1. Is air moving away or towards each of these pressure centers?
Bellwork 4/12 H L
Is air moving away or towards each of these pressure centers?
Will air circulate Clockwise or Counterclockwise around each of these pressure centers?

2. Cyclonic and anticyclonic winds in the Northern Hemisphere

3. Cyclones and anticyclones
A center of low pressure
Pressure decreases toward the center
Wind blow inward and Counterclockwise around a low (Northern Hemisphere) **CONVERGING winds
Forces air upwards
Rising air = cloud formation

4. Cyclones and anticyclones
A center of high pressure
Pressure increases toward center
Winds around a high blow outward and clockwise (Northern Hemisphere)
**DIVERGING winds
Causes air to sink
Sinking = little/no cloud formation… “Fair weather”

5. Cyclones and anticyclones
Cyclones exist for long periods of time only when air aloft is diverging
Maintains low surface pressure
Anticyclones exist for long periods of time only when air aloft is converging
Maintains high surface pressure
Diagram on next slide

6. Airflow associated with surface cyclones and anticyclones

7. How to read a weather map:

8. Symbols

9. Wind Directions

10. Quick Practice:
1. Locate the weather station in northern Iowa. From which direction is the wind blowing?
2. Locate the weather station in Chicago, Illinois. From which direction is the wind blowing?
3. Locate and label any Highs or Lows on this map.
4. Cyclones are associated with which type of system?
5. Anticyclones are associated with which type of system?

11. Which way is the wind blowing?

12. General atmospheric circulation
Underlying cause is unequal surface heating
*Much is still unknown about global circulation, but it helps to start by looking at a non-rotating Earth…

13. Circulation on a Non-rotating Earth
There would be two “cells” that distribute heat
One moving warm air to the poles,
One moving cool air the equator
…but our Earth rotates…

14. Idealized global circulation

15. General atmospheric circulation
Idealized global circulation
4 Pressure Systems associated with air distribution
Equatorial Low Pressure Zone
Subtropical High Pressure Zone
Subpolar Low Pressure Zone
Polar High Pressure Zone

16. Pressure Systems Equatorial Low pressure zone
Rising air
Abundant precipitation
Subtropical High pressure zone
Subsiding, stable, dry air = deserts
Air traveling equatorward from the subtropical high produces the trade winds
Air traveling poleward from the subtropical high produces the westerly winds

17. Pressure Systems Subpolar Low pressure zone Polar High pressure zone
Warm and cool winds interact
creates a Polar Front – an area of storms
Polar High pressure zone
Cold, subsiding air
Air spreads equator-ward and produces Polar Easterly winds
Polar easterlies collide with the Westerlies along the Polar Front

18. Idealized global circulation

20. General atmospheric circulation
Influence of continents
Seasonal temperature differences disrupt the
Global pressure patterns
Global wind patterns
Influence is most obvious in the Northern Hemisphere
WHY??? _____________________
Monsoon
Seasonal change in wind direction

21. General atmospheric circulation
Influence of continents
Monsoon
Occur over continents
During warm months
Air flows onto land
Warm, moist air from the ocean
Winter months
Air flows off the land
Dry, continental air

22. Average surface pressure and associated winds for July

23. Average surface pressure and associated winds for January

24. Homework:
Identifying Winds & Pressure systems
Applying your knowledge to make weather predictions

26. Bellwork 4/13 Why are deserts hot & dry?
Why are rainforests warm & rainy?
What connection can you make to wind patterns?

27. Surface Winds vs. Winds Aloft
Upper air winds
Generally blow parallel to isobars – called geostrophic winds
(No effect from friction)
Jet stream
"River" of air
High altitude
High velocity ( ) kilometers per hour

28. The geostrophic wind

29. Surface Winds vs. Winds Aloft
Winds Below 600m
Affected by friction
Lowers wind speeds, which in turn lowers the Coriolis Effect

30. Comparison between upper-level winds and surface winds