Dr Mark Cresswell The Atmosphere 69EG5513 – Climate & Climate Change

Lecture Topics General Points A history of the atmosphere Composition of the atmosphere Structure of the atmosphere Cycles (nitrogen and oxygen) Greenhouse Gases

The atmosphere behaves like a fluid The atmosphere is a mixture of different gases, aerosols and particles The atmosphere remains around the earth as an envelope because of gravity Much of the observed motion in the atmosphere results from solar radiation General Points

The atmosphere is like a skin

The Earth’s original atmosphere (4.6 BYA) was most likely hydrogen and helium - as well as methane and ammonia Speculation suggests that much of this early atmosphere escaped to space from the hot surface A more dense atmosphere enveloped the Earth as molten rock from volcanoes and steam vents escaped from the hot interior History of the atmosphere

Such volcanic activity produces mostly water vapour (about 80%), carbon dioxide (about 10%) and a few percent of nitrogen These new gases would have created a new atmosphere Over many more millions of years, the constant production of these gases from the hot Earth interior (outgassing) provided enough water vapour for clouds to form History of the atmosphere

After the formation of clouds, rainfall would have occurred forming the rivers, lakes and oceans of the world Large amounts of carbon dioxide was dissolved in the oceans and later locked up as carbonate sedimentary rocks such as limestone As CO 2 dwindled the concentration of chemically inactive nitrogen increased History of the atmosphere

Oxygen began increasing in concentration slowly as solar energy split water vapour (H 2 O) into hydrogen and oxygen - a process called photodissociation Lighter hydrogen would have risen and escaped into space whilst the oxygen remained in the atmosphere This initial supply of oxygen would have allowed primitive plant life to begin History of the atmosphere

Continued plant growth would have enriched the atmosphere with yet more oxygen during photosynthesis The atmosphere as we know it today would have taken a great deal of time to develop and we as a species may be altering it more dramatically through pollution and burning of fossil fuels. History of the atmosphere

Composition of the atmosphere

At the surface, there is a balance between Nitrogen/Oxygen destruction and production Nitrogen is removed from the atmosphere primarily by biological processes involving soil bacteria. It returns to the atmosphere by the decay of plant and animal matter Oxygen is removed from the atmosphere when organic matter decays - or combined with other substances to make oxides. It is also removed by animals during respiration (lung exchange). Oxygen is added to the atmosphere by plants during photosynthesis Cycles

Water vapour is one of the most important atmospheric gases. It’s concentration will vary geographically and temporally. Values as high as 4% are found in the humid tropics, but as low as a fraction of a percent near polar regions Apart from its contribution to cloud formation and precipitation, water vapour releases large amounts of heat - called latent heat when it changes from vapour to liquid Latent heat is important to the formation of thunderstorms and hurricanes. Water vapour is also an important greenhouse gas as it strongly absorbs and re-emits outgoing longwave radiation Cycles

Composition of the atmosphere

Decreasing wind speed through the friction layer This layer can be as shallow as 500ft

Any questions ?