Transcript Document

Temperature, heat,
and energy balance
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.
• 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
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)
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.
• 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
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)
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
aren’t in direct contact with the hot substance
Earth – Sun energy balance
sunlight energy reaches Earth via radiation
 it is also the way energy leaves Earth to return to space
The
Electromagnetic
Spectrum
Shorter wavelength = higher energy
“Rules” of Radiation:
1. All objects with a temperature above absolute zero (0 K) emit radiation
2. Wein’s law: The higher the temperature of the object, the shorter the
wavelength of radiation emitted
3. 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 Sun’s peak wavelength of emission is
in visible light, and earth’s is in infrared
(IR)
Fun with Stefan-Boltzman
It’s 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%
εe= emissivity of earth, 1.0 (blackbody)
σB= 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!
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
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
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
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.
Most of the sun’s 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 earth’s surface
Question: why are you more
likely to be sunburned on a
mountaintop than you are at
sea level?
The Reason for the Seasons
Why is the sun more intense in the tropics?
The daily temperature cycle
Daily maximum temperatures (usually) occur several hours after the
time of maximum incoming solar radiation? WHY?
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
Seasonal temperature variation: Graz
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
The
Atmospheric
“Greenhouse”
Effect
CO2 and
water
vapor are
the primary
greenhouse
gasses
Net result of all this talk about energy: weather
is primarily driven by energy transfer around the
planet
Another process by which energy is transferred through
the atmosphere is called “latent heat”
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!
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.