<P> It is also possible to define the tropopause in terms of chemical composition . For example, the lower stratosphere has much higher ozone concentrations than the upper troposphere, but much lower water vapor concentrations, so appropriate cutoffs can be used . </P> <P> The tropopause is not a "hard" boundary . Vigorous thunderstorms, for example, particularly those of tropical origin, will overshoot into the lower stratosphere and undergo a brief (hour - order or less) low - frequency vertical oscillation . Such oscillation sets up a low - frequency atmospheric gravity wave capable of affecting both atmospheric and oceanic currents in the region . </P> <P> Most commercial aircraft are flown in the lower stratosphere, just above the tropopause, where clouds are usually absent, as also are significant weather perturbations . </P>

Going from the equator to the north pole the altitude of the tropopause