* Reading Assignments:

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Reading Assignments
3.4 7.1 7.4 7.5 7.6 7.7 7.8 (7.9, 7.10)
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6. Transformations of Moist Air
6.1 Description of Moist Air
The equation of state for vapor
where
the specific gas constant for vapor
the ratio of molar weights of vapor and dry air
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Absolute humidity (density)
Specific humidity
Mixing ratio
Relation between the mixing ratio and vapor
pressure
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What is the value of specific gas constant
for moist air (a mixture of dry air and
vapor) ?
The equation of state for moist air (a mixture
of dry air and vapor)
The virtual temperature
Specific heats for moist air
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Saturation Properties
An indication of how far the system is from
saturation
Relative humidity
Dew point temperature
The temperature to which the system must
be cooled isobarically to achieve saturation
If the saturation is with respect to ice, this
temperature is called the frost point temperature
Dew point spread
What is the relation between the relative
humidity and dew point spread?
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6.2 Implications for the Distribution of Water
Vapor
Saturation values represent the maximum amount of
water vapor that can be supported by air for a
given temperature and pressure.
Saturation vapor pressure depends exponentially
on temperature.
Condensation in the atmosphere by isobaric
cooling
Radiation fog
Advection fog
Global distribution of water vapor
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6.3 State variables of the two-component system
6.3.1 Unsaturated moist air
Three state variables are required to specify the
moist air.
Pressure, temperature and moisture
Because there is only trace amount of vapor, the
thermodynamic processes for moist air are very
much similar to those for dry air.
The virtual potential temperature
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6.3.2 Saturated moist air
A mixture of dry air and two phases of water
(vapor and condensate)
The chemical equilibrium requires
therefore
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6.4 Thermodynamic behavior accompanying vertical
motion
6.4.1 Condensation and the release of latent
heat
Saturation mixing ratio
where
is a reference saturation mixing ratio
The saturation mixing ratio increases with
decreasing pressure, but decreases with
decreasing temperature.
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Lifting Condensation Level (LCL) As an air
parcel is lifted it can cool until condensation
begins at the cloud base or LCL.
Below LCL
Above LCL
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6.4.2 The Pseudo-adiabatic process
The pseudo-adiabatic process is nearly identical
to a reversible saturated adiabatic process,
i.e., an isentropic process.
The condensate does not precipitate out.
The heat transfer with the environment is
negligible.
The equivalent potential temperature is defined as
which is constant during the pseudo-adiabatic
process.
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6.4.3 The saturated adiabatic lapse rate
The saturated parcels temperature decreases with
height is defined as the saturated adiabatic
lapse rate, i.e.,
Because of the release of latent heat,
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On a winter day, the outside air has a
temperature of -15oC and a relative humidity of
70. a) If outside air is brought inside and
heated to room temperature of 20oC without
adding moisture, what is the new relative
humidity? b) If the room volume is 60 m3, what
mass of water must be added to the air by a
humidifier to raise the relative humidity to 40?
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Meteorology 341 Homework (5)
1. Problem
3.10 2. Problem 7.9 3. Problem 7.13 4. Problem
7.14
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6.5 The Pseudo-Adiabatic Chart
The pseudo-adiabatic chart is one of many
different thermodynamic charts, and is used to
describe the adiabatic process in the atmosphere.
Adiabats
(dark solid lines)
For the dry and unsaturated conditions,
temperature changes with height at the rate of
Pseudo-adiabats
(dashed lines)
For the saturated conditions, temperature varies
with height at the rate of
(thin solid lines)
Isopleths of saturation mixing ratio
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Meteorology 341 Homework (6)
1. Problem
7.10 2. Problem 7.11 3. Warm moist air leaves
an array of cooling towers at a power plant
situated at 825 mb. Acloud forms directly
overhead. If the ambient temperature profile is
isothermal, with T5oC and if the initial
temperature and mixing ratio of air leaving the
towers are 30oC and 25 g kg-1, respectively,
determine a) the relative humidity, and the
virtual potential temperature immediately above
the towers b) the pressure at the cloud base, c)
the equivalent potential temperature at 800 mb,
d) the mixing ratio at 700 mb, and e) the
pressure at the cloud top. Use the attached chart
Figure 5.6.
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