Chapter 2 temperature radiation energy Temperature vs Heat

Chapter 2 temperature, radiation & energy

Temperature vs. Heat • Temperature: A measure of internal energy (in this case, 1575 o. F). • Heat: Thermal energy transferred between systems at different temperatures.

Energy Transfer • Conduction, convection, and advection require molecules • Radiation is an electromagnetic phenomenon, and is able to pass through the vacuum of space.

(a) conduction: molecular vibration

(b) convection: eddy transfer 1

(b) convection 2 1

(b) convection 1 2 3

icecold advection: mass transfer 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!

radiation the solar spectrum 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 Rayleigh scattering: molecules of size r << : wavelength K(blue) / K(red) = ( red / blue)4 = (0. 64 m / 0. 47 m)4 = 3. 5 blue is scattered more than red

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 Moon

Venus visible view radar view volcanoes and dark lava rocks …. below a thick CO 2 atmosphere with sulphuric acid clouds

the Earth’s albedo is far from constant

now

The albedo of the ocean is very low Zenith angle, 7%

Interpret the global mean albedo

The solar radiation budget on earth 30% 4% 20% 100% 6% 19% 51%

puzzle the sun shines every day, the earth cumulates more solar radiation = energy = heat so the earth should become warmer every day

Answer: the Earth emits radiation as well! micrometer

We all emit IR radiation!

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) max is the wavelength at which the radiation peaks (mm) • The amount of radiation emitted (W) increases with the 4 th power of T: – Stefan Boltzman’s equation [s= 5. 67 10 -8 W/(m 2 K) ] • The atmosphere will absorb some of the radiation emitted by the Earth surface. We are closer to the concept of greenhouse gases

big window small window Absorption of radiation by the atmosphere

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 T = 59°F (15°C) without atmosphere T = 0°F (-18°C)

Pop quiz 1. 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 ? 2. What is the ratio of the manmade to the natural greenhouse warming? 1. 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 CO 2). 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% 4% 20% 100% 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 energy gained by the atmosphere 130 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 heating Condensation, freezing = latent heating The lower atmosphere is heated from below….

Evaporation takes energy

Oceans continuously heat up by net radiation uptake. They are ‘air-conditioned’ by evaporation at the surface. evaporation trade winds evaporation over the ocean

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 +30 net radiation

=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/m 2 Solar constant = 1380 W/m 2

Global mean surface energy balance: net rad = net SW rad + net LW rad R H + LE R = Sn+ Ln and R = 51 – 21 = 30 R = 7 + 23 = 30 Why are the tropics warmer than polar regions?

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

There are two reasons why the solar radiation at the surface is weaker when the Sun is lower in the sky What are these reasons?

Why is the sun stronger when it is higher in the sky? normal oblique (1) Because normal insolation provides more energy, per unit area, than does oblique insolation. Atmospheric attenuation: {scattering + absorbance} (2) Because oblique insolation is more attenuated than is direct insolation. Air Mass traversed is double at 60º

Seasonal variation of the net radiation R at the surface W/m 2 What explains the seasons?

What explains the seasons? Sun above equator Sun above 23½ºS Sun above equator Sun above 23½ºN try this animation!

Fig. 2. 17

total insolation, all day long, at various latitudes June 21: summer solstice Attenuation removes a great amount of solar energy at the pole. December 21: winter solstice Axial tilt has plunged the North Pole into 24 -hour darkness.

Axial Tilt of Earth, 21 June Tilted by 23. 5 from the perpendicular 41 r ato Equ N

Solar angle v season Length of day as function of time of year and latitude 40°N Fig. 2. 16 in textbook Fraction of solar constant

Energy Balance at the Earth’s Surface Net radiation: R = Sn+ Ln R = H + LE R warms the surface causing convective currents (H), and evaporates water (LE) R

Energy Balance at the Earth’s Surface Pop quiz • Sensible heat flux H versus latent heat flux LE. Which one is true? – – a: over the ocean LE > H; b: over a dry desert surface, at noon, H > LE; c: as a global average, LE > H; d: all of the above.

H vs. LE Globally • Over oceans, 90% of R is used to evaporate water (LE), only 10% used to warm the air (H) by conduction or convection. • On land, H LE. • Globally, LE = 23 units (77%), H = 7 units.

Energy flux These bars respresent different continents Which bar represents: Australia South America Antarctica

Energy flux

Local energy balance Inside which one is it warmer on a sunny day? Why? – a white styrofoam cooler, lid closed; – a white styrofoam cooler, lid off; – a styrofoam cooler painted black on the inside, lid off; – a styrofoam cooler, painted black on the inside, lid off, but covered by a glass pane; – a metal toolbox, painted black on the inside, covered by a glass pane, and buried in the ground so that the top is level with the surface.

results • 9 Sept 2003, Prexy lawn, 1: 15 pm. Sunny day. Air temperature: 81°F – – a: a white styrofoam cooler, lid closed: 78°F b: a white styrofoam cooler, lid off: 88°F c: a styrofoam cooler painted black on the inside, lid off: 103°F d: a styrofoam cooler, painted black on the inside, lid off, but covered by a glass pane: 189°F – e: a metal toolbox, painted black on the inside, lid off, but covered by a glass pane: 124°F – f: a metal toolbox, painted black on the inside, lid off, but covered by a glass pane, half -buried: 115°F

Summary of chapter 2 • • Electromagnetic radiation Heat transfer (convection, conduction, advection) Scattering and absorption of radiation by the atmosphere Shortwave (solar) and longwave (terrestrial) radiation The natural greenhouse effect Global energy balance (solar radiation, terrestrial radiation, and heat transfer) Seasonal/regional variations of the surface energy balance

End of Chapter 2