Conditional Instability
If you cause dry air to rise, its temperature will drop due to adiabatic expansion. This vertical temperature gradient is called the dry adiabatic lapse rate. When the temperature gradient becomes sharper than this (i.e. the temperature drops sharply as altitude increases), the atmosphere becomes unstable.
On the other hand, if you cause air saturated with water vapor to rise as the temperature falls the water vapor condenses and lets off heat, so the temperature gradient becomes smaller. This is called the moist adiabatic lapse rate.
In other words, saturated air becomes unstable under a smaller temperature gradient than dry air. Conditions with a vertical temperature distribution under which both dry and saturated air are stable are called absolutely stable, and conditions that cause instability for both dry and saturated air are called absolutely unstable, and conditions that cause instability in saturated air but not dry air are called conditionally unstable.
The lower layer of the atmosphere is in a state of conditional instability, so when water vapor reaches saturation point, it becomes unstable, and cumulus and cumulonimbus occur.