1. NATS 101-05 Lecture 11 Newton’s Laws of Motion Upper-Air Winds

2. Supplemental References for Today’s Lecture
Gedzelman, S. D., 1980: The Science and Wonders of the Atmosphere. 535 pp. John-Wiley & Sons. (ISBN )

3. PGF
Wind

4. Do Rocks Always Roll Downhill?
PGF
Upper-Level Winds
Gedzelman, p 247

5. Newton’s Laws of Motion
Newton’s 1st Law
An object at rest will remain at rest and an object in motion will remain at a constant velocity (same speed and same direction) if the net force exerted on it is zero
An external force is required to speed up, slow down, or change the direction of air

6. Newton’s Laws of Motion
Newton’s 2nd Law
The net force exerted on an object equals its mass times its acceleration
Sum of All Forces = Mass  Acceleration
Acceleration = Velocity Change / Time
Acceleration = Change in Speed and/or Direction

7. Velocity, Acceleration and Force are Vectors
Size and Direction
Original Velocity
New Velocity
Acceleration and Force
Original Velocity
New Velocity
Acceleration and Force

8. Uniform, Circular Motion Requires Acceleration
Original Velocity
New Velocity
Acceleration directed toward center of circle Centripetal
Circular Path
New Velocity
Original Velocity

9. Accelerated Frame of Reference
You are glued to car’s floor and drop an egg.
What happens if the car begins to accelerate?
Inside the car, it looks a mystery force is attracting the egg to the back of the car. Your frame of reference is accelerating.
(rest)
Someone outside the car sees that the egg is just accelerating to the floor, you are accelerating with the box car. A force is accelerating the car. Their frame of reference is not accelerating.
time
Splat!

10. Life on a Rotating Platform
From perspective of person not on merry-go-round, path of ball is straight.
From perspective of person on merry-go-round, path of ball deflects to left. There is an apparent force.
Play carousel for 3-4 loops. As the animation loops, point out that
The ball goes straight relative to ground (straight line shadows)
But it appears to curve from perspective of someone on the carousel
(left click picture for animation) World Weather Project 2010 Courtesy of M. Ramamurthy U of Illinois, Urbana-Champaign
Merry Go Round Link

11. Earth’s Rotation If viewed from space, earth is like a carousel!
Gedzelman, p 240
If viewed from space, earth is like a carousel!
Northern Hemisphere rotates counterclockwise
Southern Hemisphere rotates clockwise

12. Ball Appears to Deflect to the Right of the Observer
Deflection increases if: Rotation rate increases Speed of ball increases
Gedzelman, p 242

13. Geostrophic Adjustment
Parcel at rest initially accelerates toward lower pressure.
Coriolis Force rotates parcel to right in NH.
As parcel speeds up, Coriolis Force increases.
Eventually (about a day), PGF equals CF and flow is parallel to isobars.
World Weather Project 2010 Courtesy of M. Ramamurthy U of Illinois, Urbana-Champaign
Animate Picture

14. Geostrophic Balance Pressure Gradient Force 5640 m Geostrophic Wind
Coriolis Force
Geostrophic Wind
5640 m
5700 m
Geostrophic Wind Arises from a Balance Between the PGF and the Coriolis Force.
PGF + Coriolis Force = 0
Technically, can exist only for East-West flow and for straight contours.

15. Geostrophic Balance Pressure Gradient Force 5640 m Geostrophic Wind
Coriolis Force
Geostrophic Wind
5640 m
5700 m
The Balance Leads to the Wind Blowing Parallel to the Height Contours, with Lower Heights to the Left of the Wind Direction in the NH.
Closer the Spacing Between the Height Contours- The Faster the Geostrophic Wind Speed.

16. PGF
Cor
Geo

17. Do Rocks Always Roll Downhill?
Not if the Hill is Tall Enough!
PGF
Upper-Level Winds
Gedzelman, p 247

18. Key Concepts for Today Rotation of Earth
Accelerated Frame of Reference
Introduce Coriolis “Force”
Apparent Force to Account for Deflection Depends on Rotation, Latitude, Wind Speed
Geostrophic Balance and Wind
Balance Between PGF and Coriolis Force Geostrophic Wind Blows Parallel to Contours About One Day Required to Reach Balance

19. Assignment for Next Lecture
Topic – Centripetal force due to curved flow
Frictional force near the ground
Reading - Ahrens pg
Problems , 24