1. GEOG 1112: Weather and Climate
Atmospheric Pressure, Wind, & Global Circulation

2. Air Pressure Weight of air exerting pressure on surface of Earth
Vertical and horizontal pressure differences that lead to atmospheric motions.
Weight of air exerting pressure on surface of Earth
Air pressure decreases with altitude
Warm air is lighter than cool air, so it tends to rise – cool air sinks

3. Air Pressure & Altitude
Lower Density,
Lower Pressure
Higher Density,
Higher Pressure

4. Vertical Changes in Pressure
Pattern of pressure: a non-linear decrease with height
Why?

5. Atmospheric Pressure Systems
High-pressure system
-also called anticyclone
-circulating body of air
-descending air
-clockwise circulation in
northern hemisphere
Low-pressure system
-also called cyclone
-rising air
-counterclockwise circ.
in northern hemisphere

6. Atmospheric Pressure Map
Isobars – lines of equal air pressure on a map

7. Direction of Air Flow Unequal heating of land surfaces
Pressure gradient force – air flows from high to low
Coriolis force – deflection or change in direction caused by Earth’s rotation
Frictional forces – places a drag on that air flow

8. Isobars and Pressure Gradient
The closeness of isobars decides the gradient between high and low pressure
The closer the isobar, the higher the winds and vice versa
Winds at the surface always blow at right angles to isobars

9. H L Pressure Gradient isobars 1028 mb 1024 mb 1020 mb 1016 mb 1012 mb

10. Pressure Gradient Force

11. Coriolis Force Due to Earth’s rotation
Pulls wind to right in northern hemisphere
Pulls wind to left in southern hemisphere
Strongest at poles
None at equator

12. Geostrophic Wind
Horizontal wind in the upper atmosphere that moves parallel to isobars. Results from a balance between pressure gradient force and Coriolis force.
Low Pressure
High Pressure
500 mb
504 mb
508 mb
512 mb
PGF

13. Frictional Forces
Near surface, friction (F) works against pressure gradient force (PGF), so resulting wind direction is between pressure gradient force and coriolis force (CF)

14. Three Forces Combined

15. Rossby Waves
Figure 6.17

16. Jet Stream

17. The Polar Front and Jet Streams
Strong boundaries often occur between warm and cold air. In the mid-latitudes, the polar front marks this thermal discontinuity at the surface.

18. Global Pressure & Atmospheric Circulation
Unequal heating of tropics and poles
Global circulation on non-rotating, uniform Earth L L H

19. Global Circulation Model

20. Seasonal Changes in Circulation
ITCZ migrates with subsolar point
Trade winds, STHs, westerlies, all follow the ITCZ north & south with seasons

21. Seasonal Changes in Circulation
Monsoon
Seasonal shift of prevailing wind dir. due to land/water contrasts
Happens throughout subtropical regions
Most significant in South & SE Asia

22. South Asian Monsoon Summer Monsoon Winter Monsoon Warm air over Asia
Cold air over Asia
Sinking air/High pressure
Cool, dry NE winds
ITCZ far south
Summer Monsoon
Warm air over Asia
Rising air/Low pressure
Warm, moist wind – rain
ITCZ swings north

23. Local Wind Systems Sea Breeze
Sun-heated air over land rises – Low pressure
Cooler air over water sinks – High pressure
Land Breeze
Night air over land cools fast – High pressure
Air over water stays warmer – Low pressure

24. Local Wind Systems – Mountain-Valley Breeze

25. Subtropical High-pressure Cells

26. Global Oceanic Circulation

27. Oceanic Circulation
Thermohaline Circulation – warm water in gulf stream cools and evaporates as it heads north making it heavier – downwelling, then a long trip to the Pacific where upwelling occurs

28. ENSO
Normal
El Niño