Solar Energy and Energy Balancein the Atmosphere

Mid-chapter summary

•The temperature of a substance is a measure of the average kinetic energy (average speed) of its atoms and molecules.

•Evaporation (the transformation of liquid into vapor) is a cooling process that can cool the air, whereas condensation (the transformation of vapor into liquid) is a warming process that can warm the air.

•Heat is (thermal) energy in the process of being transferred from one object to another because of the temperature difference between them.

•In conduction, which is the transfer of heat by molecule-to-molecule contact, heat always flows from warmer to colder regions.

•Air is poor conductor of heat.

•Convection is an important mechanism of heat transfer, as it represents the vertical movement of warmer air upward and cooler air downward.

•All objects with a temperature above absolute zero emit radiation.

•The higher an object’s temperature, the greater the amount of radiation, and the shorter the wavelength of maximum emission.

Methods for heat transfer in the atmosphere

•Thermal conduction♦ It requires: physical contact, temperature difference♦ The energy flow is from a body with high T to a body with low T.

♦ It is important only for a thin layer close to the ground.

•Convection (advection) ♦ Winds in the atmosphere can transport energy.♦ Very efficient way of energy transport in the atmosphere.

•Radiation♦ All bodies with T> 0 K emit EM waves.♦ EM waves can propagate in vacuum.♦ The EM spectrum of a body is the energy it emits at different wavelengths

♦ The higher the temperature the more energy is emitted (SB Law)

♦ The wavelength at which the peak of the EM emission of a body occurs is determined by its temperature (Wien’s Law).

Radiation and Temperature•All bodies with T>0K emit radiation (electromagnetic

energy).

•The origin of the emission is the transition of the atoms (molecules) from one energy state to another.

•The wavelength and the amount of energy emitted by the body depend on its temperature.

•Higher T -> larger internal energy -> atoms vibrate faster -> the radiation has shorter wavelength and higher energy.

The Sun’s electromagnetic spectrum

Temperature and Emitted Energy

4TE σ=

4TE σ=

T

const.max =λ

Energy

The Sun emits ~(6000/288)4~188,000 times more energy than the Earth!!!

Sun/Earth radiation

•Sun♦ T=6000 K

♦ λmax=0.5 m

♦ Maximum in visible

•Earth♦ T=288 K = 15 C

♦ λmax=10 m

♦ Maximum in IR

The Sun emits (6000/288)4~188,000 times more energy than the Earth!!!

Radiative Equilibrium

Emission

Emission

Emission Absorption

Absorption

Absorption

EquilibriumT=constant

Heating

Cooling>

=

<

The Earth’s Balancing Act: Absorption, Emission and

Equilibrium•First, suppose the Earth had no atmosphere at all

♦ Energy from the Sun would be absorbed during daytime♦ Energy would be radiated away 24/7

•Average surface temperature at equilibrium: 255K=-18C=0F

•Why aren’t we freezing to death?

The greenhouse effect of the atmosphere

T=255 K T=288 K

No Atmosphere With Atmosphere

Selective Absorbers/Emitters

•Trees: absorb visible energy from Sun, radiate in infrared

•Snow away from trees: reflects visible sunlight, stays cold

•Snow near trees: reflects visible sunlight, but absorbs the infrared radiation from nearby trees, heats up and melts.

Atmospheric Greenhouse effect

•The gases in the Earth’s atmosphere are selective absorbers♦ Greenhouse gases: absorb the infrared radiation from Earth♦ Also selectively radiate in the infrared, a fraction goes

back to Earth

♦ The Earth’s greenhouse effect: H2O 60%; CO2 26 %; rest 14%

O2, O3 H2O, CO2 O3

H2O, CO2

Fig. 2-11b, p. 41

Fig. 2-11a, p. 41

Fig. 2-11c, p. 41

The greenhouse effect of the atmosphere

T=255 K T=288 K

No Atmosphere With Atmosphere

The lower atmosphere is heated from below!!!

The Incoming Solar Radiation

•Insolation = Incoming Solar Radiation

•The solar constant: the energy from the sun at the TOP of the atmosphere per unit surface per unit time (1367 W/m2)

•Scattered light: scattering from molecules, dust particles, aerosols. ♦ The scattered (diffuse) light goes in all directions.

♦ Shorter wavelengths are scattered more efficiently by the atmosphere (the sky is blue, see Chapter 19).

•Reflected light: some light is sent backwards.♦ Albedo: the percentage of the incident light that is reflected.

♦ The albedo is a measure of the reflectivity of the surface

•Absorption

Table 2-3, p. 44

The solar energy budget

The Energy Balance

•The amount of incoming energy should be exactly equal to the amount of the outgoing energy!!!♦ The Earth

•IN: Solar radiation+Cloud IR emission•OUT: Conduction+Convection+H2O Evaporation+ IR emission+Reflection of solar light

♦ The atmosphere:

•IN: from the ground (convection,conduction,latent heat,IR emission)+solar radiation

•OUT: to the ground (IR emission+reflected light)+to space(IR radiation)

♦ The Earth + atmosphere:

•IN: Solar Radiation•OUT: IR radiation (ground+atmosphere)+Reflected light

The Earth Energy Balance