sphere is trapped beneath the warmer, less dense air of the stratosphere. So the tropopause
is a barrier that keeps air from moving from the troposphere to the stratosphere. Sometimes
breaks are found in the tropopause and air from the troposphere and stratosphere can mix.
Stratosphere
Thestratosphererises above the tropopause. When a volcano erupts dust and gas that
makes its way into the stratosphere, it remains suspended there for many years. This is
because there is so little mixing between the stratosphere and troposphere. Pilots like to fly
in the lower portions of the stratosphere because there is little air turbulence.
In the stratosphere, temperature increases with altitude. The reason is that the direct heat
source for the stratosphere is the Sun. A layer of ozone molecules absorbs solar radiation,
which heats the stratosphere. Unlike in the troposphere, air in the stratosphere is stable
because warmer, less dense air sits over cooler, denser air. As a result, there is little mixing
of air within the layer.
The stratosphere has the same composition of gases as the rest of the atmosphere, with the
exception of theozone layer. The ozone layer is found within the stratosphere at between
15 to 30 km (9 to 19 miles) altitude. The thickness of the ozone layer varies by the season
and also by the latitude. The amount of ozone present in the ozone layer is tiny, only a few
molecules per million air molecules. Still, the concentration of ozone is much greater than
in the rest of the atmosphere. The ozone layer is extremely important because ozone gas in
the stratosphere absorbs most of the Sun’s harmful ultraviolet (UV) radiation.
How does ozone do this? High energy ultraviolet light, traveling through the ozone layer,
breaksaparttheozonemolecule, O 3 intooneoxygenmolecule(O 2 )andoneoxygenatom(O).
This process absorbs the Sun’s most harmful UV rays. Ozone is also reformed in the ozone
layer: Oxygen atoms bond with O 2 molecules to make O 3. Under natural circumstances,
the same amount of ozone is continually being created and destroyed and so the amount of
ozone in the ozone layer remains the same.
The ozone layer is so effective that the highest energy ultraviolet, the UVC, does not reach
the planet’s surface at all. Some of the second highest energy ultraviolet, UVB, is stopped
as well. The lowest energy ultraviolet, UVA, travels through the atmosphere to the ground.
In this way, the ozone layer protects life on Earth. High energy ultraviolet light penetrates
cells and damages DNA, leading to cell death (which we know as a bad sunburn). Organisms
on Earth are not adapted to heavy UV exposure, which kills or damages them. Without the
ozone layer to reflect UVC and UVB, most complex life on Earth would not survive long.
Above the stratosphere is the thinstratopause, which is the boundary between the strato-
sphere below and the mesosphere above. The stratopause is at about 50 km above the
Earth’s surface.