How thermals form: sun, ground and air mass

A thermal is air that warms at the ground, becomes lighter than its surroundings and therefore rises. That definition is correct but not enough to plan a flight. The useful question is: today, at this site, at this hour, will there be thermals and how high will they go? The answer comes in three separate parts.

1. The sun: where the energy comes from

The sun does not heat the air directly; its rays pass through the air and strike the ground. It is the ground that warms, and the air warms from it. That is why a thermal day depends on sun angle: a summer noon delivers more than a winter noon, a south-facing slope more than a north-facing one.

Cloud cover cuts the source directly. Even thin cirrus reduces the energy reaching the ground noticeably; a low stratus layer ends the day.

2. The ground: handing the energy to the air

Under the same sun, different surfaces behave differently. What matters is heat capacity and moisture:

• Rock, dry soil, ploughed fields, tarmac: heat fast, release fast. The first thermals of the morning start here.
• Forest: heats slowly, but produces steady, wide thermals in the afternoon.
• Green crops, wetland, lakes: spend the energy on evaporation; thermals are weak or absent. Over water there is usually sink in the afternoon.
• Slope: a face square to the sun heats more than flat ground of the same type.

Warm air does not rise at once; it clings to the ground and accumulates. It needs a trigger to break free: a ridge, a clump of trees, the edge of a stream bed, a gust moving across the ground. That is why thermals usually leave not from the middle of the warm patch but from its downwind edge.

3. The air mass: is rising allowed?

As warm air rises it expands and cools, for dry air about 1 °C per 100 metres. The surrounding air also gets colder with height, but at a different rate. The difference between the two rates decides the thermal's fate:

• If the surroundings cool quickly with height, the rising parcel stays warmer than its surroundings at every level and keeps going. The atmosphere is unstable; it is a thermal day.
• If the surroundings cool slowly, or actually warm at some level (an inversion), the parcel reaches the surrounding temperature there and stops. That level is the day's ceiling.

This is why the same site, sun and ground take you to 2,500 metres one day and 900 the next. The difference is not on the ground but in the air mass above. A morning sounding or the forecast model's temperature profile shows this third part.

Bubble or column?

With weak triggering and light wind, thermals leave the ground as bubbles: you climb for a while, the air runs out, and you wait below for the next one. With strong heating and steady triggering the bubbles merge into a column, the continuous climb from launch to ceiling. In a log the two look different: a bubble day gives short, broken thermal segments, a column day long and smooth ones. The three criteria of thermal detection turn that difference into numbers.

What the wind does

Wind tilts the thermal. Air leaving the ground drifts with the wind as it rises, and you drift with it. That drift can be read from the log and gives the wind at every altitude: wind profile from thermal drift. Strong wind also tears bubbles apart and makes thermals rough and broken.

Reading a day

Ask in order: Is there sun, and for how long? Is the ground dry, and from which edge will it trigger? Does the air above allow rising, and where is the ceiling? A day with three yeses is a good thermal day. One no cancels the other two.