Our sun is the source of all thermals. Mother Natures god the Sun powers our world, our sky’s above.
A steep lapse rate, one in which the air cools fairly quickly as your glider gains elevation, is needed to produce solid thermal lift to your aircraft. A thermal, as you probably know, is air that rises because it’s warmer and therefore less dense than the surrounding air. A parcel of warm air rising from the ground will cool more slowly then the surrounding air that the parcel is rising through. A steep lapse rate means that the rising parcel will stay relatively warmer then the air around it for a longer period of time, meaning it can rise higher through the atmosphere and produce a better thermal lift. When the parcel reaches air that is at the same temperature as it is, the parcel/thermal stops rising: end of the elevator ride.
So with this in mind, why are some days better then others for thermals?
The inversion is simply an inversion of the lapse rate: that is, the air starts warming with increasing altitude rather then cooling. This means that a thermal rising from cooler low-level air will stop rising when it reaches the warmer air of the inversion. If the layer is down low, it means the thermal elevator ride will be short indeed.