air pressure nats 101 lecture 14 air pressure. recoil force what is air pressure? pressure =...
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Recoil Force
What is Air Pressure?
Pressure = Force/Area
What is a Force? It’s like a push/shove
In an air filled container, pressure is due to molecules pushing the sides outward by recoiling off them
Air Pressure
Concept applies to an “air parcel” surrounded by more air parcels, but molecules create pressure through rebounding off air molecules in other neighboring parcels
Recoil Force
Air Pressure
At any point, pressure is the same in all directions
But pressure can vary from one point to another point
Recoil Force
Higher density at the same temperature creates higher pressure by more collisions among molecules of average same speed
Higher temperatures at the same density creates higher pressure by collisions amongst faster moving molecules
Ideal Gas Law
• Relation between pressure, temperature and density is quantified by the Ideal Gas Law
P(mb) = constant x d(kg/m3) x T(K)
• Where P is pressure in millibars
• Where d is density in kilograms/(meter)3
• Where T is temperature in Kelvin
Ideal Gas Law
• Ideal Gas Law is complex
P(mb) = constant x d(kg/m3) x T(K)
P(mb) = 2.87 x d(kg/m3) x T(K)
• If you change one variable, the other two will change. It is easiest to understand the concept if one variable is held constant while varying the other two
Ideal Gas Law
P = constant x d x T (constant)With T constant, Ideal Gas Law reduces to
P varies with d Boyle's Law
Denser air has a higher pressure than less dense air at the same temperature
Why? You give the physical reason!
Ideal Gas Law
P = constant x d (constant) x TWith d constant, Ideal Gas Law reduces to
P varies with T Charles's Law
Warmer air has a higher pressure than colder air at the same density
Why? You answer the underlying physics!
Ideal Gas Law
P (constant) = constant x d x T
With P constant, Ideal Gas Law reduces to
T varies with 1/d Colder air is more dense (d big, 1/d small)
than warmer air at the same pressure
Why? Again, you reason the mechanism!
Pressure-Temperature-Density
9.0
km
300 mb
1000 mb
400 mb
500 mb
600 mb
700 mb
800 mb
900 mb
Minneapolis Houston
9.0
km
Pressure
Decreases with height at same rate in air of same temperature
Constant Pressure (Isobaric) Surfaces
Slopes are horizontal
Pressure-Temperature-Density
Pressure (vertical scale highly distorted)
Decreases more rapidly with height in cold air than in warm air
Isobaric surfaces will slope downward toward cold air
Slope increases with height to tropopause, near 300 mb in winter
8.5
km 9.5
km
300 mb
1000 mb
400 mb
500 mb
600 mb
700 mb
800 mb
900 mb
Minneapolis Houston
COLD
WARM
Pressure-Temperature-Density8.
5 km 9.
5 km
300 mb
1000 mb
400 mb
500 mb
600 mb
700 mb
800 mb
900 mb
Minneapolis Houston
HHLL
LLHH
PressureHigher along horizontal
red line in warm air than in cold air
Pressure difference is a non-zero force
Pressure Gradient Force Pressure Gradient Force or PGF (red arrow)or PGF (red arrow)
Air will accelerate from column 2 towards 1
Pressure falls at bottom of column 2, rises at 1
AnimationSFC pressure rises SFC pressure falls
PGF
PGF
COLD
WARM
Summary
• Ideal Gas Law Implies
Pressure decreases more rapidly with height in cold air than in warm air.
• Consequently…..
Horizontal temperature differences lead to horizontal pressure differences!
And horizontal pressure differences lead to air motion…or the wind!
N. Pacific Pressure Analysis (isobars every 4 mb)
Pressure varies by 1 mb per 100 km horizontally or 0.0001 mb per 10 m
2000 km
Review: Pressure-HeightRememberPressure falls very rapidly with height near sea-level
3,000 m 701 mb2,500 m 747 mb2,000 m 795 mb1,500 m 846 mb1,000 m 899 mb500 m 955 mb0 m 1013 mb
1 mb per 10 m height
Consequently………. Vertical pressure changes from differences in station elevation dominate horizontal changes
Station Pressure
Pressure is recorded at stations with different altitudesStation pressure differences reflect altitude differences Wind is forced by horizontal pressure differences Since horizontal pressure variations are 1 mb per 100 km We must adjust station pressures to one standard level:
Mean Sea Level
Ahrens, Fig. 6.7
Reduction to Sea-Level-Pressure
Station pressures are adjusted to Sea Level PressureSea Level Pressure Make altitude correction of 1 mb per 10 m elevation
Ahrens, Fig. 6.7
Correction for TucsonElevation of Tucson AZ is ~800 m
Station pressure at Tucson runs ~930 mb
So SLP for Tucson would be
SLP = 930 mb + (1 mb / 10 m) x 800 m
SLP = 930 mb + 80 mb = 1010 mb
Correction for DenverElevation of Denver CO is ~1600 m
Station pressure at Denver runs ~850 mb
So SLP for Denver would be
SLP = 850 mb + (1 mb / 10 m) x 1600 m
SLP = 850 mb + 160 mb = 1010 mb
Actual pressure corrections take into account temperature and pressure-height variations, but 1 mb / 10 m is a good approximation
Summary
• Because horizontal pressure differences are the force that drives the wind
Station pressures are adjusted to one standard level…Mean Sea Level…to remove the dominating impact of different elevations on pressure change
Key Points• Air Pressure
Force / Area (Recorded with Barometer)• Ideal Gas Law
Relates Temperature, Density and Pressure• Pressure Changes with Height
Decreases More Rapidly in Cold air than Warm • Station Pressure
Reduced to Mean-Sea-Level to Mitigate the Dominate Impact of Altitude on Pressure Change
Summary
• Because horizontal pressure differences are the force that drives the wind
Station pressures are adjusted to one standard level…Mean Sea Level…to mitigate the impact of different elevations on pressure
Local ExampleThe station pressure at PHX is ~977 mb.
The station pressure at TUS is ~932 mb.
Which station has that higher SLP?
Correction for PhoenixElevation of PHX Airport is ~340 m
Station pressure at PHX was ~977 mb
So, SLP for PHX would be
SLP = 977 mb + (1 mb / 10 m) x 340 m
SLP = 977 mb + 34 mb = 1011 mb
Correction for TucsonElevation of TUS Airport is ~800 m
Station pressure at TUS was ~932 mb
So, SLP for TUS would be
SLP = 932 mb + (1 mb / 10 m) x 800 m
SLP = 932 mb + 80 mb = 1012 mb
The SLP was higher in TUS than PHX
Surface Maps
• Pressure reduced to Mean Sea Level is plotted and analyzed for surface maps.Estimated from station pressures
• Actual surface observations for other weather elements (e.g. temperatures, dew points, winds, etc.) are plotted on surface maps.
NCEP/HPC Daily Weather Map
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