When the distribution of temperature is such that at every level the environmental lapse rate is greater than the dry adiabatic lapse rate, the displaced parcel of air has a tendency to continuously move upward till its temperature is equal to that of the surrounding air.
Such a state of continued vertical movement of the ascending air is called absolute instability. When there is general instability in the atmosphere, an impulse, whatsoever, is needed to displace a given volume of air.
Intense surface heating by the sun’s rays or a physical barrier in the path of air steams is such a mechanism which initially produces the trigger-effect and forces the air to move upward.
Under the conditions of absolute instability, the displaced air mass continues its ascent till a level is reached where the temperature difference between it and the surrounding air is reduced to zero.