PRESSURE & WIND,
GENERAL CIRCULATION,
JET STREAMS
FORCES that move AIR:
Winds Aloft:
Surface Winds:
(top of troposphere)
1. Gravity
1. Gravity
2. Pressure gradient
2. Pressure gradient
3. Coriolis effect
3. Coriolis effect
4. Friction
1. Gravity
Earth exerts gravitational force on atmosphere.
(This causes pressure and density to be greater closer to
earth.)
Acceleration due to gravity = 9.8 m/sec2
2. Pressure Gradient
a)
Vertical
(Remember hydrostatic equilibrium)
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surface
2. Pressure Gradient
b) Horizontal (wind)
Pressure Gradient Force (PGF)
1005 1000 995 990 985
PGF is perpendicular to isobars.
high wind speed.
low wind speed.
Wind speed determined by steepness of gradient.
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Current weather map
3. Coriolis Effect / Force
Apparent deflection of moving object due to rotation of earth.

Animation

Animation
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Animation
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Animation
Deflection…
1. Is to the right of the path of motion in the northern
hemisphere and to the left of the path of motion
in the southern hemisphere.
2. Increases with latitude:
maximum at poles; zero at equator
Plane of deflecting force is parallel to earth’s
surface at poles; no component of deflection
parallel to surface at equator.
Deflection...
3. Increases with wind speed.
4. Increases with mass of object.
CE is perpendicular to path of motion.
If PG and CE were only forces on atmosphere, wind would blow
parallel to isobars.
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4. Friction

Surface provides friction to atmospheric movement;
“slows down” the air.
Minimal friction aloft
> 3000 ft in troposphere
“friction layer” : 0 – 3000 ft
Winds aloft blow parallel to isobars:
geostrophic wind
geostrophic balance
“balance between pressure gradient and Coriolis forces acting on a parcel
so that the forces are equal in magnitude but in opposite directions”
GEOSTROPHIC WIND
Northern Hemisphere
L
H
Around and clockwise
Around and counterclockwise
Southern Hemisphere
L
H
Around and counterclockwise
Around and clockwise

How do surface winds differ from these upper tropospheric
winds?
Friction and Surface Winds
Drag produced by surface.
Frictional force is applied opposite to direction of air
motion; causes wind to blow across the isobars.
At surface, friction reduces wind speed, which reduces Coriolis effect.
Coriolis can not balance PGF so wind crosses isobars.
Southern hemisphere
PG
CE
HIGH
Resulting wind direction:
Southern hemisphere
PG
Out and
counterclockwise
HIGH
FRIC
CE
S. hem, HIGH
HIGH
Southern hemisphere
PG
CE
LOW
Southern
hemisphere
CE
PG
In and
clockwise
LOW
S. hem, LOW
LOW
Northern hem, HIGH
Out and
clockwise
HIGH
Northern hem, LOW
In and
counterclockwise
LOW
General
Circulation
Global Wind Systems
driven by Highs
and Lows at surface
Where are Highs and
Lows?
Imagine the earth with no
rotation
HIGH
There would be a single
cell of convection in each
hemisphere
LOW
But the earth rotates
Coriolis deflection
causes air to be deflected
from those simple
convective pathways
Creating 3 cells in each
hemisphere and a surface
High Pressure in subtropics
Let’s look at SURFACE
Components of each cell
Hadley Cells
Strong and persistent
Warm air rising at
Intertropical Convergence
Zone (ITCZ)
At top of troposphere,
spreads poleward, sinks at
Subtropical Highs
Blows towards ITCZ at
Surface, creating…
Trade Winds
Between subtropical
Highs and ITCZ
NE in N. Hem
SE in S. Hem
Ferrel cells
Not as strong,
persistent, welldefined
Westerlies
(surface component of Ferrel cells)
35o - 60o N & S
 not
steady or
persistent
Polar Front Zone
60o - 65o N & S
zone of conflict
between differing
air masses
Polar Easterlies
65o - 80o N & S
 more
prevalent in
Southern,
variability in
Northern
Distribution of land masses disturbs this idealized
system of Highs, Lows, winds
Why?
Uneven heating of land and water creates temperature differences
and therefore pressure differences over land vs water with seasonal changes
Canadian High
Siberian High
Icelandic Low
Aleutian Low
Azores Bermuda High
Pacific High
Pacific High
Azores Bermuda High
Monsoonal Low
Upper Air
Movement
500
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750
875
1000
City
HEAT
City
COOL
Isobaric
surfaces
DECREASED DENSITY
INCREASED DENSITY
It takes a shorter column of
cold air to exert the same
surface pressure as a tall
column of warm air.
500
625
500
625
750
750
875
875
1000
1000
HEAT
COOL
2300 meters
500
Constant Pressure Map
(isobaric maps)
625
750
1100 meters
500
625
850
750
850
1000
1000
Constant pressure map
shows elevation of a
certain pressure.
Low heights and troughs
represent cold air.
High heights and ridges
represent warm air.
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5400
5580
5640
5700
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Currently:
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Current surface temperature map
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Current map of heights of 500 mb layer
Constant Altitude Map

Shows pressure at a given altitude
500
625
550 mb
750
810mb
1000m
1000m
500
625
850
750
850
1000
1000
1000m
On a constant altitude
map:
low pressures indicate Cold Air
high pressures indicate Warm Air

High heights on a constant pressure
surface map are equivalent to high
pressures on a constant altitude map
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Low 500 mb heights are associated with
low pressure at any given altitude;
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High 500 mb heights are associated with
high pressure at any given altitude.
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Therefore, high and low heights tell you where
high and low pressures are (for a given altitude)
Upper Level Winds

Westerly

in mid- and high latitudes
(20°-90° N & S)

Easterly

in Tropics (15°N - 15°S)

Upper Level Westerlies have ridges and
troughs:
 “Rossby
Waves” (Longwaves)
Wavelength

3 - 6 loops around earth
above

= 1000s km
500 mb layer
influence surface weather
c
Converging height lines make
wind speeds increase
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On warm side, pressure drops less rapidly with
altitude than on cold side; Note isobaric surfaces
slope and slope increases with altitude
Therefore, wind speed increases with altitude
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JET STREAMS : zones of high wind speed
(Narrow bands, speed increases toward
center (up to 150 mph))
 Embedded
 below
 Jet
in upper level Westerlies
tropopause
streams are located above strong
temperature contrasts at surface
Polar Jet Stream
Subtropical Jet Stream
Polar Front Jet Stream

Between midlatitude tropopause and polar
tropopause
Polar Jet

above Polar Front Zone :
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Where cold dense polar air meets warmer air from mid-latitudes
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Can see polar jet on 300 mb maps
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Current 300 mb map
Subtropical Jet Stream
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Between midlatitude tropopause and tropical
tropopause
Subtropical Jet
greatest wind speed at
North edge of Hadley
cell

due to
Conservation of
Angular
Momentum:
(smaller radius of
rotation, faster the
spin)
Enhanced warming in Arctic is affecting Rossby waves
Highs and Lows move
horizontally

Highs move towards convergence aloft

Surface pressure rises in direction High is moving
and falls in its wake

Rising barometer means air is being ADDED aloft
and sinking air (clear skies) are coming