Download Lecture 4: Heat, and Radiation

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
Æ Good isolator: thermo
Fast heat conduction Æ 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 ~ 3x108 m/s through a
vacuum (slightly slower through air).
Duality of light
• Photons
• Waves
Reflected light
t
h
g
i
l
d
c te
a
r
f
e
R
Incoming light
Newton demonstrated the
wavy nature by putting light
through a prism
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).
(Terrestrial)
(Solar)
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 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 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/m2
Radiation Concepts
What happens to sunlight
once it reaches the
Earth?
Transmission
Scattering (reflection)
Absorption
Selective absorbers
(poor absorber for shortwave,
but good absorber/emitter for longwave): snow, atmosphere, …
Absorption
Greenhouse gases: H2O, CO2, CH4, N2O, O3
Sun
Earth
Molocular absorption
Scattering—light deflected in all direction by small objects
Scattering by air molecules ~ 1/λ4
ÆBlue light scattered more Æ Blue sky when looking away from sun
ÆLong visible light left after passing a thick layer of air Æorange/red
sunset
Scattering by cloud droplets (>>λ): independent of wavelength
Æ 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