Temperature and Heat Transfer

1
Temperature and Heat Transfer
Review of concepts from last time

• Conduction

• Latent Heat vs Sensible Heat

• Convection

• Thermals

• Advection

• Radiation

• Air has low conductivity so convection and
  latent heat release are key concepts for
  transferring heat within the atmosphere.

2
Latent Heat of Fusionsolid gt liquid
If you have a glass of a cool drink, well
supplied with ice, you can expect its temperature
to drop until it is close to 0ºC. You also can
expect (and can easily check with a thermometer)
that it will remain cold, regardless of the
outside temperature, as long as there remains
some unmelted ice in the drink. Only after all
the ice has melted will the temperature of the
drink begin to rise. Why is this?
Ice to water 80 Cal g-1
3
Latent Heat of Vaporizationliquid gt solid
water to water vapor 540 Cal g-1
4
Fig. 2-2, p.28
Removes Heat
-620
-80
-540
80
540
620
Releases Heat
1 g vapor at 120C
Total 50 80 100 540 20 790 cal
1g ice at 50C
5
Latent Heat
Energy Units 4.186 Joule (J) 1 calorie (cal)
6
Phase Change, Evaporation, and Vapor Pressure
Will evaporation occur from a water surface when
the water is not at its boiling point? Is latent
heat involved in this process? How does this
relate to the equilibrium vapor pressure of water?
7
Saturation Vapor Pressure
(Remember this example from lecture 2?) At time 0
we add some water to a closed container. At some
later time we check the pressure gauge connected
to our container.
Adopted from http//wine1.sb.fsu.edu/chm1045/notes
/Forces/Vapor/Forces05.htm
8
Saturation Vapor Pressure
Adopted from http//wine1.sb.fsu.edu/chm1045/notes
/Forces/Vapor/Forces05.htm
9
Evaporation and Latent Heat
Saturation vapor pressure - the equilibrium
pressure of a vapor above its liquid (or solid)
that is, the pressure of the vapor resulting from
evaporation of a liquid (or solid) above a sample
of the liquid (or solid) in a closed container.

• Latent heat stored when molecules evaporate from
  surface -gt cool surface

• Latent heat released when molecules condense on
  surface -gt warm surface

gas-liquid
gas-solid
The number of molecules leaving the solid or
liquid surface is equivalent to the number of
molecules condensing back onto the surface once
equilibrium is reached. The number of molecules
coming and going is temperature dependent for all
substances.
10
Evaporation
Evaporation - Ordinary evaporation is a surface
phenomenon - some molecules have enough kinetic
energy to escape. If the container is closed, an
equilibrium is reached where an equal number of
molecules return to the surface. The pressure of
this equilibrium is called the saturation vapor
pressure or equilibrium vapor pressure.
Air inside the container for which evaporation
has reached equilibrium with the surface is
saturated.
In order to evaporate, a mass of water must
collect the large heat of vaporization, so
evaporation is a potent cooling mechanism.
Evaporation heat loss is a major climatic factor
and is crucial in the cooling of the human body.
http//www.vivoscuola.it/us/rsigpp3202/umidita/cop
ie/vappre.htmc3
11
Evaporation vs Boiling
Ordinary evaporation is a surface phenomenon -
since the vapor pressure is low and since the
pressure inside the liquid is equal to
atmospheric pressure plus the liquid pressure,
bubbles cannot form. But at the boiling point,
the saturated vapor pressure is equal to
atmospheric pressure, bubbles form, and the
vaporization becomes a volume phenomena.
Bubbles form and rise since vapor pressure can
overcome atmospheric pressure
Bubbles cannot form since vapor pressure is less
than atmospheric pressure
http//www.vivoscuola.it/us/rsigpp3202/umidita/cop
ie/vappre.htmc3
12
Earths Energy Budget
Review of concepts from last time
Radiant energy supplied by the sun

• Radiation

• Electromagnetic Waves

Heat transfer from surface to atmosphere

• Conduction/Convection

• Latent Heat release

• Radiation

• Atmosphere is heated from below.

13
Blackbody
Blackbody - a substance that absorbs and emits
radiation energy with 100 efficiency absorbs
all the radiation that strikes it emits all the
radiant energy possible given its temperature.
Wiens Law - wavelength (l) of peak energy
emission from a substance is inversely
proportional to its temperature (T). lpeak 1 /
T Here, sun emits at a shorter wavelength than
the earth because it is hotter.
Sun (scaled by a factor of 10-6
Radiation Flux in W m-2 ?m-1
Earth
Wavelength in ?m
14
Blackbody
Blackbody - a substance that absorbs and emits
radiation energy with 100 efficiency absorbs
all the radiation that strikes it emits all the
radiant energy possible given its temperature.
Stefan-Bolzmann Law -The greater the temperature
of an object, the more radiation is emitted per
unit time higher temperature objects emit energy
at a higher rate or intensity (I). I sT4
Here, sun emits at a greater intensity than the
earth because it is hotter.
Sun (scaled by a factor of 10-6
Radiation Flux in W m-2 ?m-1
Earth
Wavelength in ?m
15
Selective Absorber
Selective Absorber - A substance that absorbs and
emits radiation at select wavelengths absorbs
and emits only a fraction of the radiation that
strikes it.
electron
atom nucleus
electron orbits or shells
absorption
emission
http//www.uwsp.edu/geo/faculty/ritter/geog101/tex
tbook/energy/nature_of_electromagnetic_radiation_p
_2.htm
16
Scattered Light
Why is the sky blue?
Light interacts with matter. Different types of
matter absorb its wavelengths in different
proportions.
When light strikes an apple all wavelengths but
red are absorbed. The red wavelengths are
scattered and reflected back to our eyes.
When all wavelengths are absorbed an object
appears black. When all are reflected, we see
white.
When light strikes the atmosphere about half of
the rays get through to the surface. The blue
wavelengths, however, are preferentially
scattered by gas and water molecules and by dust.
Thus, the sky appears blue no matter what angle
you look at it.
Scattered light goes in all directions. Reflected
light is scattered light that is preferentially
scattered backwards.
17
Fig. 2-12, p.40
18
Most important selective absorbers in our
atmosphere
19
Fig. 2-9f, p.36
Atmospheric Window - The region of the spectrum
not absorbed by atmospheric gases. The 8-11 mm
(IR wavelengths) region.
20
Selective Absorption
Which gas absorbs radiation at the greatest range
of wavelengths?
a. nitrous oxide b. methane c. molecular oxygen
and ozone d. water vapor e. carbon dioxide
X
21
Atmospheric Greenhouse Effect
CO2 currently the primary cause of global
warming, continues to rise due to the burning of
fossil fuels and deforestation.
CH4, NO2, and CFCs concentrations are on the rise.
CFCs - 10,000 times more effective as a
greenhouse gas than CO2 and have a 100 year
residence time in the atmosphere.
22
Global Warmingpreview for Ch. 14
There is general consensus among climate
scientists that the earth has warmed since the
early 1900s.
Clouds and oceans least understood aspects of our
climate.
Satellite observations indicate that clouds cool
the planet. Uncertain weather a warmer climate
would increase or decrease the cloud cover and/or
change the balance of cloud type (high, medium,
low). Either scenario greatly affects the earth
radiation budget and thus its average temperature.
The atmospheric greenhouse effect itself is not
of concern. It is the enhancement of this effect
due to increasing levels of greenhouse gases,
especially those that absorb in the atmospheric
window, that is of concern.
23
Absorption, Emission, and the Atmospheric
Greenhouse Effect
T-18C
T15C
Fig. 2-10, p.37
No greenhouse gases
Greenhouse gases present
What is going on in this picture?
24
Effect of Clouds
Earths surface
25
Albedo
75-95
75-95
30-50
15-45
10-30
3-10
10
Thick Clouds
Thin Clouds
Sand
Water
Grass
Forest
Snow
Albedo - The fraction of solar radiation that is
reflected off, rather than absorbed by or
transmitted through an object.
26
Fate of the suns energy upon entering the
earths atmosphere
Fig. 2-13, p.41
27
Earths Energy Budget radiative-convective
equilibrium
-30 reflected
Fig. 2-14, p.42