Ch 6
Sec A
Ch 6
Atmospheric Moisture
Sec A
• States of Matter
• Other Changes of State
– Gas
– Solid
– Liquid
– Plasma (Not used in most Meteorology)
– Condensation: Gas to Liquid
Water Vapor to Liquid Water
– Deposition: Directly from Vapor to Solid
For example: Whoar Frost
– Sublimation: Directly from Solid to Vapor
• Change of State: State Transitions
• Water added to Atmosphere by
– Evaporation
– Sublination:
Ch 6
Sec A
Atmospheric Moisture
Liquid to Gas
Solid to Gas
1
2
Ch 6
Atmospheric Moisture
Sec A
• Latent Heat: That heat contained in a parcel
due to its state of matter.
• Release of Latent Heat when:
– Water Goes from Vapor to Liquid: The surrounding
vapor will get hotter
– Water Goes from Liquid to Solid: The surrounding
liquid will get hotter.
• When Latent Heat is Released something is
heated
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Atmospheric Moisture
• Absorbtion of Heat into Latent Heat of a Substance:
– Evaporation or Vaporization from liquid to vapor: the liquid
looses heat and becomes colder Example 1. A
Carburetor: liquid gas becomes a vapor. Not only cools the
liquid gas but the surrounding air and if it has moisture in it
can produce carburetor icing.
Example 2. A swamp cooler: In dry hot areas water flows
over a coil evaporating and taking heat from the coil cooling
it.
– Melting: The liquid water will absorb heat from the ice and
surrounding air to make them colder.
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1
Ch 6
Sec A
Atmospheric Moisture:
Pressure
Vapor
Sec A
• Air is a mixture of gasses. The total pressure exerted
by the gas is simple the PRESSURE
• The part of the pressure that is exerted by each
different gas in the air is called its partial pressure.
• So the pressure of the Air is composed of the:
The Partial Pressure of Nitrogen: about 70% of P
The Partial Pressure of Oxygen: about 21% of P
The Partial Pressure of Argon: about 1% of P
The Partial Pressure of H2O
between .5% and 4% of P
– Etc.
–
–
–
–
Ch 6
Sec A
Atmospheric Moisture:
Pressure
Ch 6
Atmospheric Moisture:
Pressure
• The partial pressure of H2O in Air is called its
Vapor Pressure
• Vapor Pressure of water in air can range from
0% to 4% of P, the Air Pressure.
• Saturation occurs when the amount of water
entering the air from the surface is the same as
the amount of water entering the air from the
surface.
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Vapor
• Saturation occurs when the amount of water
entering the air from the surface is the same as
the amount of water entering the air from the
surface. When this occurs the air is said to be
Saturated.
• Saturation Vapor Pressure: When the air is
saturated the Vapor Pressure of water is called
the Saturation Vapor Pressure SVP.
• At Sea Level and SA conditions SVP is about
7
0.5” of the 29.92” inches Standard Pressure
Vapor
6
Ch 6
Sec A
Atmospheric Moisture:
Pressure
Vapor
SAT
8
2
Ch 6
Sec A
Atmospheric Moisture:
Pressure
Vapor
Ch 6
Sec A
• Percentage of water required for saturation is
dependent on Temperature:
Atmospheric Moisture:
Relative Humidity
• Relative Humidity is a percentage but NOT the
percentage of water in the air!!!
• Relative Humidity is
(Current VP of water in air)
Saturated VP of water at current Temperature
• RH is usually highest at time of lowest temp.
• RH is usually lowest at time of highest temp.
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9
Ch 6
Sec A
Atmospheric Moisture:
Dewpoint Temperature
Ch 6
Sec A
• Dewpoint is the temperature at which
condensation first occurs when the air is cooled
at constant pressure.
• If the dewpoint is below freezing it is called the
frost point.
• Approximate Formulas not to be memorized:
• RH = 100 – 5(T-Tdp) for RH > 50% (Celsius)
(Approximately +/- 1 degree C)
(Celsius)
• Tdewpt = T -(100 – RH)/5
11
Atmospheric Moisture: Clouds
• Clouds are billions of droplets of water
• Things needed for cloud formation
–Water Vapor
–Condensation Nuclei (dust etc)
–Cooling from
• Contact with a cold surface(Nuclei)
• Adiabatic Expansion
12
3
Ch 6
Sec A
Atmospheric Moisture: Clouds
• Clouds are billions of droplets of water
• Fog: a cloud near the Earth’s surface
–Cooling of the surface occurs via
radiation from the surface
(usually at night)
–The surface cools the air by conduction
–This is called Radiation Fog
Ch 6
Sec A
Atmospheric Moisture: Clouds
• When moist air moves over cold
surface we get advective fog
• Advective means to move
(usually horizontally)
• For example: moist air moving North
along the California coast over the
California current make Advective Fog
13
Ch 6
Sec A
Atmospheric Moisture: Clouds
• Upslope fog: when moist air is
transported by wind up the windward
side of a hill
• Contrails: Burning Jet Fuel produces
water which is cooled to form
streaks of clouds behind the airplane.
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14
Ch 6
Sec A
Atmospheric Moisture: Clouds
• Latent Heat and Stability
• Rising Moist Air May Condense
• Rate of cooling of moist air is: SALR
Saturated Adiabatic Lapse Rate
• Not to be confused with ISALR:
International Standard ATM LR
• SALR is variable not fixed like
– DALR 3 degrees / 1000ft
– ISALR 2 degrees / 1000ft
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4
Ch 6
Lab 9 Find RH
Lab 9 Find RH
Ch 6
Sec A
Sec A
• Approx: RH = 100 – 5(T-Tdp) for RH > 50% (Celsius)
• Simplifying equation for RH
• But exp(a-b) = exp(a) * exp(-b)
• More exact equation: RH = 100 * e(Tdp)/e(oat)
where:
• e(Tdp) is saturated vapor pressure of water at Tdp
• e(OaT) is saturated vapor pressure of water at OaT
• So e(T)= 6.11 *exp( 5417/T0)*exp(-5417/T)
Remember RH = Current Vapor Pressure given by Tdp
Vapor pressure at Saturation at OAT
(Outside Air Temp)
•
e(T)= 6.11 *exp( 5417*(1/T0 – 1/T))
where
T0=273.15 and T is the Temperature in Kelvin.
!!!
= e(Tdp) / e(OAT)
T is Kelvin Temperature !!!
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Ch 6
Lab 9 Find RH
18
Ch 6
Lab 9 Find RH
Sec A
Sec A
• Simplifying equation for RH
• Columns should be KSTA, Tc, Tdpc, Tk, Tdpk,
e(OaT), e(Tdp), RH, RH NWS,
• So e(T)= 6.11 *exp( 5417/T0)*exp(-5417/T)
RH =
100*Current Vapor Pressure given by Tdp
Vapor pressure at Saturation at OaT
RH = 100*e(Tdp) / e(OAT)
RH = 100*6.11 *exp( 5417/T0)*exp(-5417/Tdp)
6.11 *exp( 5417/T0)*exp(-5417/OaT)
Canceling RH=100*exp(5417*(1/OaT - 1/Tdp))
(All Temperatures are in Kelvin)
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RH_Approximation (C=Celsius Tc, K=Kelvin T)
• For
KABQ,KPHX,KDEN,KRDD,KSFO,KSLC,KPWM
• Use Metars and RMK T field
• Print with Lines: see print preview
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