Lecture 4: Heat, and Radiation

Heat

• Heat is a transfer of energy from one object to another.

• Heat makes things warmer.

• Heat is measured in units called calories.

• A calorie is the heat (energy) required to raise one cubic centimeter of water by 1˚C.

Heat in the Atmosphere

There are four ways in which heat is transferred.

1. Conduction – heat transfer by direct contact.

2. Convection – heat carried by currents.

3. Radiation – heat transfer by electromagnetic waves, which are emitted by all objects.

4. Latent heat – hidden heat associated with changes of state (aka phase).

Conduction

Good isolator: thermo

Fast heat conduction cool feeling

Convection

Advection: Transfer of heat, water vapor and other properties byhorizontal wind (e.g., passage of a cold front)

Latent heat – heat required/released as a substance changes from one state to another.

Latent heat may be transported from one place to another: e.g., much of precipitation in polar regions is due to water vapor transport from lower latitudes.

cooling

heating

Important Heat Concepts

• Sensible heat – heat that can be measured by a thermometer.

• Latent heat – heat required/released when a substance changes from one state to another.(Latent heat when added/removed from a substance does not change its temperature when a change in state does not occur.)

Almost a daily occurrence in Hawaii over the mountains --caused by surface heating, rising buoyant plumes, and the release of latent heat in clouds

Moist Convectionfuels hurricanes

Radiation• Radiation - energy leaving a body in the form

of electromagnetic waves.

• Light is a form of electromagnetic radiation.

• The speed of light is ~ 3x108 m/s through a vacuum (slightly slower through air).

Newton demonstrated the wavy nature by putting light through a prism

Duality of light• Photons• Waves

Incoming light

Refracted light

Reflected light

Radiation Quality and Quantity

• The amplitude corresponds to the energy carried

• The wavelength corresponds to the type

Heat in the Atmosphere

Latent heat of evaporation

Radiation

—Micrometer (µm)

—Nanometer

Planck’s LawElectromagnetic spectrum E (λ)

Blackbody - is a theoretical object that absorbs all incident radiation and emits the maximum possible radiation for its temperature (according to Planck’s Law).

(Solar)(Terrestrial)

Laws of Radiation

1. All objects emit radiation (except at 0°K).2. Hotter objects emit more energy per unit

area than colder objects.3. The hotter the object the shorter the

wavelength of maximum radiation.4. Objects that are good absorbers of radiation

are good emitters of radiation.

Laws of RadiationSTEFAN BOLTZMAN LAW

E = σ T4 (E is in Watts/m2)

As T increases, E increases by a power of 4. If T doubles, E increases by 16 times!

Does everything emit radiation?

WIEN’S LAW

λmax ~ 3000/T (λmax is in µm and T is in Kelvin)

Wavelength of peak radiation emitted by an object is inversely related to temperature

Radiation ConceptsSolar constant -Amount of solar radiation passing through a unit area at the top of the earth's atmosphere perpendicular to the direction of the radiation at the mean Earth-sun distance. Solar constant ~ 1400 W/m2

Radiation ConceptsWhat happens to sunlight

once it reaches the Earth?

TransmissionScattering (reflection)Absorption

Selective absorbers(poor absorber for shortwave, but good absorber/emitter for longwave): snow, atmosphere, …

SunEarth

AbsorptionGreenhouse gases: H2O, CO2, CH4, N2O, O3

Molocular absorption

Scattering—light deflected in all direction by small objects

Scattering by air molecules ~ 1/λ4Blue light scattered more Blue sky when looking away from sunLong visible light left after passing a thick layer of air orange/red sunset

Scattering by cloud droplets (>>λ): independent of wavelength white cloud

Reflection

Fig. 2-13, p.41

Key wordsHeatSensible heat, latent heatConductionConvection, thermals, advectionRadiation, electromagnetic waves, photonsVisible region, ultraviolet (UV) & infrared (IR) radiationBlack bodySelective absorbers, atmospheric windowSolar constant, scattering, reflection, albedo