The image above displays the sun baking the NW side of Signal Hill in the afternoon. The yellow and red circles indicate areas where the sun will warm some features more than others (such as rock-faces or large buildings), and these set off thermals which rise through the air.
Thermals are columns of hot air that rise from the ground. As the sun heats air near the ground, that air expands and rises.
When this hot air expands enough it will release either a column of rising air or “bubbles”. The air will rise until it reaches the same temperature level as higher up in the sky. The rate that this column of warm air rises depends largely on the “lapse rate” (the rate that air cools as it rises) and can be influenced by a number of things but mostly the surrounding air mass. This is why we experience the best time for thermal development in post frontal conditions.
The red arrows on the image above displays the wind direction. The yellow line that goes straight up shows the stronger thermal and the more curved yellow line the weaker thermal. The weaker thermal gets blown downwind as it is not strong enough to resist the wind. The strongest thermals can usually be found upwind of a “drifting thermal”
The green telephone cord lines indicate the flight path that the Paragliding Pilot should follow to climb in the Thermal.
Once a pilot finds a thermal, he or she will begin to fly in a circle, trying to center the circle on the strongest part of the thermal or the “core” of the thermal, where the air is rising the fastest.
Most pilots use a variometer, which indicates climb rate with beeps and/or a visual display, to help core-in on a thermal. Often there is strong sink surrounding thermals, and there is often also strong turbulence resulting in wing collapses as a pilot tries to enter a strong thermal. Good thermal flying is a skill which takes time to learn, but a good pilot can often core a thermal all the way to cloudbase.
In our next article we will discuss Wave lift