicators.
Centring a thermal
Thermals are not uniform. The strongest lift is usually in a core. Use your vario and feel to turn toward the strongest lift and tighten or widen your circles to stay in the core. Practise on days with mild thermals.
Safety around thermals
- Strong thermals bring strong sink and turbulence around them.
- Cumulonimbus and developing storms are hazardous: keep away.
- Keep a mental picture of other aircraft, and follow the circling-direction rule when in a thermal with others.
Learning curve
Thermalling is a skill learnt with practice and mentoring. Start with small, gentle thermals and gain experience gradually.