Why could changes in air temperature interfere with the Cavendish experiment?

In the Cavendish experiment, Henry Cavendish sought to measure an incredibly faint gravitational force—approximately 1/50,000,000 of the weight of the lead balls used in his torsion balance.12

Because the force being measured was so minuscule, even minor environmental disturbances could interfere with the results. Differences in air temperature between the two sides of the balance would create air currents or convection, which would "swamp" the tiny gravitational attraction he intended to record. To mitigate this and other sources of error, Cavendish placed the entire apparatus in a sealed room and operated it from the outside, observing the balance through a telescope.1