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Atmospheric Processes

The structure of the atmosphere

• 8th June 1898 – French scientist Léon Teisserenc de Bort used balloons carrying thermometers to record temperature variations within the atmosphere

• Temperature decreased with height to 11.8 km above sea level (ASL), then remained constant to 13 km ASL

• 206 further balloon ascents over next 4 years showed that temperature begins to increase above 13 km ASL

• Teisserenc De Bort had discovered the stratosphere

The structure of the atmosphere

Atmosphere can be divided into four layers:

Thermosphere

Mesosphere

Stratosphere

Troposphere

Layers reflect differences in ability to absorb solar radiation

Atmospheric pressure

• Rapid fall in air density and pressure with increasing height above Earth’s surface

• 50% of atmospheric mass lies within lowest 5.6 km

• >99% of atmospheric mass lies within lowest 40 km

• Atmospheric pressure units = millibars (mb or mbar)

• Normal pressure at sea-level = 1013.2 mb

Troposphere

• Derived from the Greek word ‘tropos’ meaning ‘turn’ – referring to the layer’s convective and mixing characteristics

• Lowest layer of atmosphere – contains 75% of atmospheric mass

• Pressure decreases with altitude

• Approximately uniform decrease in temperature with height of c.6.5°C per 1000 m – environmental lapse rate

• Contains virtually all the atmosphere’s water vapour, clouds and pollution

• Contains the weather

Tropopause

Troposphere

Tropopause • Temperature inversion that acts as a ‘ceiling’ to air rising within the troposphere

• Latitudinal variations in height of tropopause ASL:

– Equator = 16-18 km (11 miles)

– Poles = 8-9 km (5 miles)

• Distortions in tropopause occur above mid-latitude jetstreams & tropical cyclones

– Dust and water vapour may penetrate up into stratosphere

– Dry ozone-rich air drawn down from stratosphere into troposphere

Stratosphere

• Derived from the Greek word ‘stratum’ meaning ‘layer’ – referring to the layer’s stratified, non-convective nature

• Located above troposphere – between c. 10 and 50 km (30 miles) ASL

• Temperature increases with height from c. –60OC (lower stratosphere) to c.0°C at c.50 km ASL (stratopause)

• Contains small amounts of ozone (Ozone Layer) – absorbs ultraviolet (UV) solar radiation, causing warming of stratosphere

• Extremely dry layer with no weather (stable conditions due to cold air below)

• Most meteorites entering the atmosphere burn out above the stratosphere

Ozone Layer• Discovered by Charles Fabry and

Henri Buisson, 1913

• From the Greek – “to smell”

• Absorbs 93-99% of the sun’s UV light which could be damaging to life on Earth

• Approximately 11-50 km in altitude

• Approximately 90% of Ozone in our atmosphere is found here

• Created by the Ozone-oxygen cycle

Nacreous clouds

• Also known as ‘mother of pearl’ or polar stratospheric clouds

• Form at c.15-25 km (9-16 miles) ASL during polar winter when lower stratosphere temperatures fall to c. –80OC

• Consist of droplets / ice crystals of water, nitric acid and / or sulphuric acid

• Only visible before dawn / after dusk as clouds receive sunlight from below horizon and reflect it to ground surface

Nacreous clouds

Nacreous clouds

Mesosphere

• Derived from the Greek word ‘meso’ meaning ‘middle’

• Located above stratosphere – between c.50 and 85 km (50 miles) ASL

• Temperature again decreases with height from c.0OC (at stratopause) to c.–90OC at c.80 km ASL (mesopause) (lowest value within layers)

• There are no gases, particles or water vapour to absorb UV radiation

Thermosphere

• Derived from the Greek words ‘thermos’ meaning ‘heat’

• Located above mesosphere – between c.85 and 500 km ASL

• Temperature increases with height to c.1200OC at 350 km ASL – due to oxygen molecules absorbing ultraviolet (UV) solar radiation

Ionosphere - upper mesosphere and thermosphere (above 80km)

• Contains many ions (electrically charged particles)

• Absorbs UV radiation

• Can reflect radio signals

• Give rise to the northern and southern lights

There is no clearly defined boundary which marks the end of the atmosphere – gases are continuously lost into space (Exosphere) which is mainly made up of Helium and Hydrogen

Noctilucent clouds

• Polar mesospheric clouds

• Form at c.85 km (50 miles) ASL – highest clouds in atmosphere

• Formation not fully understood – mainly composed of water ice

• Most commonly observed between 50O and 60O north during summer months

• Only visible before dawn / after dusk when lower atmosphere is in Earth’s shadow – otherwise too faint to be seen

• First reported in 1885 – shortly after eruption of Krakatoa

Noctilucent clouds

Noctilucent clouds

Aurora borealis

Aurora borealis

Fly through - NASA

• http://svs.gsfc.nasa.gov/vis/a010000/a010000/a010014/

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