1. Winds Professor Jeff Gawrych De Anza College

2. Q: What drives the weather in the atmosphere? The uneven heating of the earth Q: What causes the weather to change? Pressure differences and winds We must analyse the predominant forces in the atmosphere, namely: pressure and pressure gradients, gravity rotation of the earth friction Principle forces

3. Determines the direction and speed of winds Can help explain general circulation of atmosphere (next lecture) These differences are brought on by Temperature and density changes Gas law p=RT p-Pressure (Pa),  - density (g m-3) R - Gas Constant (287 J K-1 kg-1) T - Temperature (K) Cold air is more dense and warm air is less dense, so cold air sinks and warm air rises!!! Pressure differences

4. It’s the difference in pressure that causes a fluid like air to move.

6. Pressure gradient: Difference in pressure between different locations. Force blows from higher pressure to lower pressure Force is directed perpicular to isoba Stronger presure gradients produce stronger winds Pressure Gradient Force

7. Pressure Gradient Force

9. The Rotation of the Earth • Rockets, migrating birds, and large scale weather systems are all deflected due to the rotation of the Earth. • The Earth’s rotation causes • Rotational movement that deflects the winds. • The Coriolis Force is the name of this rotational force that deflects motion.

11. Coriolis Force • Affects direction, not speed of object • Maximum at the poles • Zero at the equator • Not the reason the toilets flush a certain way in each hemisphere. Always acts to the right of the motion in Northern Hemisphere. Always acts to the left of the motion in the Southern Hemisphere.

13. 3 main types of wind • 1) Geostrophic: • winds aloft, where frcition is minimal • 2) Surface winds • Frition cannot be ignored • 3)Local/regional winds • Slope induced winds, thermal circulations, etc,monsoon, etc.

14. Geostrophic Flow • Assumes there is no friction at all! • Not 100% realistic, but neraly true at high altitudes • PGF and Coriolis Force in balance • Flow is parallel to isobars • The jet stream is nearly geostophic

15. GeostrphicFlow

17. SurfaceFlow

18. Aloft, winds are nearly geostrophic At surface, friction alters direction of winds

19. Why do low-pressure systems cause rain? • Aloft: flow is counter clockwise in NH and flow is geostrophic • Surface: flow is ~Clockwise, but NOT geostrophic, • Instead flow is inward towards low causing convergence • Leads to clouds/rain

20. Why do high-pressure systems cause clear conditions? • Aloft: flow is clockwise in NH and geostrophic • Surface: flow is ~clockwise but NOT geostrophic • Instead flow is outward from high causing divergence • Leads to sinking motion (subsidence) and clear skies • Sinking air WARMS and DRIES OUT

22. Surface Winds • Friction is now a major factor, and slows down the wind. • Trees, buildings, monuntqains,air resistance, etc. all contribute to friction • Winds DO NOT blow parallel to isobars, but • Inward towards low pressure • Outward from high pressure

23. Slope Winds Day: warmer air rises and ascends upslope. Often causes cloud formation Night: Cooler air descends downslope.

24. Slope flow II: Slope flow significantly alters precipataion patterns

25. Thermal Circulations • Localized events • sea breeze • land breeze • country breeze • Surrounding areas heat/cool at different rates. • Surface winds blow from the colder place towards the warmer place

26. Monsoon circulation