Microsoft Word - chapter03.docxChapter 3: Properties of Fluids Thermodynamic Properties of Air If two samples of air can exist in contact with each other without a change in properties, the two samples are said to have the same state. What properties define the atmospheric state?
- Temperature - Pressure - Density - Water vapor content - Other chemical consituents
Equation of state – an equation that relates properties of state to one another
Ideal gas law:
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= R, a constant
Composition of the Atmosphere How does the composition of the atmosphere vary in space? The Dry Atmosphere What are the constituents of the dry atmosphere?
Constituent
Fractional Concentration by Volume of Dry Air
Nitrogen (N2) 28.013 78.08% Oxygen (O2) 32.000 20.95% Argon (Ar) 39.95 0.93%
Carbon Dioxide (CO2) 44.01 380 ppm Neon (Ne) 20.18 18 ppm
Helium (He) 4.00 5 ppm Methane (CH4) 16.04 1.75 ppm
Krypton (Kr) 83.80 1 ppm Hydrogen (H2) 2.02 0.5 ppm
Nitrous oxide (N2O) 44.013 0.3 ppm Ozone (O3) 48.00 0-0.1 ppm
Water vapor is present in the atmosphere in varying concentrations from 0 to 5%. Each gas that makes up the atmosphere obeys the ideal gas law:
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Molecular mass for dry air: Md = 28.966 g mol-1
Gas constant for dry air:
Ideal gas law for dry air: Example: Calculate the density from the observed pressure and temperature at the ATOC weather station https://sundowner.colorado.edu/weather/atoc8/ The Moist Atmosphere Why do we care about water in the atmosphere?
- Water exists in all three phases (solid, liquid and vapor) in the atmosphere
- Energy (latent heat) is associated with water phase changes - Unlike most other atmospheric consituents water varies in space and
time - Water is what forms clouds and precipitation
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+
#$" ≈ !"
#
Application of the ideal gas law to a moist atmosphere: Virtual temperature
The ideal gas law for moist air: Virtual temperature (Tv): The temperature dry air would need to have if it were to have the same density as a sample of moist air at the same pressure
% = ,(1 − ) + % " 1 = (1 + 0.61)
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= ρRd ρd ρ
#
$ %
&
p = ρRdTv
Static Stability Air parcel – a small mass of air which we can follow through the atmosphere First law of thermodynamics: %&
%' = #
%( %' − %#
%' or = & −
Adiabatic process – a process in which an air parcel exchanges no heat (Q) with its surroundings (environment) In an adiabatic process dQ = 0 Can T and p change for an adiabatic process? Potential temperature (q) – the temperature an air parcel would have if brought to a reference pressure (p0) (usually 1000 hPa) through an adiabatic process Potential temperature is conserved (i.e. it does not change) if an air parcel does not exchange heat with its environment!!! Derivation of the equation for potential temperature – conservation of heat Combine the ideal gas law and first law of thermodynamics to give:
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Use T=q for p=p0 to find C:
This gives us an equation for potential temperature:
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cp Rd lnθ − ln p0 =
cp Rd lnT − ln p
ln θ cp Rd
Rd cp
Forces acting on an air parcel that is displaced vertically: Start with an air parcel with volume V at height z. Assume that this air parcel has the same temperature as its environment (=Tz). For both the air parcel and the environment at height z:
How does the temperature and potential temperature of the air parcel change as it is lifted adiabatically from height z to height z+dz?
The temperature of the environment at height z+dz is:
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p0
p0
ρ parcel,z+dzV
We can rewrite the expression for this force using the ideal gas law and noting that V of the air parcel and displaced air are equal.
Noting that qparcel,z+dz=qz:
Brunt-Väisälä frequency (N): the frequency at which an air parcel will oscillate if displaced vertically and acted upon by the restoring force arising from the buoyancy of the parcel (also known as the buoyancy frequency) What property of the environment determines the sign and magnitude of the buoyancy force? The vertical gradient of potential temperature 9"'
"( : determines the sign and
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Tz+dz
θz+dz
θz+dz
= −N 2dz
Dry adiabatic process – an adiabatic process in which no phase change of water occurs A dry adiabatic process does not refer to process in which no water vapor is present, only one in which the water vapor that is present does not undergo any phase changes!!!! What happens to an air parcel that is displaced vertically by a dry adiabatic process if:
Static stability classes
Stable Unstable Neutral
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dθ dz < 0
The Continuity Equation Continuity – atmospheric mass is not created or destroyed Divergence – air expands (flows outwards) Convergence – air compresses For an air parcel with volume dxdydz and a spatially varying density r the mass of air is given by:
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+ (ρv)y −∫∫ (ρv)y+dy dxdz
+ (ρw)z −∫∫ (ρw)z+dz dxdy
This can be rewritten to give the flux form of the continuity equation:
The continuity equation is a conservation equation for atmospheric mass. We can also express the continuity equation as:
What do terms (1), (2), and (3) represent physically?
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If in addition density does not vary in space:
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