A "thermal" or "updraft" ...
Solar heating of the ground warms air near the ground
Once triggered, this warm air "bubbles" upwards
Mathematical models exist to determine thermal size & shape
Frequency and intensity are also predictable
Thermal Cross Section
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Example simulation of a single thermal,
shown is the vertical ascent rate.
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Sensing Thermals
To find thermals, a "variometer" is used. The vario differentiates the altitude reading over time to get a vertical ascent/descent rate.
A pilot can pull back on the stick and exchange airspeed for altitude and produce a false sensor reading, called a "stick thermal" ... a Total Energy Compensation (TEK) nozzle eliminates this effect. It has a special geometry to balance airspeed to altitude changes, thereby removing pilot effects from the sensor reading. Now we have a sensor that reads the airmass movement directly!
As a higher accuracy alternate, the aircraft's energy state can be estimated from airspeed and altitude, allowing a more analytical method of determining the local atmospheric contributions.
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TEK Nozzle
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