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 by horizontal wind (e.g., passage of a cold front)
Latent heat heat required/released as a substance changes from one state to another. cooling heating 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.
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.)
Moist Convection fuels hurricanes Almost a daily occurrence in Hawaii over the mountains -- caused by surface heating, rising buoyant plumes, and the release of latent heat in clouds
Radiation Radiation - energy leaving a body in the form of electromagnetic waves. Light is a form of electromagnetic radiation. The speed of light is ~ 3x10 8 m/s through a vacuum (slightly slower through air).
Duality of light Photons Waves Reflected light Refracted light Newton demonstrated the wavy nature by putting light through a prism Incoming 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 Law Electromagnetic 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 Radiation STEFAN BOLTZMAN LAW E = σ T 4 (E is in Watts/m 2 ) 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 λ max ~ 3000/T (λ max is in µm and T is in Kelvin) Wavelength of peak radiation emitted by an object is inversely related to temperature
Radiation Concepts Solar 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/m 2
What happens to sunlight once it reaches the Earth? Transmission Scattering (reflection) Absorption Radiation Concepts Selective absorbers (poor absorber for shortwave, but good absorber/emitter for longwave): snow, atmosphere,
Absorption Greenhouse gases: H 2 O, CO 2, CH 4, N 2 O, O 3 Sun Earth Molocular absorption
Scattering light deflected in all direction by small objects Scattering by air molecules ~ 1/λ 4 Blue light scattered more Blue sky when looking away from sun Long 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 words Heat Sensible heat, latent heat Conduction Convection, thermals, advection Radiation, electromagnetic waves, photons Visible region, ultraviolet (UV) & infrared (IR) radiation Black body Selective absorbers, atmospheric window Solar constant, scattering, reflection, albedo