direction. Instead, there are two climatologically occurring breaks in each hemisphere, one containing the subtropical jet stream (near 30° N), and the other containing the polar jet stream (near 45° N). These jet streams are not stagnant in location, but shift with season, moving closer to the equator in winter and more poleward during summer.
The jet streams are important distributors of air pollutants for two reasons. They create major areas of air exchange between the troposphere and stratosphere. And the strong winds of the jet stream can rapidly transport pollutants. For example, if one assumes an average wind speed of 35 m s–1 (~ 70 kt) at 40° latitude, an eastward-moving air parcel will circumnavigate the globe in only 10 days. The stratosphere, which extends to ~ 50 km, has regions of strong winds, but virtually no turbulent mixing except for occasional overshooting thunderstorms, certain types of lightning, and occasional thin clouds.
The atmosphere’s vertical temperature profile plays the dominant role in controlling whether and how quickly an air pollutant will be dispersed upward from its point of emission. The change of temperature with height or lapse rate is used to quantify vertical temperature profiles. The average midlatitude tropospheric temperature lapse rate is 6.5°C km−1, with actual values constantly changing in both time and three-dimensional space. Large lapse rates (like those near the surface on a sunny day) are associated with atmospheric instability, which promotes turbulence. Conversely, small lapse rates near the surface (as would occur on a cold, windless night) denote