1. Atmospheric Motions & Climate
Horizontal Atmospheric Motion
Geostrophic Wind Approximation
High and Low Pressure System Winds
Boundary Layer
Convergence-Divergence
Science Concepts
Newton’s Laws of Motion
Horizontal Forces
Pressure Gradient
Force
Coriolis Force
Friction Force
The Earth System (Kump, Kastin & Crane)
• Chap. 4 (pp , Fig. 4-13)

2. Atmospheric Motions Of what is this a picture?

3. Atmospheric Motions
Note similar patterns between Bermuda grass colorations and clouds in the lower right-hand corner?

4. Atmospheric Motions Forces Newton's Laws
• Forces are a vector quantity - they have a direction as well as a magnitude.
Newton's Laws
• First Law
- An object's velocity (direction or speed) will remain unchanged unless
acted upon by a force.
• Second Law
- Object's acceleration = net force
object's mass
- Acceleration is defined as the change in velocity with time and it also is
a vector
a = (Change in velocity) = ( V2 - V1 )
(Change in time) ( t2 - t1 )
> Change in velocity can be change in direction or speed or both

5. Atmospheric Motions Horizontal Motion • Forces and accelerations
- Pressure gradient force (PGF)
- Coriolis force (CF)
- Friction force (FF)
- Thus,
Acceleration on a parcel = ( PGF + CF + FF ) / Mass of parcel
• Pressure gradient force (PGF)
- Gradient is defined as the change of a quantity with change of location,
i.e., the pressure gradient is the change in pressure with change in
position.

6. Atmospheric Motions Horizontal Motion (Con’t)
• Pressure gradient force (PGF) (Con’t)
PG = (Change in pressure) = ( p2 - p1 )
(Change in location) ( L2 - L1 )
Pressure gradient force is the force exerted on an air parcel by the pressure
gradient, i.e., the force which causes air to accelerate from an area of high
pressure (too much mass) toward an area of low pressure (too little mass) in
an attempt to reduce the pressure gradient.
1020 mb
1016 mb
Pressure
Gradient
Force
High
Pressure
Low
Pressure L 1 L 2
Location 1
Location 2
Isobars - Lines of equal pressure

7. Gradient Force is largest
Atmospheric Motions
Horizontal Motion (Con’t)
• Pressure gradient force (PGF) (Con’t)
- Example
< Note PGF is larger where isobars are closely packed and smaller
where isobars are spaced further apart
1016
Pressure
Gradient
Force
Isobars
1020
1024
Note: The Pressure
Gradient Force is largest
where the Pressure
Gradient is largest. H

8. Atmospheric Motions Horizontal Motion (Con’t) • Coriolis Force
- Example: Merry-Go-Round and Ball

9. Atmospheric Motions Horizontal Motion (Con’t) • Coriolis Force
- Example: Merry-Go-Round and Ball

10. Atmospheric Motions Horizontal Motion (Con’t) • Coriolis Force
- Example: Fire missile with speed of 1 mile per second from the North
Pole toward New York City. Flight would take 55 minutes.

11. Atmospheric Motions
Horizontal Motion (Con’t) • Coriolis Force Example (Con’t) - During that 55 minutes the Earth would have turned about 15°. 90 60 30
120
150 NP
CHI NY

12. Atmospheric Motions
Horizontal Motion (Con’t) • Coriolis Force (Con’t) - How fast is a spot on the Equator moving? Speed = Distance / Time = Circumference of Earth at the Equator / Time = ( 2 • π • 6,378.4 km ) / 24 h = ( 2 • • 6,378.4 km ) / 24 h = 1,670 km / h - How fast is a spot with half the radius (60° Latitude) moving? Speed = ( 2 • π • 3,189.2 km ) / 24 h = 835 km / h
N P

13. Atmospheric Motions
Horizontal Motion (Con’t) • Coriolis Force (Con’t) - How fast is a spot with a 1 km radius moving? Speed = ( 2 • π • 1 km ) / 24 h = 0.26 km / h - Object moving from the Equator toward the Pole is moving faster than the surface and thus,appears to turn eastward - Object moving from the Poles toward the Equator is moving slower than the surface and thus, appears to turn westward
N P

14. Atmospheric Motions Horizontal Motion (Con’t) • Coriolis Force (Con’t)
- Apparent force resulting from the Earth’s rotation
- Causes objects to deflect to the right of their direction of motion in the
Northern Hemisphere
- Zero for objects at rest, increasing as an object’s velocity increases
- Zero for objects located at the equator, increasing as the object moves
toward either pole
• Geostrophic Wind Approximation
- Geostrophic - Earth turning
- Balance of the PGF and the Coriolis Force
- Approximation to the “real” wind - have neglected friction
- Approximation improves as one proceeds upward out of the boundary
layer where friction is most important

15. Atmospheric Motions Horizontal Motion (Con’t)
• Geostrophic Wind Approximation (Con’t)
PGF
PGF V CF
996 mb
PGF V CF
1000 mb
PGF CF V
PGF V CF
1004 mb
1008 mb
Balance of the Pressure Gradient Force (PGF) and the Coriolis Force (CF)

16. Atmospheric Motions Horizontal Motion (Con’t)
• Geostrophic Wind Circulations
- Implications
> In the Northern Hemisphere and at levels where friction is not a
dominate force, wind blows clockwise (in the anticyclonic direction)
with lower pressure to the left, around High pressure areas
dominate force, wind blows counterclockwise (in the cyclonic
direction) with lower pressure to the left, around Low pressure
areas
Anticyclone
Cyclone

17. Atmospheric Motions Horizontal Motion (Con’t) • Friction force
- Always acts in the opposite direction to the velocity
- Two types
> Mechanical (similar to forced convection)
> Thermal (similar to free convection)
- Important within the boundary or well-mixed layer

18. Atmospheric Motions Horizontal Motion (Con’t) • Friction force (Con’t)
- Important within the boundary or well-mixed layer (Con’t)
> Depth of the mixed layer is affected by
‡ Surface heating (more heating -
deeper layer)
‡ Wind speed (higher wind speed -
‡ Surface roughness or terrain
(rougher surface - deeper layer)
600
Strong
Mixing
400
Height (m)
Weak
Mixing
200 5 10
Wind Speed (kt)

19. Atmospheric Motions Horizontal Motion (Con’t)
• Wind approximation with friction
PGF
PGF
996 mb
1000 mb V FF
1004 mb
1008 mb CF
Friction slows the wind speed which reduces the CF thus
allowing the PGF to pull harder toward Low pressure than the CF pulls to the right.

20. Atmospheric Motions Horizontal Motion (Con’t)
• Wind Circulations with Friction
- Implications
> In the Northern Hemisphere near the surface, wind blows clockwise
and slightly out of High pressure areas
> In the Northern Hemisphere near the surface, wind blows
counterclockwise and slightly into Low pressure areas

21. Atmospheric Motions Horizontal Motion (Con’t)
• Circulations in the Northern and Southern Hemispheres
- Coriolis force is to the left in the Southern Hemisphere
> In the Southern Hemisphere near the surface, wind blows
counterclockwise and slightly out of High pressure areas
clockwise and slightly into Low pressure areas
Northern Hemisphere Low Southern Hemisphere Low
natural_hazards_v2.php3?img_id=10657
natural_hazards_v2.php3?img_id=2108

22. Atmospheric Motions Horizontal Motion (Con’t) H L • Buys-Ballot Rule
Study/NAO/NAO_2.html
Horizontal Motion (Con’t)
• Buys-Ballot Rule
- When one has his or her back to the
wind, low pressure will be to the left
and slightly ahead.
• Convergence - Divergence
Divergence out of the center of the High causes downward motion that warms the air and decreases its Relative Humidity
Convergence into the center of the Low causes upward motion that cools the air and increases its Relative Humidity H L