Thermals
A thermal is a bubble or column of air that the ground has warmed, rising because it is warmer than the air around it.
Heat from the ground
Section titled “Heat from the ground”The sun warms the ground, and the ground warms the air above it. How much warmth reaches the air depends on:
- The sunshine that gets through. Cloud, and high cloud in particular, cuts it.
- The ground. Dark, dry ground and bare fields pass on more heat than forest or wet meadows; water passes on almost none.
- How wet the soil is. Water in the soil uses the sun’s energy to evaporate, leaving less to heat the air.
- The slope. A slope facing the sun gets more sunshine than flat ground; one facing away gets less.
The thermal layer grows
Section titled “The thermal layer grows”Through the morning the warmed air mixes upward and the thermal layer grows deeper. It stops growing where it meets air that is too warm for the thermals to rise into, such as an inversion. In the late afternoon, as the heating fades, the layer collapses.
From heat and depth to climb
Section titled “From heat and depth to climb”The more heat the ground passes on, and the deeper the layer, the stronger the thermals. Take off what the glider itself sinks while circling, and you have the climb you would see on your vario.
What weakens them
Section titled “What weakens them”Some things make the day worse than heat alone suggests:
- Rain, which ends the thermals and leaves wet ground behind.
- Heavy shading from cloud.
- A day that overdevelops into showers.
- Strong wind, which breaks thermals up.
- A lid of stable air the thermals cannot break through.
- Snow on the ground.