Transcript September 15th: Station Model & Heat Transfer

AOS 100: Weather and
Climate
Instructor: Nick Bassill
Class TA: Courtney Obergfell
Miscellaneous
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Review of September 10th: Ideal Gas
Law Continued
Pressure = Density * Gas Constant * Temperature
• Pressure is basically a measure of the weight of
the atmosphere above a given location
• Therefore, pressure is a useful vertical
coordinate
• Example: If the surface pressure is 1000 mb,
then a value of 500 mb means that half of the
atmosphere is above and half is below (in terms
of weight)
• The troposphere contains almost all “weather”,
and is comprised of approximately the lowest
80% of the atmosphere (by weight)
Review Continued
Remember our example of one
cold air column and one warm air
column …
• There is more weight
(higher pressure) above any
given level in column 2 than
column 1
• This will cause air to move
from column 2 to column 1
to attempt to correct this
500 mb
• This results in more air
molecules within column 1
than column 2
Surface
1
2
• Which means the pressure
is higher at the surface of
column 1!
Review Continued: Station Models
Station Models
For pressure, only the last three digits
are shown (including the first digit
after the decimal point)
If the first number begins with a 0, 1,
2, 3, 4, or 5, then put a ‘10’ in front
If the first number begins with a 6, 7,
8, or 9, put a ‘9’ in front
Meanings
*
*
*
*
New Topic – Heat Transfer
• 1st Law of Thermodynamics - Heat can be
thought of as a form of energy (transfer)
• 2nd Law of Thermodynamics – Heat always goes
from hotter objects to colder objects
• The amount of heat lost by the warm object is
equivalent to the heat gained by the second
object
From: http://en.wikipedia.org/wiki/Laws_of_thermodynamics
More Heat
• When you touch an object, heat is transferred
• This is why certain objects feel warm and others
feel cold
• Note: although heat is similar to temperature, it
is technically incorrect to say that we feel
another object’s temperature
• Rather, we observe the transfer of heat from
ourselves to other objects
Specific Heat
• This is a measure of the heat energy needed to
heat 1 g of an object 1º C
• Different objects have different specific heat
values
−1K−1)
Substance
Value
(Jg
Examples:
Liquid Water
4.183
Ice
Wood
Sand
Air
2.050
0.420
0.835
1.012
More Specific Heat
• Therefore it takes more energy to warm water by
1º C than it does to warm air by 1º C
• This is why the water temperature in a swimming
pool stays fairly constant during the day, while
the air might change temperature much more
Mechanisms of Heat Transfer
(1) Conduction
(2) Convection
(3) Advection
(4) Radiation
Conduction
• Conduction is the transfer of heat through
an object molecule by molecule
• For example, if you were to hold a metal
poker in a fire, eventually the handle will
warm as well, primarily due to conduction
Heat Conductivity
• This is a measure of the effectiveness of
different materials at conducting heat
Examples:
Material
Value
(W/mK)
Air
0.023
Water
0.6
Ice
2.1
Iron
80
Heat Conduction Continued
• This is why some materials “feel” colder
than others, even if they are at the same
temperature
• For example, 40º F water “feels” colder
than 40º F air, even though they are the
same temperature
• This is because water is more effective at
conducting heat (i.e. more effective at
taking heat from your body)
Convection
• Convection is heat transfer by the
movement of fluids (gases and liquids) in
the vertical (along gravity)
• As shown in the ideal gas law, warm air is
less dense than cold air
• This means warm air tends to rise, and
cold air tends to sink
Convection Continued
• As air in contact with the ground gets
heated during the day (through
conduction), it will rise (convection)
• However, because the pressure decreases
with altitude, as the air rises, it also cools
• If it becomes colder than the surrounding
air, it will sink again
• This process is called convection
Lapse Rate
• The lapse rate
is a measure of
the rate of
change of
temperature
with height
• In this case, the
lapse rate is
about 65 ºC per
10000 meters,
or 6.5ºC/km
Convection Continued
• As dry air rises, it cools at a rate of 9.8º C
per km
• Moist (saturated) air will cool at a slower
rate due to the conversion of water vapor
to liquid water (more on this later in the
semester)
• Convection is the process that produces
“fair weather” cumulus clouds and
thunderstorms
Advection
• Advection is the transfer of heat through
the horizontal movement of a fluid (gases
or liquids)
• For example, a south wind might “advect”
warmer air from the southwest to Madison
“Cold Air
Advection”
“Warm Air
Advection”