G109:
10. Pressure and Forces
10. PRESSURE AND FORCES
A&B: Chapter 4 (p 93-114)
Review: Atmosphere section, Pressure sub-section
Pressure: varies in space and time
⇒ spatial & temporal patterns
⇒ need a way to depict variations
⇒ two types of charts used to depict horizontal
pressure variation
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10. Pressure and Forces
1. Constant Height -• most common pressure at the surface
• Altitude corrections: all corrected to one level e.g. sea
level
Prevents mountains appearing to have lower pressure
because of elevation :
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2. Constant Pressure - show variation along actual
pressure surface.
Isobaric surface
--Spatial variation
of the height
(altitude) along a
pressure surface
Pressure is equal
all along this
surface
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10. Pressure and Forces
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Horizontal Pressure Gradients
• when surface under one air column is heated:
o air column expands, following
P = ρ Rd T
o example: height at which 500 mb is reached: 5500 m,
say (before heating)
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o after heating and expansion: height at which 500 mb
pressure is reached is now higher up, at 5600 m, say
(this is like a hill of air above the heated spot)
o at 5500 m the pressure is now less than 500 mb.
o Gradual poleward decrease in mean temperature
o Denser air at higher latitude
o More rapid decrease of pressure with height
G109:
10. Pressure and Forces
Result:
o at a given height in the atmosphere, pressure varies
horizontally
(if no air is flowing away or into the air column, the surface
pressure does not vary)
o with a horizontal pressure gradient created in this way,
air can start to flow from higher pressure to lower
pressure
o the weight of the entire air column (surface pressure) is
changed as well, because of the air flow
o there are now horizontal pressure gradients at the
surface and at height. Give rise to pressure gradient
force.
⇒ Pressure differences cause WINDS.
⇒ Pressure differences occur because of T differences
which cause ρ differences
Representation of Pressure Distribution:
Two ways:
• isobars (= pressure variation at constant height)
• contours of geopotential (= height variation of given
pressure level)
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Newton’s II Law of Motion:
Acceleration of an air parcel is equal to the net force
action on it and the mass.
net force
acceleration =
mass
r
r Δv ∑ Fi
=
a=
Δt
m
net Force = mass * acceleration
• acceleration: change of velocity over (unit) time
ƒ speed and/or direction change
• net force: vector sum of all component forces
• forces in the atmosphere (most important ones):
o Pressure Gradient Force (FPG) :
driving force – affects speed and direction
o Coriolis Force : (Fc)
deflecting force – affects only direction
o Friction Force : (Ff)
retarding force – affects only speed (friction
increases with increasing wind speed)
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10. Pressure and Forces
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Pressure Gradient Force (FPG)
• Force resulting from the horizontal difference in pressure
To get anything to accelerate - need a net unbalanced force
in one direction
• proportional to the horizontal pressure gradient: depends
on the change in pressure and the distance
net
H
PGF =
force
L
1004
1000
ΔP change in PRESSURE
=
Gradient
d
DISTANCE
• the closer the isobars , the stronger the pressure gradient,
and thus the FPG
1004
1000
1004
1000
• FPG goes from higher pressure towards lower pressure
o At right angles to Isobars
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10. Pressure and Forces
• can set a stationary air parcel in motion
• Mostly responsible for the magnitude of the wind
If PGF was the only force - H ⇒ L in a straight line
• air would move at right angles to isobars
But the earth rotates ⇒ causes deflection of the air
• Coriolis effect
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10. Pressure and Forces
Coriolis Force – Coriolis Effect (see CD!!)
• Apparent deflection in the direction of the wind as a
result of the earth’s rotation.
• an un-accelerating object moves in a straight
line..Balanced forces
• Because of Earth’s rotation, a “straight line motion” on
Earth as viewed from (e.g.) another planet, leaves a
curved trace on Earth: the motion appears to be
accelerated (curved!)
• Apparent acceleration accounted for by apparent force:
Coriolis force FC
• Northern Hemisphere: (ccw rotation) deflection to right
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• Southern Hemisphere: (clockwise rotation) deflection to
left
• FC (on Earth) dependent on:
o latitude: strongest twisting motion at poles, no
twisting motion at the equator. Strength FC increases
with latitude from equator to poles
o velocity: proportional to velocity. FC increases with
wind speed (m s-2)… the greater the distance
traveled per unit time the greater the deflection
Coriolis force
o always at right angles to the direction of air flow
o affects only the wind direction not wind speed
o affected by wind speed.
o Stronger the wind speed the greater the force
o Strongest at the poles and weakest at the equator
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10. Pressure and Forces
FC= 2 ν Ω sin φ
ν - wind speed
Ω - earth's angular rate of spin (constant )
φ - latitude
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Geostrophic wind
-
wind aloft
evenly spaced isobars
coriolis force balances the pressure gradient flow
Ideal model (rearly case in real world but good
approximation of winds aloft)
NH
PGF = FC
net force = 0
Wind flows in a straight path parallel to the isobars at a
constant speed (no acceleration) proportional to the
pressure gradient force. (steep gradient strong winds.-weak gradient light winds)
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10. Pressure and Forces
Gradient wind
⇒ above the level of frictional influence.
⇒ wind that blows at a constant speed parallel to curved
isobars –
NH
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In upper atmosphere pressure height variations are
distributed as :
Ridge:
Trough:
Greatest surface instability (thunderstorms) is usually immediately
ahead of (to the right of) the 500 mb trough. –see current upper air
map-Friction – surface effect within first few km
Roughness of the surface retards the airflow
⇒ wind speed reduced
• this impacts Fc (lower wind speed less deflection)
• Pressure gradient force not affected →dominates
Angle wind crosses isobars depends on friction
Smooth ocean air only at slight angle 10-20°
Rough terrain - greatest 45°
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Anticyclones
• enclosed areas of H --circular isobars or height contours
NH
• winds rotate clockwise as Fc deflects to right and FPG
directs it outward
SH
• winds rotate counter clockwise as Fc deflects to left
and FPG directs it outward
Divergence
Convergence
Cyclone
• enclosed areas of L -circular isobars or height contours
NH
• winds rotate counter clockwise as Fc deflects to right
and FPG directs it inward
SH
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10. Pressure and Forces
• winds rotate clockwise as Fc deflects to left and FPG
directs it inward
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