Temperature vs. Heat Why is this man unharmed? • Temperature: A measure of internal energy (in this case, 1575oF). • Heat: Thermal energy transferred between systems at different temperatures.
Conduction, convection, and advection require molecules Radiation is an electromagnetic phenomenon, and is able to pass through the vacuum of space. Energy Transfer
1 (b) convection: eddy transfer
(b) convection 1 2
1 (b) convection 2 3
advection: mass transfer icecold cool
Pop quiz • It is a balmy winter day in Chicago. This is because of warm air …………. by winds from the Gulf of Mexico. • conduction; • advection; • convection; • radiation. • You can burn your hand holding it above a candlelight because of … • Convection!
radiationthe solar spectrum l Blue has a shorter wavelength than red
colors in the sky … • Why is the clear sky blue? • Why are sunsets red?
Scattering of Visible Light K : scattering efficiency K ~ -4 K(blue) / K(red) = (lred / lblue)4 = (0.64m / 0.47m)4 = 3.5 blue is scattered more than red Rayleigh scattering: molecules of size r << l: wavelength
Scattering of Visible Light Mie scattering: haze, dust r little color variation
Three forms of light scattering: Rayleigh : r << Mie : r ~ geometric : r >> Geometric scattering: r >> (water droplets, ice crystals) light is reflected or refracted
Question: • How much of the solar radiation reaching the earth, is reflected into space? 30%
Planet Earth’s Albedo: 30% Albedo: the fraction of solar radiation that is reflected or scattered back into space How bright is the moon?
Mars Albedo = 17% Moon Albedo = 6%
visible view Venus radar view ….below a thick CO2 atmosphere with sulphuric acid clouds volcanoes and dark lava rocks Albedo = 78%
the Earth’s albedo is far from constant 3-10% 10-30% 5-20% 75-95% 15-45%
The albedo of the ocean is very low Zenith angle, 7%
The solar radiation budget on earth 30% 100% 4% 20% 6% 19% 51%
the sun shines every dayevery day, the earth cumulates more solar radiationradiation = energy = heat so the earth should become warmer every day puzzle
Answer: the Earth emits radiation as well! micrometer micrometer
radiation • solar radiation (0.5 mm) terrestrial radiation (10 mm) solar terrestrial Now we can connect to the concept of greenhouse gases
Terrestrial radiation emitted • Each surface emits radiation, at capacity (‘blackbody’) • The most likely type of radiation emitted depends on temperature T (K): • Wien’s displacement law (b= 2900) • lmax is the wavelength at which the radiation peaks (mm) • The amount of radiation emitted (W) increases with the 4th power of T: • Stefan Boltzman’s equation [s= 5.67 10-8 W/(m2 K) ] • The atmosphere will absorb some of the radiation emitted by the Earth surface. We are closer to the concept of greenhouse gases
Absorption of radiation by the atmosphere big window small window
If we had no atmosphere … … the global mean temperature would be 0°F
Our atmosphere acts as a greenhouse, and causes the air temperature to be 33 K (59°F) above the Earth’s ‘radiative equilibrium’ temperature with an atmosphere without atmosphere T = 59°F (15°C) T = 0°F (-18°C)
Pop quiz • Is the greenhouse effect of the Earth’s atmosphere: • manmade (mainly due to the burning of fossil fuels); • or mostly natural and existed before human history ? • What is the ratio of the manmade to the natural greenhouse warming? • Answer: about 1:33, but rising (Source: Climate Research Unit, Univ. of East Anglia, UK)
A petroleum geologist told me this … • In the last 100 years or so, we have been burning a lot of coal and oil and gas, fossil fuels. That produces heat. That heat adds up and spreads globally. That causes the global warming. • 3. Is his argument right or false? Why? • 4. What (else) does cause global warming? • Answer (3): False. The heat generated by burning of fossil fuels is insignificant compared to other terms in the global energy balance. The heat that was generated by cars and industry years ago has long been dissipated into space as terrestrial radiation. • Global warming is largely due to the greenhouse gases contained in the burnt fossil fuels (mainly CO2). These gases alter the Earth’s radiative balance.
How long does it take for the Earth to cool, if the Sun suddenly went out? • Without the oceans, the Earth would cool from the current average (59ºF) to freezing (32ºF) in 7 days. • The oceans store a lot of heat. Depending on the rate at which this is released, the cooling down to freezing would probably take some 59 days. • The heat associated with the burning of all fossil fuels in the past century corresponds with all the solar radiation received by the Earth in just 4 days !
reminder: the solar radiation budget 30% 100% 4% 20% 6% 19% 51% The Earth surface is emitting IR radiation, but then some of it is absorbed by the atmosphere.
The Earth’s energy budget +70 130 energy gained by the atmosphere NET infrared radiation lost at the earth surface -117+96=-21 => There is net deficit of 30 units in the atmosphere, and a net excess of 30 units at the surface
Global energy balance • At the top of the atmosphere, outgoing terrestrial radiation is balanced by incoming solar radiation. • At the earth surface, the net longwave radiation emitted (21%) is insufficient to offset the net solar radiation (51%) received. • The atmosphere continuously cools by radiation: the net longwave radiation lost (49%) exceeds the net solar radiation (19%) received • So what prevents the earth surface from heating up & the atmosphere from cooling down?
Non-radiative atmospheric heating:Conduction + convection = sensible heatingCondensation, freezing = latent heating The lower atmosphere is heated from below….
Oceans continuously heat up by net radiation uptake. They are ‘air-conditioned’ by evaporation at the surface. evaporation trade winds evaporation over the ocean
condensation (latent energy release) evaporation (cooling)
Satellite IR image shows cold anvils on top of thunderstorms evaporation Thunderstorms! Inter-tropical convergence zone evaporation
The Earth’s energy budget -30 net radiation +30 net radiation -30
=100% Fig 2.20 in the textbook. The units are NOT % of the incoming radiation at the top of the atmosphere, but rather in W/m2 Solar constant = 1380 W/m2
Global mean surface energy balance: Why are the tropics warmer than polar regions? net rad = net SW rad + net LW rad R = Sn+ Ln RH + LE and R = 7 + 23 = 30 R = 51 –21 = 30
net outgoing terrestrial radiation net incoming solar radiation
Why are the tropics warmer than polar regions? • net radiation R is positive in the tropics, negative at poles. • heat transfer: • atmospheric currents (especially near fronts) • ocean currents • in winter, the high-latitude radiation deficit is even larger, • therefore the pole-to-equator temperature difference is larger, • therefore the currents need to transport more heat poleward