Lecture 7

• temperature


  determines which way heat flows and represents the amount of heat a body contains (K, C)
• heat (energy)


  the quantity of energy required to give an object its temperature (J)
• (heat) flow/flux

  the rate of energy transfer (between objects)(Js^-1 =W) possibly per unit area (WM^-2)
• heat from the sun is 340 Wm^-2
• on average the Earth reflects about 30% of the incoming energy from the Sun back to space largely from clouds. oceans are darker while land is lighter
• what determines the temperature of a planet?


  power of incident sunlight; albedo of the planet; net power absorbed = incident power x (1-albedo)
• suns temperature is 6000 K - it emits radiation at visible wavelengths
• earth's temperature is ~255 K - it emits radiation at infrared wavelengths
• what is wrong with planetary temperature predictions?

  simple energy balance theory accounts pretty well for mean surface temperature of Mars and Enceladus but poorly for Earth and Venus because of the atmosphere
• mars has almost no atmosphere - volume = 2% of earth's
• moon and Enceladus have no atmosphere
• Earth and Venus do have atmospheres - Venus's is 90 times (mass) that of Earth's and is mostly CO2
• 'tropo' means 'turning'

  • troposphere is mainly heated from below
  • rising warm air, descending cool air
  • vertically mixed (convection and turbulence)
  • 6-10 degrees C decrease in temp per km altitude
• 'pausis' means 'ceasing'

  • tropopause: height where vertical mixing stops
  • temperature stops decreasing with height
• 'strato' means 'layered'

  • stratosphere is heated by absorbing UV light
• the oceans take time to respond to the change in energy balance and hence for the surface temperature to change. the whole ocean takes a long time to respond. the mixed layer (20 km) responds much faster
• most radiation absorbed near the surface of the ocean (20 m)
• temperature of planets and moons calculated using simple 'box' model
• 'box' model balances incoming solar energy against outgoing infrared energy
• simple model fails to predict observed temperatures of Venus and Earth. OK for Mars, Enceladus and the moon
• atmosphere important - for Earth, infrared radiaton emitted about 5.5 km up
• ocean - most heat absorbed in upper 20 m but mixed to about 100 m