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

Supplemental References for Today’s Lecture Gedzelman, S. D., 1980: The Science and Wonders of the Atmosphere. 535 pp. John-Wiley & Sons. (ISBN 0-471-02972-6)

PGF Wind

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

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

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

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

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

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!

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

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

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

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

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.

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.

PGF Cor Geo

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

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

Assignment for Next Lecture Topic – Centripetal force due to curved flow Frictional force near the ground Reading - Ahrens pg 155-158 Problems - 6.23, 24