THE STRATOSPHERE In this region, which extends upwards from the tropopause to the limit of the atmosphere, the temperature lapse-rate, characteristic of the troposphere, is zero or may even be reversed. The height of the tropo pause varies with season, latitude, and other factors. The bottom of the stratosphere may be found at an altitude of about io km., though under special conditions it may reach temporarily to much nearer ground-level.
The dust of the stratosphere is believed to be meteoric and to have entered the Earth's atmosphere from space. It is generally believed that terrestrial dust, including organic spores, is almost, if not entirely, con fined to the troposphere—except for occasional incursions in air currents dragged up into the stratosphere by volcanic eruptions (or hydrogen bombs). However, recent studies of atmospheric circulation, discussed by Machta (19J9), may point to exchange of air between troposphere and stratosphere—with rising air over the equator and descending air in middle latitudes.