the ozone ‘hole’

The Ozone ‘Hole’

The Heat Balance

What is the purpose of ozone?

• Located in stratosphere 25-30km a.s.l.• Acts as protective shield against ultra-

violet (UV) radiation from the sun

What damage can UV radiation cause?

- Sunburn – skin cancer – snow blindness – cataracts – eye damage – ageing of skin – wrinkling of skin – 1997 suggested link to blood cancer

What is the ozone ‘hole’?• Depletion 1st observed by British Antarctic

Survey in 1977

• First ‘hole’ noted in 1985

• There is no actual hole in the layer of gases, they have just been depleted by over 50%

• Each spring (Sept-Nov) over Antarctic the very low temperatures cause ozone to be destroyed in chemical reaction with chlorine

Where does the chlorine come from?

• Chloroflorocarbons – aerosols, refridgerator coolant, manufacture of foam packaging (long term)

• Major volcanic eruptions – (short term)

How bad is the problem?

• 1993 Antarctic - over an area the size of the USA ozone was reduced to between half or two thirds of its 1970 levels

• Over the Arctic ozone ‘hole’ first notes in 1989, ozone decreased 10% in 1990s

• In Britain increase in UV rays 6.8% per decade since 1979

• 1% less ozone = 5% increase in skin cancer

Can ozone appear else where?

• Car exhausts generate ozone close to the earth’s surface

• Damages plants & causes health problems eg. asthma (London 1994, Paris 1995)

• Ground level ozone increases during warm, sunny, anticyclonic conditions

• Nitrogen oxides from exhausts can react with Volatile Organic Compounds (VOCs) in sunlight to produce petrochemical smog under extreme conditions

How is the troposphere heated?• The earth’s atmosphere is not directly heated by the

sun.• The rays from the sun are mainly short-wave light rays

(insolation).• SW rays are absorbed by the earth’s surface,

converted into heat and returned to the atmosphere as long-wave heat rays by:

a) Radiation – transfer of heat by electromagnetic wavesb) Conduction – transfer by contact eg. poker in a firec) Convection – mass movement of a gas or liquid

These LW heat rays are trapped by the water vapour in the atmosphere – this is called the Greenhouse Effect.

Therefore, the atmosphere is heated from below and temperature decreases with altitude in the troposphere

The Heat Budget• The earth’s temp. remains

fairly constant • Due to balance between SW

(incoming) and LW outgoing radiation

• Net gain every where on earth’s surface except polar areas (high latitude & albedo) – curve A

• Net loss throughout atmosphere – curve B

• Net surplus between 35OS – 40ON (positive heat balance)

• Net deficit at high latitudes (polar regions) and high altitudes

(negative heat balance)

So how is the heat surplus between 40oN&S transferred to other areas?

2 major heat transfers take place to stop tropical areas overheating.

1.Horizontal Heat Transfers from equator to poles

80% by winds20% by ocean currents

• 2. Vertical Heat Transfers• Conduction• Convection• Radiation• Latent heat – the amount of

heat energy needed to change the state of a substance without affecting its temperature.

• Heat is used up in melting & evaporation

• Heat is released by condensation & freezing (warming the atmosphere)

Heat Transfers

Global factors affecting insolation(Long term)

• 1. height above sea level (altitude)

• Why does temp decrease with height?

• Decreased area of land surface from which to heat the air

• Density or pressure of air decreases – air not as able to hold heat (molecules fewer and more widely spaced)

• 2. Altitude of the sun• As move towards pole

land area to be heated by each ray increases due to angle

• & depth of atmosphere to pass through increases (more chance for scattering, absorption, reflection)

Global factors affecting insolation(Long term)

• 3. Land and sea• Diff. abilities to absorb,

transfer & radiate heat energy

• Sea absorbs heat to 10m depth

• Sea transfers heat to greater depths via waves & currents

• Oceans act as thermal reservoirs in winter

• Sea has greater specific heat capacity

• SHC is the amount of energy required to raise the temp of 1kg of a substance by 1oC

• SHC of water 1 kilocalorie• SHC of land 0.5 kc• SHC of sand 0.2 kc

Mean annual range in temperature oCCoastal areas have a smaller annual range of temp. than locations at

centre of continents

Global factors affecting insolation(Long term)

• 4. prevailing winds• Temp determined by area of origin and the surface over

which it has blown

Global factors affecting insolation(Long term)

• 5. ocean currents• Horizontal transfer of heat energy• Warm currents head towards poles• Cold currents from poles to equator

Circular currents:

Clockwise in N

Anticlockwise in S

Temperature Anomaly = this term is used to describe temperature differences from a meanEg. Stornoway has a mean Jan temp 20oC higher than the aver for other places at 58oNNE Siberia 24oC colder than the mean at its latitude. WHY?

Controls on temperature at global level:

• Latitude – angle of the sun

• Revolution – earth revolves around the sun in 365 and a quarter days

• Rotation – earth takes 24 hours to rotate on its own axis

• Tilt – earth is tilted at 66.5o to the horizontal path of the earth’s travel

Seasonal changes:

• Solstices – June 21st is the summer (sun at 54o and 17.5 hrs sunlight) – winter is December 22nd (sun at 12o and 6.5 hrs sunlight)

• Equinoxes – March 21st & September 22nd – sun overhead at the equator and all places have equal day and night

Local influences on insolation

• Aspect • Cloud cover- Clouds reduce day time temps- Provide insulating layer to

retain heat at night- World’s greatest diurnal temp

range found in tropical deserts

• Urbanisation- Affects the albedo- Creates urban ‘heat islands’

South facing - ubac

North facing - adret