About tropospheric temperature and altitude
The Earth's atmosphere is divided up into a number of layers (troposphere, stratosphere,
etc.) based upon the variation in temperature with elevation within that layer. The troposphere (closest to the
Earth's surface) has a large negative temperature gradient, decreasing from about 15° C at the Earth's surface
to about -50° C at the top of the troposphere (called the tropopause).
The graph shows a linear decrease in temperature with altitude, which should seem odd to
students after some reflection. Shouldn't the slope be positive? At higher altitudes, the atmosphere is closer
to the Sun and therefore should be hotter. Temperature decreases with altitude because the troposphere is warmed
from below, through absorption and re-emission of incoming solar radiation by the Earth's surface, rather than
being warmed from above by incoming solar radiation. The Earth's surface emits longwave (thermal infrared) radiation
that is absorbed by water vapor and carbon dioxide in the atmosphere, much more so than the incoming solar radiation.
Clouds also play an important role.
The amount of incoming radiation is balanced by the amount of outgoing radiation. If the
Sun were to decrease its solar output, the Earth would re-emit less radiation, and the atmosphere would cool. This
process of cooling of the Earth's surface and corresponding decrease in longwave thermal radiation would proceed
until the incoming and outgoing radiation budgets were once again balanced. And the atmosphere would be colder.
The "data" in the table and graph are not actual measurements of temperature, but
are numbers generated by a linear model (function) chosen to represent a standard or "typical" atmosphere.
The "data" show no scatter and return an absurdly high correlation coefficient upon regression. Students
should recognize immediately that no single set of measurements can represent air temperature, especially near
zero altitude. The Y-intercept is revealing; whose temperature is this?
Note also that above 11 km, temperature is constant for many kilometers, and then begins
to increase with increasing height until about 50 km; a very complex function.
Reference: Aguado E and Burt JE (1999), Understanding Weather and Climate; Prentice Hall, Saddle River, New
Jersey, 474 pp.
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