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Dr Sylvia Knight answers questions on Climatic Systems
1. What are the different layers in the atmosphere, and how do they differ?
All our ‘weather’ – clouds, rain, wind, and most of the mass of the atmosphere is found in
the bottom layer, the troposphere. This extends from the surface of the Earth up to about
10km – higher in the Tropics than near the poles. The air in the troposphere is warmest near
the ground and is colder with height.
Above this, the stratosphere extends up to about 50km. The stratosphere is where ozone is
formed (and destroyed), and temperature increases with height.
The mesosphere, thermosphere and exosphere are found above, with ever smaller
concentrations of air.
In the ionosphere, an area covering the top of the thermosphere and the bottom of the
exosphere, the Sun’s radiation causes the gas molecules to become positively or negatively
charged. When this layer is disturbed, we see the ‘Northern Lights’ are found.
(Image from NOAA and Wikimedia Commons)
2. Why are there different layers in the atmosphere?
It’s basically down to a question of stability – if a parcel of air rises (maybe because it’s being
pushed over a mountain), cooling as it expands, does it end up cooler or warmer than the air
around it? If it’s cooler, it will tend to sink back down to where it came from (it’s stable), if
it’s warmer, it will tend to carry on rising (it’s unstable).
The troposphere is warmest near the ground, where radiation from the sun is absorbed.
Heat is transmitted to the air by conduction and radiation as well as through evaporation
and subsequent condensation of water vapour. The tropospheric air cools with height, so it
is possible for parcels of air forced to rise from low down to end up warmer than the air
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around them, making them unstable. This gives us the clouds and rainfall that we are used to. However,
the ozone in the stratosphere absorbs incoming solar radiation, and reemits it as heat, warming the upper
stratosphere up. This means that, in the stratosphere, temperatures increase with height and the
stratosphere is therefore always stable. All parcels of air which are pushed upwards, immediately tend to
bob back down again. The stratosphere is therefore a lot less turbulent and mixed.
3. Does what happens in one layer have any effect on what happens elsewhere?
Yes! The most obvious example would be the effect of releasing CFCs in the troposphere on the
concentrations of ozone in the stratosphere, and the subsequent consequences for the amount of
ultraviolet radiation getting through to the surface of the Earth.
4. How does information get passed between the layers?
Figure: Schematic representation of the
Brewer-Dobson circulation. Source
This is relatively poorly understood and
the area of much active research. Actual
matter (air molecules and particles),
electromagnetic radiation and other
forms of energy can be exchanged
between the layers.
A very small amount of air moves from
the troposphere into the stratosphere in
the tropics. The tropics receive most
energy from the Sun per unit area than
the rest of the world. This heats the
ground, and subsequently the air above
it. Warm air rises, and we get the
vigorous convection, clouds and rain we
associate with the Inter Tropical
Convergence Zone. The up draughts of
air in some of the cumulonimbus clouds
are strong enough to be able to push
relatively moist air through the tropopause into the stratosphere – the air then gradually moves around in
the Brewer Dobson circulation. In the same way, very large volcanic eruptions, particularly in the Tropics,
can push ash and soot particles into the stratosphere.
The biggest events in the stratosphere are the so-called sudden stratospheric warmings (SSWs). They
usually occur over the North Pole in winter. The polar vortex – a cyclonic vortex of stratospheric westerlies
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surrounding the winter pole, and enclosing the coldest air temperatures, sinking air and high surface
pressure, suddenly, over the course of a couple of days, slows down or even reverses. Stratospheric
temperatures over the Arctic then rise by as much as 50°C. SSWs are caused by the effects of waves
propagating up from the troposphere and releasing their energy in the stratosphere (in the same way that
breaking ocean waves release their energy, moving beach materials and creating noise). These waves are
vast Rossby waves – with just a few wavelengths being enough to circle the globe. SSWs can have an
important impact on the distribution and amount of ozone in the stratosphere.
5. Where in the atmosphere do you find ozone?
0% of ozone is found in the troposphere, where it is produced by the combustion of fossil fuels and has a
toxic effect on animals and vegetation.
Taken from the 2006 Scientific Assessment
of Ozone Depletion, WMO Image Source
Polar stratospheric clouds, taken from a
British Antarctic Survey base on 27/7/07
Most ozone, about 90%, is found in the stratosphere. It is mainly formed in the tropical stratosphere and
transported polewards. Ozone is extremely important to life on Earth as it absorbs ultraviolet radiation
(UV-B) radiation from the Sun which can cause cell damage. In the pre-industrial world, stratospheric
ozone concentrations would have increased with latitude, peaking at the Poles. However, since
industrialisation, people have been emitting ozone depleting chemicals, such as Chlorofluorocarbons
(CFCs). These gases are very long-lived and so become well mixed in the troposphere and slowly are
transported into the stratosphere, where the Sun’s energy converts them into more reactive gases, eg
chlorine and bromine, and they destroy ozone. The chemical reactions which most effectively destroy
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ozone can only happen in very cold temperatures – so cold that polar stratospheric clouds form. These
clouds, which are not clouds of water vapour but of nitric acid, are usually found in the within the Antarctic
(and Arctic) polar vortices. Instead of
having the highest concentrations, the
polar regions therefore currently have
virtually no ozone in the stratosphere, in
winter at least.
Total ozone above the Antarctic on 26th
October 2010 – clearly showing the
ozone hole which is at its worst at the
beginning of Spring.
6. Is the increase of greenhouse gases in the troposphere having any impact on the
recovery of the ozone hole?
As the concentration of greenhouse gases in the troposphere increases and the troposphere warms, the
stratosphere actually gets colder – think of putting a woolly jumper on, it keeps you warmer, but the air
around you becomes cooler as you lose less heat. As the stratosphere cools, more polar stratospheric
clouds form in the winter and the loss of ozone is exacerbated. In some years, all the ozone over a wide
range of altitudes within the Antarctic region has gone by spring.
7. As the ozone hole recovers, what impact will it have on the climate we experience?
The effects on the ‘greenhouse effect’ are quite complex!
Ozone is an important gas for the atmosphere, because it absorbs electro-magnetic radiation in two
‘bands’ – it absorbs incoming ultraviolet radiation from the sun as well as being a ‘greenhouse’ gas,
absorbing outgoing infra-red (heat) radiation from the Earth. This means that, as the ozone hole recovers,
less incoming radiation gets through to the surface – with benefits for all living things that do not tolerate
UV radiation well. On its own, this would lead to a cooling. However, the increased ozone will contribute to
the ‘greenhouse effect’ – ozone absorbs wavelengths of radiation that are not intercepted by water
vapour or carbon dioxide and re-emits them down to the surface of the Earth. Also, the gases currently
responsible for destroying stratospheric ozone are themselves greenhouse gases; as their concentrations
fall in response to international agreements, their contribution to the greenhouse effect also falls.
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To add to all this, the contribution to the greenhouse
effect made by ozone in the troposphere far outweighs
all of the above.
(a) Energy received and transmitted by the earth. (b)
Absorption by various gases (after Houghton). Image
8. What are the current best estimates for when
the ozone hole will recover?
As a result of the Montreal Protocol, the effective abundance of ozone depleting gases should fall to the
levels seen before the Antarctic ozone hole began to form in the 1980s by the 2050s.
Stratospheric ozone concentrations are expected to return to pre-1980 values soon after, depending on
whether countries stick to emission agreements and, to some extent, on what the Sun and big volcanoes
9. What are the career opportunities for people interested in climatology?
For a career in atmospheric science (whether that’s atmospheric physics, climate science or chemistry) you
really need a good science or maths degree. However, there are some opportunities for studying climate,
and particularly the impacts of climate and climate change, for people with a background in geography.
Similarly most people in the weather forecasting industry will have either an undergraduate or masters
degree in science. There is a lot of information about the sorts of jobs meteorologists have on the website
Dr Sylvia Knight’s biography
Dr Sylvia Knight FRMetS joined the Royal Meteorological Society as Head of Education in 2007, since
when her working life has been incredibly varied and interesting. Her career in meteorology started
when, after completing a degree in Natural Sciences at Cambridge University, she started a PhD at
Reading University modelling the development of waves in different kinds of atmospheric flow. After
this she spent three years using a state-of-the-art computer model to investigate how changes in
atmospheric greenhouse gases and ozone interact with stratospheric temperature, then moved on to
work with the climateprediction.net project, distributing climate models to be run as screensavers on