Instability

The likelihood that showers will develop is largely determined by how 'stable' the atmosphere is. When the atmosphere is particularly warm near the ground and/or or unusually cold higher up, it is 'unstable', meaning 'bubbles' of air can rise up through it. If sufficiently moist, these bubbles (seen as familiar cumulus clouds) can eventually condense into shower clouds (cumulus) or even thunder clouds (cumulonimbus).