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Meteorology 10 - Weather and Climate
Fall 2008
CHAPTER 6
FOCUS: forces of air motion
Air Pressure and Winds
Which way will the wind blow and how strong will it be?
1. Pressure Gradient Force
2. Coriolis Force (Earth’s Rotation)
3. Centrifugal Force
4. Friction
We are going to focus on one of these at a time…here goes
Which way will the wind blow and how strong will it be?
1. Pressure Gradient Force
2. Coriolis Force (Earth’s Rotation)
3. Centrifugal Force
4. Friction
Pressure Gradient Force (PGF)
The source of all wind….
Define PGF, word for word. First:
Pressure is the force exerted by the air per unit area
from the weight of the column of air above it.
99% of mass
1013.25 millibars
70% of mass
14.7 lbs/sq. inch
1 atmosphere
Pressure Gradient Force (PGF): The source of all wind….
Two ways to further describe pressure:
Either as HIGH (H) or LOW (L)
Look at each set in the figure and see how the pressure is
defined relative to the other column of air.
Fig. 6-2, p. 143
Pressure Gradient Force
Pressure Gradient = change in pressure
distance
In this simplified figure, there is no pressure “gradient” in
the horizontal direction
Height
200mb
400mb
600mb
800mb
1000mb
Ocean
Land
Pressure Gradient Force
Newton’s First Law of Motion
“A force is required to make something move”
Pressure Gradient Force (PGF)
The source of all wind….
200mb
Cool the air in Column 1 and
the air gets dense
P= RT
When P is
constant 
T are together
a constant
such that the
following must
be true
T

200mb
1000mb
1000mb
Pressure Gradient Force (PGF)
The source of all wind….
Pressure Gradient Force driven by temperature:
•Two primary examples follow.
•But here are the basic physical principals
Afternoon
Height
L
200mb
H
400mb
Horizontal
PGF
600mb
H
Colder (Ocean)
800mb
1000mb
L
Warmer (Land)
Thermal Circulation - Pressure Gradient Force
Sea Breeze
Onshore Wind
Thermal Circulation - Pressure Gradient Force
Land Breeze
Offshore Wind
Take home message: air flows from H to L
pressure
Fig. 6-11, p. 151
Surface and Upper Air Charts
•
•
•
•
isobaric maps
contour lines
ridges
troughs
• Color-filled contour maps are the same as ordinary
contour maps, except that the area between adjacent
lines is filled in with color.
Surface of
constant
pressure
Warm
Cold
Constant
pressure
chart
5520 m
5580 m
5640 m
Editable Text
Figure 3, p. 150
Figure 2, p. 150
Which way will the wind blow and how strong will it be?
1. Pressure Gradient Force
2. Coriolis Force (Earth’s Rotation)
3. Centrifugal Force
4. Friction
Other forces on the atmosphere:
“Coriolis Force”
• Air moves in the reference frame of the earth.
• But, the earth is rotating...
The atmosphere is not in a “fixed” frame of reference
Coriolis Force
•
•
•
•
real and apparent forces
Coriolis force
strength and direction of the Coriolis force
factors that affect the Coriolis force
• It is sometimes claimed that “water swirls down a bathtub
drain in opposite directions in the northern and southern
hemispheres”. This is not true.
Deflection of ball’s path in rotating reference frame
Is the ball
moving in a
straight or curved
path???
Depends on your
frame of
reference!!
Fig. 6-14, p. 153
Coriolis Force in a rotating reference frame
http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/fw/gifs/coriolis
.mov
Coriolis Force due to twisting of North-South reference frame
as Earth turns
Fig. 6-15, p. 153
Coriolis Force due to rotation of North-South reference frame on
spherical Earth
Objects are deflected to the
right of their path
in the Northern Hemisphere
Greatest near poles
No change at equator
Fig. 6-15, p. 153
Earth reference frame changes orientation due to Earth rotation
A moving object appears to be deflected from its original course
The earth rotates, so
All moving objects are deflected by the Coriolis force
• Coriolis force is ‘noticable’ for objects that are in
motion for a significant fraction of the time it takes
for the earth to rotate once (a day).
- an airplane flying to Paris?
- air moving across the U.S.?
- water spinning down a drain?
Coriolis Force depends on
•Earth’s rotation
•Speed of object
•Latitude of object
Coriolis Force = 2Vsin
 = Earth’s rotation rate
V = object’s velocity
 = latitude where object is located
Coriolis Force = 2Vsin

Northern
Hemisphere
V
CF
 = 0o
Equator
Southern
Hemisphere
 = 30o
CF
V
 = - 30o
Moving objects “deflected” to the right of their path in the
Northern Hemisphere (to the left in Southern Hemisphere)
Straight-line Flow Aloft
• combination of the pressure gradient and
Coriolis forces
• geostrophic wind
• Geostrophic winds can be
observed by watching the
movement of clouds.
Colorado-sized area in the Northern Hemisphere
• Motionless air
• Uniform pressure
NO FORCES, so NO WIND
Colorado-sized area in the Northern Hemisphere
Low Pressure
PGF
High Pressure
Colorado-sized area in the Northern Hemisphere
Which way will the wind blow and how strong will it be?
1. Pressure Gradient Force
2. Coriolis Effect (Earth’s Rotation)
3. Centrifugal Force
4. Friction
Curved Winds Around Lows and
Highs Aloft
• cyclonic and anticyclonic flow – not in
geostrophic balance  curved
• centripetal force – force ‘felt’ by the air parcels
• gradient wind
Gradient Wind: General case of wind along curved path
Centrifugal Force (CE)
Acts away from center of
curvature
PGF
CF
CE
PGF = CF + CE
• Pressure gradient is
stronger around a Low
e.g., hurricanes
PGF
CF
Centrifugal Force
Gradient Wind: General case of wind along curved path
Centrifugal Force (CE)
Acts away from center of
curvature
CF
PGF
CF = PGF + CE
CE
• Pressure gradient is
weaker around a High
PGF
CF
Centrifugal Force
Winds on Upper-level Charts
• gradients in contour lines
• meridional and zonal winds
• Height contours on upper-level charts are interpreted
in the same way as isobars on surface charts.
Figure 6.19: An upper-level 500-mb map showing wind direction, as indicated by lines that parallel the wind.
Wind speeds are indicated by barbs and flags. (See the blue insert.) Solid gray lines are contours in meters above
sea level. Dashed red lines are isotherms in °C.
Fig. 6-19, p. 158 Stepped Art
Which way will the wind blow and how strong will it be?
1. Pressure Gradient Force
2. Coriolis Effect (Earth’s Rotation)
3. Centrifugal Force
4. Friction
Surface Winds
• planetary boundary layer
• friction
• frictional effects on the wind
• Most people rarely venture out of the planetary boundary layer.
Winds and Vertical Motions
• divergence and convergence
• hydrostatic equilibrium – gravity balanced
with vertically directed PGF