Air Pressure and Winds II

1. Air Pressure and Winds II

2. RECAP • Ideal gas law: how the pressure, the temperature and the density of an ideal gas relay to each other. • Pressure and pressure measurements • Constant height charts: isobars • Constant pressure charts: contour lines • Note wind direction in NH • Low P -> Cyclones • High P -> Anticyclones

3. Isobaric chart • Constant pressure, P=500 mb, then plot heights: contour lines • Pressures on surface maps <-> Heights on isobaric charts • Low <-> Low • High <-> High • Pressure <-> Temperature • Low <-> Low • High <-> High • Note wind direction here • Different from surface winds • Along the contour lines, no crossing

4. Newton’s Laws of Motion F G • Newton’s First Law: an object will • stay at rest, or • maintain its motion at a constant velocity and in a straight line as long as • no force is exerted on the object, or • all forces cancel each other • Says who? An observer at rest

5. Newton’s Second Law • The force (F) experienced by an object is equal to its mass (m) times the resulting acceleration (a). • Acceleration: change in velocity (magnitude or direction) • Examples of accelerated motion • Speeding up • Slowing down • Making a turn • A body with a largemass is more difficult to accelerate (or stop). Objects with large masses are more inertial.

6. Forces acting in the atmosphere. • Gravity force. • Vertical force in a downward direction • Atmospheric drag force (friction). • Acts against the motion • Proportional to velocity squared • Pressure force. • From high to low pressure regions • The bigger the pressure gradient the larger the pressure force • Centripetal force • Acts perpendicular to the wind velocity, towards the center of the curvature. • It changes the wind direction not the magnitude. • Larger velocity and larger curvature (smaller radius) correspond to a bigger centripetal force. • Coriolis force: due to the Earth’s rotation

7. Pressure gradient force • Example: two tanks • Tank A: full • Tank B: half-full • Pressure at the base of tank A is higher than the pressure at the base of tank B • The pressure gradient force causes the water to flow from A to B (from high pressure to low pressure) • Analogous phenomenon in the atmosphere

8. Pressure Gradient • Vector: It has a magnitude and direction. • Magnitude: how fast the pressure is changing in a given direction. • Direction: the direction of the fastest increase of the pressure. • Two examples: • Color gradient • Mountain slope

9. What is the direction of the color gradient?

10. Topographic maps • Elevation contours, index contours (bold), slope angle. • Steep slope: dense contours • Gentle slope: contours are further apart • Elevation -> pressure; slope -> gradient

11. In which direction is the pressure gradient?

12. Pressure Gradient Force • The pressure gradient force is analogous to the gravity force on a mountain slope • Its magnitude is proportional to the pressure gradient. • Its direction is opposite to the direction of the pressure gradient. It is from HIGH to LOW pressure. • It is perpendicular to the lines of constant pressure (isobars). • The closer to each other the isobars are the larger the gradient is and the larger the pressure force is. • The pressure force along an isobar is ZERO!

13. In which direction is the pressure gradient force?

14. Wind and Pressure Map

15. Coriolis Force (Effect) • It is an apparent force; • Due to the rotation of the coordinate system (Earth); • It makes a moving object deflect from a straight line even in the absence of any forces acting on it.

16. The Magnitude of the Coriolis Force • The rotation of the Earth • The faster the planet rotates the bigger the force • The speed of the object • Bigger V -> bigger effect • The latitude: • Min. at the equator • Max. at the poles

17. Coriolis force as a function of: • The speed of the object • The latitude: • Min. at the equator • Max. at the poles

18. The Direction of the Coriolis Force • In the Northern hemisphere the deflection is to the right of the direction of motion. • In the Southern hemisphere the deflection is to the left of the direction of motion. • The winds in the Northern hemisphere will be deflected to the right and in the Southern hemisphere they will be deflected to the left. • Hurricanes spin differently in the Northern and Southern hemisphere

19. The Coriolis Force and the Earth • The Coriolis effect is important when moving over LARGE distances (air plane travel), with large velocities, away from the equator.