Temperature, heat, and energy balance

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Temperature, heat, and energy balance
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But what do we really mean by temperature?

• The particles (atoms and molecules) that make up
  all substances are always in motion
• We call this energy of motion kinetic energy
• The particles will not all have the same energy,
  and the energy of the particles is constantly
  changing as they undergo changes in speed
• So, for a given sample of matter, we can only
  talk about the average kinetic energy of the
  particles.
• Temperature is a measure of the average kinetic
  energy of the particles in a substance.

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• When we refer to an object as being hot, we
  usually mean it has high temperature
• Temperature is more easily measured and defined
  than heat
• Heat is a measure of the total molecular energy
  of a substance
• The heat of an object depends on three
  characteristics of a substance its temperature,
  its mass, and its composition

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• To drive the atmosphere, heat must be transferred
  from place to place and from substance to
  substance.
• There are three ways by which heat is
  transferred
• Conduction
• (contact)
• Convection
• (motion of
• currents)
• Radiation
• (energy
• transfer)

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Conduction occurs when energy is passed directly
from one item to another. - If you stirred a
pan of soup on the stove with a metal spoon, the
spoon will heat up through conduction Metals
are excellent conductors of heat energy. Wood or
plastics are not. These "bad" conductors are
called insulators. That's why a pan is usually
made of metal while the handle is made of a
strong plastic.
Conduction can occur in solids, liquids, or
gases. However, because the molecules in gases
are far apart, gases are much poorer conductors
that either solids or liquids.
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• Convection transfers heat energy as fluid
  parcels move.
• Depends on the fact that, in general, fluids
  expand when heated and thus undergo a decrease in
  density
• Warmer less dense portion of the fluid will tend
  to rise through the surrounding cooler fluid.
• If heat continues to be supplied, the cooler
  fluid that flows in to replace the rising warmer
  fluid will also become heated and also rise.

Convection can occur in liquids or gases
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BOTH conduction convection at work here
Pavement warms faster than grass air touching
pavement heats through _______, then rises and
transfers heat to air above it through _______
Depending if the rising air is saturated or not,
convection can be dry or moist Moist convection
is associated with cloud formation (but not the
only way clouds form, as we will see later)
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Radiation is the process by which heat is
transferred in wave form - this process
transfers heat at the speed of light We can feel
heat transferred by radiation, even though we
arent in direct contact with the hot substance
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• Earth Sun energy balance
• sunlight energy reaches Earth via radiation
• it is also the way energy leaves Earth to return
  to space

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The Electromagnetic Spectrum
Shorter wavelength higher energy
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• Rules of Radiation
• All objects with a temperature above absolute
  zero (0 K) emit radiation
• Weins law The higher the temperature of the
  object, the shorter the wavelength of radiation
  emitted
• Stefan-Boltzman equation The higher the
  temperature of the object, the greater the total
  radiation emitted

As temperature increases, peak wavelength gets
shorter and total energy emitted increases
The Suns peak wavelength of emission is in
visible light, and earths is in infrared (IR)
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Fun with Stefan-Boltzman
Its possible to rearrange the S-B equation
(leave as an exercise for you), equating the
amount of energy received by the earth and the
amount of energy emitted by the earth. This
yields an equilibrium temperature, Te, that can
be solved for.
Fs solar constant, 1355 W m-2 A albedo
(reflectivity) of earth, 30 ee emissivity of
earth, 1.0 (blackbody) sB Boltzman constant,
5.67 x 10-8 W m-2 K-4 Substituting gives T 255
K, or -18 C. Brrr! Cold! But what gives? We
measure the earth to be 288 K, not 255
K. Answer? The emissivity is not 1.0, but rather
0.75, due to greenhouse gasses. Furthermore, if
concentration of these gasses increases,
emissivity will decrease, and Te will increase.
Now you have a basic equation to describe global
warming. Go forth and impress your friends!
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Atmospheric spectral transmission implications
Just a couple of points 1- UV waves are strongly
absorbed 2- IR waves are either transmitted or
absorbed, depending on wavelength 3- Weather
satellites rely these spectral properties of the
atmosphere satellite tunes its sensor to a
particular wavelength band to observe clouds,
etc. on earth
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• The solar radiation that reaches Earth is
  affected in one of the following ways
• it can be reflected
• it can be scattered
• it can be absorbed

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Why is the sky blue? _________ gives us our blue
sky

• The small gas atoms and molecules that make up
  the atmosphere preferentially scatter the shorter
  wavelengths of light
• Blue is shorter than green or red (remember ROY
  G BIV?), and thus blue is scattered first

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As the sun gets lower in the sky, the sunlight
has to travel through more atmosphere. This
gives the orange and red light more of a chance
to be scattered.
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Most of the suns energy is absorbed by the upper
levels of our atmosphere (even though not much
air exists there, it absorbs almost 100 of the
very harmful rays Visible, Infrared, and some
Ultraviolet (UV) light does reach the earths
surface Question why are you more likely to be
sunburned on a mountaintop than you are at sea
level?
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The Reason for the Seasons
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Why is the sun more intense in the tropics?
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The daily temperature cycle
Daily maximum temperatures (usually) occur
several hours after the time of maximum incoming
solar radiation? WHY?
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Not only daily temperature lag but a seasonal
temperature lag, too
Seasonal variation of temperature depends
strongly on latitude but also on location
within the continent
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Seasonal temperature variation Graz
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What comes in, must go out Over the course of a
year, Earth must emit as much radiation as it has
received from the Sun
Energy in Energy out
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The Atmospheric Greenhouse Effect
CO2 and water vapor are the primary greenhouse
gasses
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Net result of all this talk about energy weather
is primarily driven by energy transfer around the
planet
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Another process by which energy is transferred
through the atmosphere is called latent heat
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Hurricane energy source comes from the
evaporation of warm ocean water. When this water
vapor condenses into clouds in the atmosphere, a
tremendous amount of energy is released in the
form of latent heat 3 x 1012 Watts, equal to
rate of power consumption in the US in the year
2000!
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Ocean currents also play a significant role in
transferring heat. Major currents, such as the
northward flowing Gulf Stream, transport
tremendous amounts of heat poleward and
contribute to the development of many types of
weather phenomena.