About The Tropopause

Temperature tends to decrease with height in the troposphere, while the stratosphere it is either isothermal or temperature increases with height. The Tropopause is the boundary between the two regions. Its height above the surface varies with latitude, season and over shorter time periods as well, with the average height at the equator being about twelve miles and only 6 miles over the poles.

Why is this so?

John Thuburn discusses this in detail in a JAS paper from 2000 called Stratospheric Influence on Tropopause Height: The Radiative Constraint. He has the details, but the short answer is that troposphere and stratosphere are dominated by different heat budgets. The troposphere is heated mainly from below and convection dominates energy transport. The stratosphere is heated both from above and below and radiation does almost all the work of energy transport. The thickness of the troposphere depends both on the temperature at the surface and the amount of infrared absorption between surface and space. High tropical surface temperature and an atmosphere full of highly absorbent water vapor lead to a high tropopause. Opposite conditions prevail at the poles.

One of the clearest signals of anthropogenic global warming is the differential effect added CO2 has on the troposphere and the stratosphere. In the troposphere, infrared opacity is so large that its thermodynamically necessary for convective transport to dominate, increasing CO2 increases that opacity, increases the effective depth of the troposphere, and consequently warms the planet’s surface. In the stratosphere, convection doesn’t occur, and temperature is limited partly by the scarcity of infrared radiators available to cool it. Additional CO2 increases the opacity and hence its ability to radiate, thus cooling the stratosphere.

It may seem counterintuitive that an increase in opacity can create greater heating in one place and greater cooling in another, but that fact has its roots in Einstein’s principle of detailed balance – absorption and emission coefficients are equal. Whether absorption or emission dominates depends on the density of absorbers. In the troposphere, there are lots of emitters but so many absorbers that a typical photon doesn’t get far. In the stratosphere, infrared absorbers are relatively few and a photon emitted has a good chance to escape from the planet.

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