Transcript Slide 1

Energy is defined as “the ability to do work.”
The standard unit of energy in the International System (SI)
used in scientific applications is the joule (J).
Power is the rate at which energy is released,
transferred, or received. The unit of power is the watt (W),
which corresponds to 1 joule per second
(1 joule = 0.239 calories).
All forms of energy fall into the general
categories of kinetic energy and potential energy.
Kinetic energy can be viewed as energy in use
and is often described as the energy of motion.
Potential energy is energy that has not yet been used,
such as a cloud droplet that occupies some position
above Earth’s surface. Like all other objects, the
droplet is subject to the effect of gravity. The higher
the droplet’s elevation, the greater its potential energy.
Energy can be transferred from one place to another
by three processes: conduction, convection, and radiation.
Conduction is the movement of heat through a substance
without the movement of molecules in the direction of
heat transfer. Conduction is most effective in solid materials,
but it also is an important process in a very thin layer of air
near Earth’s surface.
The transfer of heat by the mixing of a fluid is called
convection. Unlike conduction, convection is accomplished
by displacement (movement) of the medium.
During the daytime, heating of Earth’s surface warms a
very thin layer of air in contact with the surface. Above this
thin laminar layer, air heated from below expands and rises
upward because of the inherent buoyancy of warm air
(the tendency for a light fluid to float upward
when surrounded by a heavier fluid).
Of the three energy transfer mechanisms,
radiation is the only one that can be propagated
without a transfer medium. Unlike conduction or
convection, the transfer of energy by radiation
can occur through empty space.
Virtually all the energy available on Earth originates
from the Sun. However, radiation is emitted by all matter.
In the case of radiation, quantity is associated with the
height of the wave, or its amplitude. Everything else
being equal, the amount of energy carried is
directly proportional to wave amplitude.
The quality, or “type,” of radiation is related to another
property of the wave, the distance between wave crests
or wavelength, which is the distance between any
two corresponding points along the wave.
Electromagnetic radiation consists of an
electric wave (E) and a magnetic wave
(M). As radiation travels, the waves
migrate in the direction shown by the pink
arrow. The waves in (a) and (b) have the
same amplitude, so the radiation intensity
is the same. However, (a) has a
shorter wavelength, so it is qualitatively
different than (b). Depending on the
exact wavelengths involved, the radiation
in (a) might pass through the
atmosphere, whereas that in (b)
might be absorbed.
It is convenient to specify wavelengths using
small units called micrometers (or microns).
1 micrometer equals one-millionth of a meter.
Perfect emitters of radiation, so-called blackbodies
are purely hypothetical bodies that emit the
maximum possible radiation at every wavelength.
Earth and the Sun are almost blackbodies.
The single factor that determines how much energy a
blackbody radiates is its temperature. Hotter bodies
emit more energy than do cooler ones.
The intensity of energy radiated by a blackbody
increases according to the fourth power
of its absolute temperature.
This relationship is represented by the
Stefan-Boltzmann law, expressed as
I = σT4
where I is the intensity of radiation in
watts per square meter,
σ is a constant (5.67 x 10-8 watts per square meter)
and T is the temperature of the body in kelvins.
Celsius Temperature = (oF - 32) / 1.8
Fahrenheit Temperature = (1.8 x oC) + 32
Kelvin Temperature = oC + 273
For any radiating body, the wavelength of peak emission
(in micrometers) is given by Wien’s law:
max = constant (2900)/T
where max refers to the wavelength of energy
radiated with greatest intensity.
Wien’s law tells us that hotter objects radiate energy
at shorter wavelengths than do cooler bodies.
Solar radiation is most intense in the visible portion of
the spectrum. Most of the radiation has wavelengths less
than 4 micrometers which we generically refer to as
shortwave radiation. Radiation emanating from
Earth’s surface and atmosphere consists mainly of that
having wavelengths longer than 4 micrometers. This type
of electromagnetic energy is called longwave radiation.
Energy radiated by substances occurs
over a wide range of wavelengths.
Because of its higher temperature,
emission from a unit of area of the
Sun (a) is 160,000 times more intense
than that of the same area on Earth (b).
Solar radiation is also composed of
shorter wavelengths than
that emitted by Earth.
As the distance from the Sun
increases, the intensity of the
radiation diminishes in proportion
to the distance squared
(inverse square law).
The solar constant is the amount of
solar energy received by a surface
perpendicular to the incoming rays
at the mean Earth–Sun distance
and is equal to 1367 W/m2.
Atmospheric gases, particulates, and droplets all reduce the
intensity of solar radiation (insolation) by absorption,
a process in which radiation is captured by a molecule.
It is important to note that absorption represents an
energy transfer to the absorber.
This transfer has two effects:
the absorber gains energy and warms, while the
amount of energy delivered to the surface is reduced.
The reflection of energy is a process whereby radiation
making contact with some material is simply redirected
away from the surface without being absorbed.
The percentage of visible light reflected by an object
or substance is called its albedo. When light strikes
a mirror, it is reflected back as a beam of equal intensity,
in a manner known as specular reflection.
When a beam is reflected from an object as a
larger number of weaker rays traveling in different
directions, it is called diffuse reflection, or scattering.
In addition to large solid surfaces, gas molecules,
particulates, and small droplets scatter radiation.
Although much is scattered back to space,
much is also redirected forward to the surface.
The scattered energy reaching Earth’s surface
is thus diffuse radiation, which is in contrast to
unscattered direct radiation.
Scattering agents smaller than about one-tenth the
wavelength of incoming radiation disperse radiation
through Rayleigh scattering, which is particularly
effective for those colors with the shortest wavelengths.
Thus, blue light is more effectively scattered by
air molecules than is longer-wavelength red light.
Microscopic aerosol particles are considerably larger
than air molecules and scatter sunlight by a process
known as Mie scattering, which does not have nearly
the tendency to scatter shorter wavelength radiation
that Rayleigh scattering does. Mie scattering causes
sunrises and sunsets to be redder than they would due
to Rayleigh scattering alone, so episodes of heavy
air pollution often result in spectacular sunsets.
The sky appears blue because gases and particles in the atmosphere
scatter some of the incoming solar radiation in all directions. Air molecules
scatter shorter wavelengths most effectively. Thus, we perceive blue light,
the shortest wavelength of the visible portion of the spectrum.
Sunrises and sunsets appear red because sunlight travels a longer path
through the atmosphere. This causes a high amount of scattering to remove
shorter wavelengths from the incoming beam radiation. The result is sunlight
consisting almost entirely of longer (e.g., red) wavelengths.
The water droplets in clouds are considerably larger than
suspended particulates reflecting all wavelengths of
incoming radiation about equally, which is why clouds
appear white or gray. Because of the absence of
preference for any particular wavelength, scattering
by clouds is sometimes called nonselective scattering.
Incoming solar radiation available is subject to a number of processes
as it passes through the atmosphere. The clouds and gases of the
atmosphere reflect 19 and 6 units, respectively, of insolation back to
space. The atmosphere absorbs another 25 units. Only half of the
insolation available at the top of the atmosphere actually reaches
the surface, of which another 5 units are reflected back to space.
The net solar radiation absorbed by the surface is 45 units.
The difference between absorbed and emitted longwave
radiation is referred to as the net longwave radiation.
Shortwave and longwave radiation are not
separate entities as far as the heating of the atmosphere
and surface are concerned. When either is absorbed, the
absorber is warmed. It is therefore natural to combine
longwave and shortwave into net allwave radiation,
or simply net radiation, defined as the difference between
absorbed and emitted radiation, or equivalently,
the net energy gained or lost by radiation.
Net radiation is the end result of the absorption of insolation and the
absorption and radiation of longwave radiation. The surface has a net
radiation surplus of 29 units, while the atmosphere has a deficit of 29 units.
Convection is a heat transfer mechanism involving the mixing of a fluid.
In free convection, local heating can cause a parcel of air to rise
and be replaced by adjacent air.
Forced convection (also called mechanical turbulence) occurs when
a fluid breaks into disorganized swirling motions as it undergoes a
large-scale flow. Air is forced to mix vertically because of its
low viscosity and the deflection of wind by surface features.
When energy is added to a substance, an increase in
temperature occurs that we physically sense
(sensible heat).
The magnitude of temperature increase is related to
two factors, the first of which is specific heat,
defined as the amount of energy needed to produce
a given temperature change per unit mass of the substance.
The temperature increase resulting from a surplus of energy
receipt also depends on the mass of a substance.
Latent heat is the energy required to change the
phase of a substance (solid, liquid, or gas).
In meteorology we are concerned with the heat
involved in the phase changes of water.
In the case of melting ice, the energy is called
the latent heat of fusion. For the change of phase
from liquid to gas, the energy is called
the latent heat of evaporation.
Both the surface and atmosphere lose exactly as much energy as they
gain. The surface has a surplus of 29 units of net radiation, which is
offset by the transfer of sensible and latent heat to the atmosphere.
The atmosphere offsets its 29 units of radiation deficit by the
receipt of sensible and latent heat from the surface.
The interactions that warm the atmosphere are often
collectively referred to as the greenhouse effect,
but the analogy to a greenhouse is not strictly accurate.
The greenhouse gases of the atmosphere do not impede
the transfer of latent and sensible heat. Thus, it
would be more accurate if the term “greenhouse effect”
were replaced by “atmospheric effect.”
The greenhouse effect keeps Earth
warmer by absorbing most of the longwave radiation
emitted by the surface, thereby warming the
lower atmosphere, which in turn emits radiation downward.
One of the most immediate and obvious outcomes of radiation gain or loss
is a change in the air temperature. The map depicts differences between
mean temperatures in January and July through the use of isotherms,
which are lines that connect points of equal temperature.
Certain geographical factors combine to influence temperature patterns across
the globe. These factors include latitude, altitude, atmospheric circulation patterns,
local conditions, continentality, (the effect of an inland location that favors greater
temperature extremes) and ocean current characteristics along coastal locations.
The daily mean is defined as the average of the
maximum and minimum temperature for a day.
The daily temperature range is obtained by
subtracting the minimum temperature from the maximum.
The monthly mean temperature is found by
summing the daily means and dividing by
the number of days in the month.
The annual mean temperature is obtained by
summing the monthly means for a year and dividing by 12.
The annual range is obtained as the difference
between the highest and lowest monthly mean temperatures.
If low temperatures are accompanied by windy conditions,
a person’s body loses heat much more rapidly than it would
under calm conditions due to an increase in sensible heat loss.
It is common for weather reports to state both the actual
temperature and how cold that temperature actually feels,
the wind chill temperature index.
Thermodynamic diagrams (such as the Stuve above) depict the vertical
profiles of temperature and humidity with height above the surface enabling
forecasters to determine the height and thickness of existing clouds and the
ease with which the air can be mixed vertically. The data on the charts are
obtained from radiosondes that are carried aloft by weather balloons
twice a day at weather stations across the globe.
Conduction - Heat Transfer
Conduction of
heat energy
occurs as
warmer
molecules
transmit
vibration, and
hence heat, to
adjacent cooler
molecules.
Warm ground
surfaces heat
overlying air
by conduction.
Figure 2.5
Convection - Heat Transfer
Figure 2.6
Convection is heat energy moving as a fluid from hotter to cooler
areas.
Warm air at the ground surface rises as a thermal bubble, expends
energy to expand, and hence cools.
Radiation - Heat Transfer
Radiation
travels as waves
of photons that
release energy
when absorbed.
All objects
above 0° K
release
radiation, and
its heat energy
value increases
to the 4th
power of its
temperature.
Figure 2.7
Longwave & Shortwave Radiation
Figure 2.8
The hot sun
radiates at
shorter
wavelengths
that carry more
energy, and the
fraction
absorbed by the
cooler earth is
then re-radiated
at longer
wavelengths, as
predicted by
Wein's law.
Electromagnetic Spectrum
Figure 2.9
Solar radiation has peak intensities in the shorter wavelengths,
dominant in the region we know as visible, but extends at low
intensity into longwave regions.
Absorption & Emission
Figure 2.10
Solar radiation is selectively absorbed by earth's surface cover.
Darker objects absorb shortwave and emit longwave with high
efficiency (e.g. Kirchoff's law).
In a forest, this longwave energy melts snow.
Atmospheric Absorption
Solar radiation passes rather freely through
earth's atmosphere, but earth's re-emitted
longwave energy either fits through a narrow
window or is absorbed by greenhouse gases
and re-radiated toward earth.
Figure 2.11
Greenhouse Effect
Figure 2.12B
Figure 2.12A
Earth's energy balance requires that absorbed solar radiation is
emitted to maintain a constant temperature.
Without natural levels of greenhouse gases absorbing and
emitting, this surface temperature would be 33°C cooler than the
observed temperature.
Warming Earth's Atmosphere
Figure 2.13
Solar radiation passes first through the upper atmosphere, but only
after absorption by earth's surface does it generate sensible heat to
warm the ground and generate longwave energy.
This heat and energy at the surface then warms the atmosphere
from below.
Scattered Light
Solar radiation
passing through
earth's
atmosphere is
scattered by
gases, aerosols,
and dust.
At the horizon
sunlight passes
through more
scatterers,
leaving longer
wavelengths
and redder
colors revealed.
Figure 2.14
Incoming Solar Radiation
Figure 2.15
Solar radiation is scattered and reflected by the atmosphere, clouds,
and earth's surface, creating an average albedo of 30%.
Atmospheric gases and clouds absorb another 19 units, leaving 51
units of shortwave absorbed by the earth's surface.
Earth-Atmosphere Energy Balance
Figure 2.16
Earth's surface absorbs the 51 units of shortwave and 96 more of
longwave energy units from atmospheric gases and clouds.
These 147 units gained by earth are due to shortwave and longwave
greenhouse gas absorption and emittance.
Earth's surface loses these 147 units through conduction,
evaporation, and radiation.
Convection:
Air or Water Moving Upward by Heating
Vertical motion of
Air (gases) or water
Caused by heating is
Called convection
(Convectional Cells)
Convection in a pan of water
Warmer materials
Move upwards &
Cooler materials fall
Convection
Heat is transferred by the
movement, from place to place, of
a gas or liquid. The principal motion
is vertical.
All heated gases and
liquids experience
this kind of “mixing”
heat exchange.
Classifying
Radiation
• Classified based on
wavelength, 
Radiation Laws
#1 Stefan-Boltzmann law:
• All objects emit radiation.
more radiation
less radiation
• Hot objects emit more radiation
(per unit area) than cold objects.
#2 Wien’s law:
The hotter the radiating
body, the shorter the wavelength of maximum
radiation.
Sun: 6000ºC (11,000ºF)
Earth:
15ºC (60ºF)
The hotter sun radiates
more energy than the
cooler earth
&
radiates the
majority of
its energy
at much
shorter
wavelengths.
Absorption:
Assimilation & Conversion of
Energy
Absorption is the processes of changing
sunlight into heat at Earth’s surface
Darker surfaces absorb more sunlight &
convert it to heat (they have a low Albedo,
like asphalt and basaltic lava flows)
Lighter surfaces reflect more light & therefore
convert little Light energy to heat (like Snow)
Albedo:
Reflection as a Percentage
Earth’s Average
Albedo is 31%
Fresh Snow
80% - 95%
Grass
25% - 30%
Water Bodies
10% - 60%
(depends on angle)
Forests
10%-20%
Asphalt
5% - 10%
Absorption of radiation by gases in the atmosphere. The
shaded area represents the percent of radiation absorbed. The
strongest absorbers of infrared radiation are
water vapor & carbon dioxide.
Sunlight warms
earth's surface only
during the day
surface constantly emits
infrared radiation
With greenhouse gases…
Air in the lower atmosphere is heated from below...
Scattering & light
Scattering
• short  , scatter
• result?
 distance , scatter
Scattering
(diffuse radiation)
Backscattering
(albedo)
• result?
• Energy “bounces off” particles in the atmosphere.
Scattered Light
Solar radiation
passing through
earth's
atmosphere is
scattered by
gases, aerosols,
and dust.
At the horizon
sunlight passes
through more
scatterers,
leaving longer
wavelengths
and redder
colors revealed.
Figure 2.14
The earth-atmosphere
energy balance.
Next time… Controls of Temperature
Earth's air temperature is governed by length of day
and intensity of insolation, which are a function of:
1) latitude
Additional controls are:
2) land and water
3) ocean currents
4) elevation