Composition and structure of Earth’s Atmosphereiceage.umeqs.maine.edu › ERS323 › pdf ›...
Transcript of Composition and structure of Earth’s Atmosphereiceage.umeqs.maine.edu › ERS323 › pdf ›...
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Composition and structure of Earth’s Atmosphere
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the atmosphere
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composition of the atmosphere
400.0 0.040
0.00018 1.8
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atmospheric CO2 concentrations at
Mauna Loa, Hawaii
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Earth-Sun Relationships
• Energy received from the Sun drives weather and climate, so it is obvious to start with changes associated with the Sun and our orbit around the Sun
• The regular changes of Earth’s orbit around the Sun, and Earth’s rotation about its tilted axis, control seasonal and diurnal cycles and dominate the climatology of Earth
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indirect rays
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Earth’s surface, and lower atmosphere,
are unevenly heated
low latitudes receive more energy per unit
area than higher latitudes
distribution of incoming solar radiation
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heat budget
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caused by Earth’s orientation-inclination of the axis seasons
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Length of day
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length of day
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daily variations in air temperature
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heat (energy, W/m2) temperature (average energy, oC)
humidity (water vapor, g/kg) pressure (force/area, mb = 100 N/m2)
winds (circulation, m/s) cloud cover (liquid/solid water)
precipitation rate (mm/day)
Environmental Variables
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Figure 1.1
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temperature
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Figure 1.2
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seasonal temperature differences
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Figure 1.3
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structure of the atmosphere
temperature vs. height
in the troposphere temperature decreases with height
the average lapse rate is about 5-9 degrees C per kilometer depending on the humidity
in the stratosphere temperature increases with height
in the stratosphere ozone absorbs incoming ultraviolet radiation
80% of the mass of Earth’s atmosphere is in the troposphere,
we all live in the troposphere
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ozone
ozone layer maximum at 20-25 km
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tropopause height varies with latitude
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Figure 1.6
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atmospheric pressure pressure is a force per unit area exerted by the weight of air above
-- about 1 kg/cm2 or 14.7 lb/in2 at the surface of Earth (equivalent to a 10 meter column of water)
units of pressure are N/m2 where N = Newton = force required to accelerate a 1 kg mass 1 m/s2
The SI unit for pressure is the Pascal (Pa) 1 Pa = 1 N/m2
a standard atmosphere (and average typical of a mid latitude location) exerts a pressure of 101,325 Pa at sea level
the unit adopted by the National Weather Service is the millibar (mb) 1 mb = 100 Pa
--> standard sea level pressure = 1013.25 mb
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factors affecting atmospheric pressure
1. gas molecules are able to fill space available
2. gas molecules bounce off one another when they collide (and off the wall if in a container)
3. the atmosphere is bounded above (gravity) and below (ground)
Air pressure is the force per unit area exerted against a surface by continuous collision of gas molecules
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pressure is partly determined by temperature consider a change in temperature while holding density (volume) constant
temperature increase --> speed of molecules increases (force increases, higher pressure)
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pressure is partly determined by density
Density (# molecules/volume) increase --> increase in # of collisions (higher pressure)
consider a change in density (volume) while holding temperature constant
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ideal gas law Boyle's Law - at constant temperature, the volume of gas varies inversely with pressure
p1V1 = p2V2
Charles' Law - at constant pressure, the volume of a given mass is directly proportional to absolute temperature--> increase in temperature results in increase in volume
V1 / V2 = T1 / T2
Combine these two laws to obtain ideal gas law, or equation of state
p = ρ R T
where p = pressure, ρ = density, R = gas constant, and T = temperature
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pressure changes with altitude
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Figure 1.9
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pressure and density vs. height
pressure at sea level is around 1000 millibars
pressure decreases with height exponentially
50% of the mass of Earth’s atmosphere is below an
altitude of 5-6 km (the 500 millibar height)
pressure is a force per unit area
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Figure 1.7
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reducing pressure to sea level
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sea level pressure on Earth
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seasonal pressure and wind patterns
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Figure 1.15
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pressure changes with temperature
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pressure changes with temperature
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hydrostatic equilibrium
concept: – the vertical pressure gradient force is equal and opposite to the gravitational force
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hydrostatic equation
Where: Δp represents change in pressure, Δz, change in altitude, ρ, air density, and g, acceleration of gravity
(Δp / Δz) = - ρg
the rate at which pressure decreases with height equals the product of air density times the acceleration of gravity
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hydrostatic equation examples
cold column: ρ = 1.3 kg/m3
(Δp / Δz) = - 1.3 (9.8) = -12.8 Pa/m
thus:
pressure declines more rapidly in a cold, dense air column than in a warm air column
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water’s changes of state
Water’s Changes of State
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amount of water vapor in air
1. absolute humidity = mass of water vapor per volume of air (g/m3)
2. mixing ratio = mass of water vapor in a unit mass of dry air (g/kg)
3. relative humidity = ratio of air's water vapor content to its capacity
4. dewpoint temperature is the temperature at which air is saturated
(100% relative humidity)
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humidity
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relative humidity
changes with added moisture
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changes with temperature
relative humidity
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saturation mixing-ratio
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Figure 1.11
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Which has more Moisture?
Which has higher Relative Humidity?
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Higher Temperature Lower Relative Humidity with MORE Moisture!
Lower Temperature Higher Relative Humidity
with LESS moisture!
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diurnal changes in relative humidity
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cloud cover