#### Transcript Ch 2 WeatherAndHydrology CE 424

```Air parcels
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A parcel is a “blob” of air
Small enough to have only one value of T, p, ρ,
etc.
Large enough to contain a significant number of
molecules. (Are there enough particles to talk
about temperature as average kinetic energy, for
example?)
Lapse Rates
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Parcel lapse rate – the rate at which temperature
changes as the parcel is lifted to a higher
altitude
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Environmental lapse rate – the rate at which the
air surrounding the parcel changes as altitude
increases
Rate
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An adiabatic process is one during which no
heat is exchanged between the substance in
question and its surroundings
Many atmospheric motions occur rapidly enough
that parcels do not exchange a significant
amount of heat with the environment
Examples:
•
•
rising air in a thunderstorm
Air rising over a topographic barrier (like a mountain)
Rate
The adiabatic lapse rate for DRY air on Earth is
Γd = g/cp
Γd = 9.81 m s-2 / 1004 J kg -1 C-1
Γd = 0.00977 C m-1
Γd = 9.77 C km-1
Rate
This means that a rising(sinking) air parcel will
cool(warm) at a rate of about 10 oC per km of
ascent(descent) unless:
•
•
•
It exhanges significant mass or heat with the
environment
It becomes saturated with respect to water vapor
It rises(sinks) so slowly that radiation heat transfer is
possible
Humidity,
Condensation and
Clouds
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Circulation of water in the atmosphere
Evaporation, condensation and saturation
Humidity
Dew and frost
Fog
Clouds
Circulation of Water in the
Atmosphere
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evaporation
condensation
precipitation
hydrologic cycle
• The total amount of water vapor stored in the
atmosphere amounts to only one week’s supply of
precipitation for the planet.
Stepped Art
Fig. 4-1, p. 80
Evaporation, Condensation
and Saturation
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saturation
condensation nuclei
• In very clean air, about 10,000 condensation nuclei
are typically found in one cubic centimeter of air,
a volume approximately the size of your fingertip.
Humidity
Mixing Ratio (w)
The ratio of the mass of water vapor in air to the mass of
dry air:
w = mv / md
 Usually expressed in g kg-1
 Some typical values:
• Tropical marine boundary layer air: w ≈ 18 g kg-1
• Polar air: w ≈ 1 g kg-1
• Stratospheric air: w ≈ 0.1 g kg-1
Specific Humidity
The ratio of the mass of water vapor in air to the total mass
of the air (dry air plus water vapor):
SH = mv / (md + mv)
w = SH / (1 – SH)
SH = w / (1 + w)
Vapor Pressure
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actual vapor pressure
saturation vapor pressure
• “Saturation” describes a condition of equilibrium:
liquid water is evaporating at exactly the same rate that
water vapor is condensing.
Vapor Pressure
Saturation vapor pressure depends only on temperature…
Formula:
 L  1 1 
es es0 exp    
 Rv  T0 T 
e s : Saturation vapor pressure
e s0 : Saturation vapor pressure at
273 K = 6.11 mb
L : Latent heat of vaporization =
2.5x10 J kg
Rv : Gas constant for water vapor
= 461 J kg K
T0 : 273 K
T : Temperature
6
-1
-1
-1
Vapor Pressure
Saturation vapor pressure depends only on temperature…
Formula:

 1
1 
es  6.11 exp 5423
 
273 T 


Vapor Pressure
Saturation vapor pressure depends only on temperature…
Graph:
Relative Humidity
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definition of relative humidity
saturation and supersaturation
condensation
relative humidity and temperature
• When the general public uses the term “humidity”,
they mean “relative humidity.”
Relative Humidity
The ratio of the actual vapor pressure to the
saturation vapor pressure.
f = e / es
Since es depends on temperature, the relative
humidity measures closeness to saturation, not
actual water vapor content.
Fig. 4-5, p. 83
Fig. 4-7, p. 85
Relative Humidity and Dew
Point
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dew point temperature: the temperature to
which air must be lowered to reach 100%
relative humidity
dew point depression and relative humidity
• The dew point temperature is useful for forecasting heat
index, precipitation probabilities, and the chance of frost.
Measuring Humidity
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psychrometers
hygrometers
Topography and Clouds
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orographic uplift
• The rain shadow works for snow too. Due to frequent
westerly winds, the western slope of the Rocky Mountains
receives much more precipitation than the eastern slope.
Collision and Coalescence
Process
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terminal velocity
coalescence
warm clouds
• A typical cloud droplet
falls at a rate of 1
centimeter per second.
At this rate it would take
46 hours to fall one mile.
Instruments
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standard rain gauge
tipping bucket rain gauge
• It is difficult to capture rain in a bucket when the
wind is blowing strongly.