The graph below shows how the temperature of air varies with altitude. The air pressure is critical as it affects the way that air behaves when the ground is heated. When the Sun is shining it warms the ground. The warm ground warms the air immediately above it. This produces a bubble of warm air that begins to rise.
Warm air rises because warm air is less dense than cold air.
The green line in the graph shows, in theory, how a bubble of air at ground level will cool as it rises and expands. When the atmosphere is at low pressure, the rising air bubble is always warmer than the surrounding air. On the graph, the green line is always above the red line. Under conditions of low pressure, the heated air will continue to rise. As it is always warmer than the surrounding air, it continues rising.