The air flow generated by a temperature difference ΔT is directed upwards where the weight is warmer, ΔT > 0 , than the surrounding air, and downwards where it is cooler ΔT < 0 . The air flow causes friction forces on the vertical surface of a weight,and pushing or pulling forces on its horizontal surfaces, resulting in a change conv Δm of the apparent mass. The load receptor of the instrument is also contributing to the change, in a manner not yet fully investigated.