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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
tropopause height varies with latitude
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pressure 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
Higher Temperature Lower Relative Humidity with MORE Moisture!
Lower Temperature Higher Relative Humidity
with LESS moisture!
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
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
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)
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
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
pressure 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