As we climb from ground level our ears
pop and it gets colder. The air pressure, density and temperature
The atmosphere's temperature is the most important property
controlling its structure.
The air in the first few miles of the atmosphere, the troposphere,
does not significantly absorb solar
radiation, instead it is warmed by contact with the ground.
The surface heated air expands as it warms, becomes less dense
than surrounding cooler air and rises as buoyant and turbulent
bubbles. This is convection and is the main process by which
the troposphere mixes and heats.
Although convection stirs and mixes the troposphere, the higher
it is the colder it becomes. Why?
Imagine an isolated bubble
of air heated by the ground and bobbing upwards.
As it climbs the pressure falls and so the bubble expands to
equalise its pressure with the air around it. To expand,
bubble must exert a force on the surrounding air and move it
away. The work done requires energy and the only
source is the internal molecular energy or heat
content of the bubble's air. The internal energy decreases and
the temperature, which is purely the measure of it, falls*.
The bubble will rise until its temperature is the same as
the surrounding air. If we visualize the atmosphere as made
up entirely of such bubbles we see that they would rise and descend
until a natural equilibrium state is reached where the temperature
falls smoothly with increasing height**.
If the air were never heated by solar radiation its temperature
would continue to fall as we climb. However, at a height of
~12 km a minimum of ~-55°C is reached, the tropopause***. Above
that the temperature starts to increase again because
the stratospheric air contains a sunlight absorber, ozone.
The tropopause minimum acts as a barrier^ between
the troposphere and stratosphere because mixing and heat transport
by convection can only occur when temperature decreases with height.
The troposphere - with convection allowed - is turbulent and well
mixed. The stratosphere with its temperature
increase with height is stable, stratified into layers
and relatively poorly mixed^^.
At high latitudes the tropopause and lower stratosphere temperature
can plunge to ~ -85°C to provide the conditions
for PSCs, polar stratospheric clouds of which the incredibly
bright and colourful nacreous clouds are
is a measure of the molecular internal energy which derives
from kinetic, rotational and vibration al motion. the
more energetic the motion, the higher the temperature.
of fall is known as the 'lapse rate. Its mean
value is 6.5°C/km. Actual rates depend on temperature
and humidity. High temperature humid air can have a rate
of only 4°C/km.
atmospheric temperature profile is latitude
dependent. The tropopause height varies from ~16 km at
the equator to only ~8 km at the poles. It depends
also on sea level temperature and season. Officially the
tropopause starts at the lowest level when the lapse rate
falls to 2°C/km.
tropopause is not a complete barrier, it leaks. Strongly
convective tropospheric storms transport water vapour up
across the tropopause. There are breaks in the tropopause
near jet stream westerlies allowing interchange of stratospheric
and tropospheric air.
the tropopause is helped by gravity
waves. Much higher
still, molecular diffusion becomes important.
|View across the
tropopause from a high flying research aircraft. Below
are familiar tropospheric clouds, above are rare nacreous clouds formed at the low temperatures of the tropopause
and lower stratosphere. Imaged by Paul Newman of NASA
Goddard Space Flight Center. Image ©Paul
Newman, shown with permission.