Chapter 2

temperature, radiation & energy

Temperature vs. Heat

• Temperature: A measure of internal energy (in this case, 1575oF).

• 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

11

11

22

(b) convection

11

22

33

(b) convection

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

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

Rayleigh scattering: molecules of size r <<

K ~ -4

K(blue) / K(red) = (red / blue)4

= (0.64/ 0.47)4

= 3.5 blue is scattered more than red

K : scattering efficiency

: wavelength

Mie scattering: haze, dust r little color variation

Scattering of Visible Light

Geometric scattering: r >>

(water droplets, ice crystals)

Three forms of light scattering:

Rayleigh : r << Mie : r ~ geometric : r >>

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

volcanoes and dark lava rocks

radar view

visible view

….below a thick CO2 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%100%

4% 20% 6%

19%

51%

the sun shines every day

every day, the earth cumulates more solar radiation

radiation = energy = heat

so the earth should become warmer every day

puzzle

Answer: the Earth emits radiation as

well!

micrometer micrometer

We all emit IR radiation!

radiation

• solar radiation (0.5 m) terrestrial radiation (10 m)

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 (m)

• The amount of radiation emitted (W) increases with the 4th power of T:– Stefan Boltzman’s equation [= 5.67 10-8 W/(m2 K) ]

• The atmosphere will absorb some of the radiation emitted by the Earth surface.

T

bmax

4TW

We are closer to the concept of greenhouse gases

Wavelength (micrometer)

Absorp

tion

0.01 0.05 10.50.1 10 1005 50

100%

0%

Wavelength (micrometer)

Radia

tion Inte

nsi

ty

0.01 0.05 10.50.1 10 1005 50

Ultra Violet InfraredVisible

Absorption of radiation by the atmosphere

big window

smal

l win

dow

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 quiz1. 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 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 !

30%100%

4% 20% 6%

19%

51%

reminder: the solar radiation budget

The Earth surface is emitting IR radiation, but then some of it is absorbed by the atmosphere.

The Earth’s energy budget

130

NET infrared radiation lost at the earth surface

energy gained by the atmosphere

-117+96=-21

=> There is net deficit of 30 units in the atmosphere, and a net excess of 30 units at the surface

+70

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 over the ocean

evaporation

trade winds

Satellite IR image shows cold anvils on top of thunderstorms

Inter-tropical convergence zone

evaporation

evaporation

Thunderstorms!

The Earth’s energy budget

-30 +30 net radiation

-30 net radiation

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

=100%

Solar constant = 1380 W/m2

Global mean surface energy balance:

R = Sn+ Ln

and R H + LE

R = 51 –21 = 30

R = 7 + 23 = 30

Why are the tropics warmer than polar regions?

net rad = net SW rad + net LW rad

net incoming solar radiationnet outgoing terrestrial 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?

(1) Because normal insolation provides more energy, per unit area, than does oblique insolation.

Atmospheric attenuation: {scattering + absorbance}

Why is the sun stronger when it is higher in the sky?

(2) Because oblique insolation is more attenuated than is direct insolation.

Air Mass traversed is double at 60º

normal

oblique

Seasonal variation of the net radiation R at the surface

What explains the seasons?

W/m2

What explains the seasons?

Sun above equator

Sun above equator

Sun above 23½ºNSun above 23½ºS

try this animation!

Fig. 2.17

total insolation, all day long, at various latitudes

June 21: summer solstice December 21: winter solsticeAttenuation removes a great amount of solar energy at the pole.

Axial tilt has plunged the NorthPole into 24-hour darkness.

Axial Tilt of Earth, 21 June

41N

Equator

Tilted by 23.5 from the perpendicular

Solar angle v season

Length of day as function of time of year and latitude

40°N

Fraction of solar constant

Fig. 2.16 in textbook

Energy Balance at the Earth’s Surface

R = H + LE

R warms the surface causing convective currents (H), and R evaporates water (LE)

Net radiation: R = Sn+ Ln

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.

Energy Balance at the Earth’s Surface

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

Which bar represents:Australia

South AmericaAntarctica

These bars respresent different continents

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.

– 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