The environmental lapse rate (the actual rate at which temperature drops with height, formula_12) is not usually equal to the adiabatic lapse rate (or correspondingly, formula_13). If the upper air is warmer than predicted by the adiabatic lapse rate (formula_14), then when a parcel of air rises and expands, it will arrive at the new height at a lower temperature than its surroundings. In this case, the air parcel is denser than its surroundings, so it sinks back to its original height, and the air is stable against being lifted. If, on the contrary, the upper air is cooler than predicted by the adiabatic lapse rate, then when the air parcel rises to its new height it will have a higher temperature and a lower density than its surroundings and will continue to accelerate upward.