The Earth s Global Energy Balance

Transcription

1 The Earth s Global Energy Balance Electromagnetic Radiation Insolation over the Globe World Latitude Zones Composition of the Atmosphere Sensible Heat and Latent Heat Transfer The Global Energy System Global Energy Budgets of the Atmosphere and Surface Net Radiation, Latitude and the Energy Balance

2 Electromagnetic Radiation Systems and reservoirs have inputs or outputs of both matter and energy Energy occurs in different forms, e.g. radiant, chemical, gravitational, etc. Energy can be transformed from one type to another Energy occurs in some form everywhere, is essential for life and all processes it cannot be created or destroyed (first law of thermodynamics)

3 Pressure Molecules bumping into an object create a force on that object Pressure is the force applied per unit area P = Force/Area; where force is mass*gravity Which box below is exerting the greatest pressure upon the ground? 1 kg 1 kg

4 Density Same number of molecules and mass Sample 1 takes up more space Sample 2 takes up less space Sample 2 is more dense than sample 1 Sample 1 Sample 2 less dense more dense

5 Pressure and Density Gravity holds most of the air close to the ground The weight of the overlying air is the pressure at any point

6 How do we measure pressure? Sea Level Value (average) Units of Pressure: 1 atmosphere 760 mm. of mercury in. of mercury 33.9 ft. of water millibars Why does pressure decrease with altitude? Remember: Pressure = mass*gravity/unit area As you go higher, you have less mass above you.

7 Hydrostatic balance What keeps air from always moving downwards due to gravity? A balance between gravity and the pressure gradient force. rg DP/ Dz = rg DP/ Dz What is the pressure gradient force? Pushes from high to low pressure.

8 Vertical Structure The world is a big place, but the atmosphere is very shallow. Consider In Billings, about 12% of the mass of the atmosphere is below our feet At the top of Long s Peak (14000 ft), you are above 40% of the atmosphere s mass You are closer to outer space than to Bozeman!

9 What is Air Temperature? Temperature is a measure of the kinetic (motion) energy of air molecules K.E. = ½ mv 2 m = mass, v = velocity So temperature is largely a measure of air molecule speed, but also can be a measure of vibrations of molecular bonds (more important for solids) The sensation of warmth is created by air molecules striking and bouncing off your skin surface and transferring heat to your skin The warmer it is, the faster molecules move in a random fashion and the more collisions with your skin per unit time Could you feel warm in a place where the temperature is low?

10 Temperature Scales In the US, we use Fahrenheit most often Celsius (centigrade) is a scale based on freezing/boiling of water Kelvin is the absolute temperature scale

11 How do we measure temperature? Conventional thermometry - Liquid in glass. Electronic thermometers - Measures resistance in a metal such as nickel. Remote sensing using radiation emitted by the air and surface (particularly, though not exclusively, from satellites). Units of temperature: Celsius, Kelvin What is the coldest possible temperature? Why?

12 Atmospheric Soundings Helium-filled weather balloons are released from over 1000 locations around the world every 12 hours (some places more often) These document temperature, pressure, humidity, and winds aloft

13 The atmosphere is layered according to its temperature structure In some layers the temperature increases with height In others it decreases with height or is constant Why? pause is a level sphere is a layer

14 Heat transfer processes Conduction - Where molecules transfer energy by coming into contact with one another. Convection - Where a fluid moves from one place to another, carrying it s heat energy with it. In atmospheric science, convection is usually associated with vertical movement of the fluid (air or water). Advection is the horizontal component of the classical meaning of convection. Radiation - The transfer of heat by radiation does not require contact between the bodies exchanging heat, nor does it require a fluid between them.

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16 Electromagnetic Radiation - Everything above -273 C (absolute zero, 0 kelvin) emits radiation - hotter objects emit more energy at shorter wavelengths - unit measurement is a micrometer (one millionth of a meter) Figure 2.2, p. 53

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18 Electromagnetic Radiation - hotter sun emits shortwave radiation (ultraviolet, visible and shortwave infrared) - cooler Earth emits longwave radiation (infrared) - much is absorbed by the Earth s atmosphere before it leaves (e.g. by carbon dioxide) Figure 2.4, p. 55

19 Insolation over the Globe - insolation (incoming solar radiation) - measured in units of watts per square meter (Wm 2 ) - varies by latitude and by season Figure 2.5, p. 57

20 Insolation over the Globe the angle of the Sun s energy determines the intensity of insolation on the ground for square B, the same amount of energy as represented by square A is spread over a larger area (b x c) on the ground and therefore represents a lower insolation intensity than area a x c Figure 2.6, p. 57

21 Insolation over the Globe the sun s path across the sky varies in position and height above the horizon seasonally (equator) Figure 2.7c, p. 58

22 Insolation over the Globe Equinoxes - at noon the Sun is 50 degrees above horizon Solstices - June solstice has a higher angle than the December solstice Figure 2.7b, p. 58

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25 World Latitude Zones globe divided into broad latitude zones based on the seasonal patterns of daily insolation observed globally Figure 2.11, p. 61

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28 Composition of the Atmosphere Constant gases in the Troposphere Nitrogen 78% (converted by bacteria into a useful form in soils) Oxygen 21% (produced by green plants in photosynthesis and used in respiration) Argon ~1% (inert) Figure 2.12, p. 62

29 Composition of the Atmosphere Carbon dioxide (CO 2 ) (0.035%) used by green plants during photosynthesis produced by respiration and burning of fossil fuels Figure 2.12, p. 62

30 Composition of the Atmosphere Variable atmospheric gases include: -water vapour = 0.1-4% -methane (produced by cows, termites, swamps etc.) -CFCs (entirely human-made gas) - dust and particulates (from pollen, sea-salt, volcanic dust, soil, etc.)

31 Sensible Heat and Latent Heat Transfer Sensible heat the quantity of heat held by an object that can be sensed by touching or feeling Latent heat - heat that is used and stored when a substance changes state from a solid to liquid (or directly to a gas) or liquid to gas (e.g. evaporation of water) Latent heat transfer the transfer of heat from an evaporating surface to the atmosphere

32 Conduction - Heat Transfer Conduction of heat energy occurs as warmer molecules transmit vibration, and hence heat, to adjacent cooler molecules. Warm ground surfaces heat overlying air by conduction.

33 Water phase changes

34 The Global Energy System Albedo - percentage of solar radiation reflected - fresh snow = 85-95% - dry sand = 35-40% - tropical forest = ~13% - Earth s average albedo = ~30%

35 The Global Energy System in theory, this could affect the climate e.g. more snow, higher albedo, less energy reaches surface, temperature decreases, more snow = ice albedo feedback effect more sunlight is reflected from the surface it gets colder we get more snow we get more snow it gets colder more sunlight is reflected from the surface

36 Planetary Energy Balance Energy In = Energy Out S(1 ) R 4 R T o T 18 C But the observed T s is about 15 C

37 Greenhouse Effect Earth's energy balance requires that absorbed solar radiation is emitted to maintain a constant temperature. Without natural levels of greenhouse gases absorbing and emitting, this surface temperature would be 33 C cooler than the observed temperature.

38 Global Energy Budgets of the Atmosphere and Surface Diffuse Radiation - under clear skies, 80% of insolation may reach the ground - under cloudy skies, only 45 to 10% of insolation may reach the ground Figure 2.13, p.66

39 Global Energy Budgets of the Atmosphere and Surface 49% of insolation = direct radiation (radiation that goes directly to Earth s surface) 31% of insolation reflected back to space (3% by scatter 19% by clouds, 9% by ground) Figure 2.15, p. 67

40 Global Energy Budgets of the Atmosphere and Surface - 20% of insolation absorbed by the atmosphere (3% by clouds 17% by dust and gases) Figure 2.15, p. 67

41 Global Energy Budgets of the Atmosphere and Surface The Greenhouse Effect greenhouse gases include carbon dioxide, ozone, water vapour, methane, CFCs - absorb longwave radiation and re-radiate it back to the Earth s surface (counter radiation) - the Earth is warmer ( by ~35 degrees) than it would be without these gases

42 Greenhouse gas emissions Human activities have caused dramatic increases in greenhouse gas concentrations

43 The U.S. is a major contributor to greenhouse gas emissions

44 Earth's annual energy balance between solar insolation and terrestrial infrared radiation is achieved locally at only two lines of latitude A global balance is maintained by transferring excess heat from the equatorial region toward the poles Earth's Energy Balance

45 Temperature Lags Earth's surface temperature is a balance between incoming solar radiation and outgoing terrestrial radiation. Peak temperature lags after peak insolation because surface continues to warm until infrared radiation exceeds insolation.

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