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MET 61 Introduction to Meteorology - Lecture 2
“The atmosphere (II)”
Dr. Eugene CorderoSan Jose State University
W&H Chapter 1
Class Outline:
ThermodynamicsHydrostatic balanceHypsometric equation
Functions of the AtmosphereFunctions of the Atmosphere
What is the structure of the atmosphere?What is the structure of the atmosphere?
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Discussion QuestionsDiscussion Questions• What is the weight of the atmosphere?
• What is the altitude where 99% of the atmosphere exists below?
• What is the approximate temperature of the atmosphere at 5 km above the surface?
• How is the atmosphere different on the top of Mt. Everest compared to San Jose in terms of – Composition– Density– Pressure
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Principal Atmospheric Gases
GAS MASS
Nitrogen 3870 (78%)
Oxygen 1185 (21%)
Argon 67 (1%)
Water Vapor 17 (0.3%)
CO2 3 (0.06%)
TOTAL 5140
Unit: 1018 g
Permanent gases
Variablegases
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ThermodynamicsThermodynamics
• Definition: The study of heat and its transformation into mechanical energy.
• Definition: The study of the processes that involve the transformation of heat into mechanical work, of mechanical work into heat, or the flow of heat from a hotter body to a colder body.
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ThermodynamicsThermodynamics
• The thermodynamic state of the atmosphere can be described by three variables:– ________________– ________________– ________________
Pressure
Density
Temperature
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• Environmental lapse rate, .
– From radiosonde (sounding)
• Dry adiabatic lapse rate, d
– Constant (9.8°/1000m)
• Moist adiabatic lapse rate, s
– Varies with temperature/moisture
– 1°/1000m - 9°/1000m; typical = 6°/1000m
Lapse Rate
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PressurePressure
Measured in Measured in
millibars (mb) or millibars (mb) or
hPa (hecto pascals)hPa (hecto pascals)
inches of mercuryinches of mercury
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Air PressureAir Pressure• The weight of the air about us.
• Pressure equals – a force over a given area (P = F/A)
• Weight is the force exerted by gravity (g) on a unit mass
• The average pressure at sea level is about ~• ~ 14.70 pounds force per sq inch• 1013 mb; 101.3 kPa or 1013 hPa• ~ 29.9 in Hg
Pa – N/m2 (N-Newton is Kg m/s2)
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Pressure measured Pressure measured via a:via a:Barometer Barometer
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Pressure (I)Pressure (I)• What does pressure represent physically?
• How does atmospheric pressure change
with height?
pHzepp 0
– p0 is average sea level pressure
– Hp is the pressure scale height (~ 7km)
– Assumption is that temperature is
constant within that layer.
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Scale HeightScale Height• The scale height is proportional to the
average temperature in a layer:
0g
TRH
• R- gas constant for dry air (287 J K-1kg-1)• g – gravitational acceleration (9.8 m s-2)
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Question 1Question 1Question:
• What is the pressure at 10km?
• At what altitude is the pressure 50% of the
surface value?
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Atmospheric Pressure and Atmospheric Pressure and DensityDensity
• Density and Pressure related…
• Density refers to number of air molecules per volume– Units are given in Kg/m3
• Pressure refers to the ‘weight’ of the atmosphere above.
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DensityDensity• What does density represent physically?
• How does atmospheric density change with height?
ρHz0eρρ
0 is average sea level pressure
– H is the density scale height (~8km)
– Assuming temperature constant
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TemperatureTemperature• What does temperature represent
physically?
• How does atmospheric temperature change with height?
2vamT w– a is a constant
– mw is the molecular weight
– v is the average molecular speed
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Equation of StateEquation of State• Ideal Gas Law; relates the thermodynamic
states of a gas
ρRTP
For moist air one can use the virtual
temperature; Tv=T(1+0.61r)
r- water vapor mixing ratio
Virtual temperatures allows for the use of R
for dry air in ideal gas law.
p-Pressure (Pa)p-Pressure (Pa)
- density (kg m- density (kg m-3-3))
R - Gas Constant for dry air R - Gas Constant for dry air (287 J K(287 J K-1-1 kg kg-1-1))
T - Temperature (K)T - Temperature (K)
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Hydrostatic Balance (I)Hydrostatic Balance (I)• Pressure gradient: the change in
pressure with distance.
• Horizontal variations in air pressure are much less than the vertical ones.
• However, horizontal pressure differences drive our weather, while vertical pressure changes don’t exert as much influence
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Hydrostatic Balance (I)Hydrostatic Balance (I)• Pressure decrease produces a vertical pressure gradient force.
• Vertical pressure gradient force is directed __________.
• Vertical pressure gradient force is balanced by gravity.
ρgdz
dp
p+p
p
z
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Hydrostatic Balance (II)Hydrostatic Balance (II)• Hydro – fluids• Static – balance
• Atmosphere is nearly always in hydrostatic balance.
• Exceptions are in severe weather conditions - “non-hydrostatic”
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Hypsometric EquationHypsometric Equation• Combination of ideal gas law with hydrostatic balance.
• Relates atmospheric thickness with average temperature.
• Thickness of atmosphere relates to difference between two atmospheric layers; z t (m) = thickness between two pressure levels
2
112 lnZ
p
pT
g
RZ d
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Question 2Question 2
• What is the thickness between 500hPa and 200hPa if the average temperature of the layer is 10F?
• Compare the average thickness at latitudes between 60N-70N with the thickness between 20N-30N.
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Principle forcesPrinciple forces
Q: What drives the weather in the atmosphere?Q: What drives the weather in the atmosphere?
Q: What causes the weather to change?Q: What causes the weather to change?
We will develop a foundation for answering these questions We will develop a foundation for answering these questions through understanding the predominant forces in the through understanding the predominant forces in the atmosphere, namely:atmosphere, namely:
• pressure and pressure gradients, pressure and pressure gradients, • gravitygravity• rotation of the earthrotation of the earth• friction friction
TodayToday
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Pressure ChangesPressure Changes
• Caused byCaused by– Temperature or density changesTemperature or density changes
• Determines the direction and speed of windsDetermines the direction and speed of winds
• Can help explain general circulation of atmosphere.Can help explain general circulation of atmosphere.
• General guidelines:General guidelines:– High pressure: clear skies/fine weatherHigh pressure: clear skies/fine weather– Low pressure: cloudy skies/changeable weatherLow pressure: cloudy skies/changeable weather
Why is this?Why is this?
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Pressure (II)Pressure (II)• Pressure can be used as a vertical coordinate.
Convenient as many instruments have pressure sensors
• Example questions:
Compare the pressures at 5 km above sea level for average layer temperatures of 25C and 35C.