Margie Morris, age 14, of Waverly, Tennessee, for her question:

Where exactly is the tropopause?

The atmosphere is the complete round shell of air that enfolds the globe from ground level to the brink of outer space. The quantity and quality of the air varies on the way up and the atmosphere is arranged in distinct layers. These airy shells are spheres of various depths    the weathery troposphere at ground level, the calm stratosphere above, the higher ionosphere and the outer exosphere. These layers of the atmosphere range from five to hundreds of miles in depth. But between the lower troposphere and the stratosphere above it is a layer too slim to be measured in miles. This is the tropopause.

The turbulent troposphere has a distinct upper boundary located from 10 to 11 miles above the equator and about five miles above the poles. Here the storms cease and most of the winds grow calm. The weather seems to pause in the tropopause. This slim layer does little more than mark the boundary between the breezy troposphere and the cool, calm stratosphere above it.