1. PRESSURE, WINDS AND CIRCULATION PATTERNS
C.J. Cox/ Air Pressure & Winds
PRESSURE, WINDS AND CIRCULATION PATTERNS
Geography Dept. (916)

2. 5+ Basic Elements of the Atmosphere
First, Let’s Recall the
5+ Basic Elements of the Atmosphere
– the main ingredients of weather and climate
-- Also called Elements of Weather and Climate
Solar Energy
-- Insolation and Heat Energy Transfer
Temperature
In this segment, we’ll discuss the next two elements:
Pressure
Wind + Circulation
Precipitation
+ Air Masses (and Fronts)

3. pressure, Winds and circulation patterns
Atmospheric Pressure
Basic Pressure Systems – High & Low Pressure
Pressure Variations – Vertical and Horizontal
Mapping Pressure Distribution – Isobars, Pressure
Gradients, Global Pressure Belts
Winds and Circulation Patterns
Pressure – Wind Relationships
Wind Modifiers – The Coriolis Effect, Friction
Surface Wind Systems – Global, Upper Air, Seasonal, Local
Ocean – Wind Relationships – Ocean Currents, El Nino,
La Nina and the Sothern Oscillation

4. Pressure Pressure is the weight of the atmosphere
Atmosphere as a whole has considerable weight and exerts average pressure of about 14.7 lbs. /sq. in.
A device used to measure air pressure is called a barometer – hence, barometric pressure
Pressure is usually measured in millibars (mb) –
14.7 lbs. of pressure causes mercury in a barometer to rise inches, which is translated into millibars. ← And this Standard sea-level pressure is also considered to be “normal” pressure.
PRESSURE also describes the tendency of air to rise or sink at any given place or time [as we will see later]
Atmospheric Pressure is important because it directly affects → atmospheric circulation –
the differences in pressure create our wind systems
and the movement of the winds drive our ocean currents

5. Mercurial Barometer
At sea level a column of air one inch square weighs
14.7 lbs
This 14.7 lbs of weight exerts pressure forcing mercury in a tube to rise inches or 1013 mb

6. basic Pressure systems
C.J. Cox/ Air Pressure & Winds
basic Pressure systems
There are two basic types of pressure systems:
the Low, or Cyclone, or “L”  Convergent, ascending air
the High, or Anticyclone, or “H”  Divergent, descending air
Geography Dept. (916)

7. Low Pressure Pressure below 1013.2 mbs., characterized by:
warm moist air
air movement is up and in
“convergent air”
Also called Cyclone – a low pressure center

8. High Pressure Pressure above 1013.2 mbs., characterized by:
Cold dry air
Air movement is down and out
“divergent air”
Also called
Anticyclone – a high pressure center

9. Pressure variations . . . Vertical Variations –
Air pressure decreases with
elevation – pressure on top of
Mt. Everest is only about 1/3rd
of that at sea level
The higher we go, air density
decreases as the air molecules
become more diffused and widely
spaced
Air tends to rise or sink as a result of its density – at ground level, air density is governed by its temperature – as air is heated, it expands, becomes less dense, thinner, and rises
Reduced air pressure also means less oxygen per
breath – that’s why airplane cabins are pressurized

10. Pressure variations . . . Horizontal Variations – grouped into two:
Thermal (determined by temperature):
As earth’s surface heats the air in contact
with it, the air expands in volume and
decreases in density – when this warm air
rises, there is less air near the surface, resulting in decrease in surface pressure  Such low pressure occurs regularly along the Equator  The opposite occurs in the Polar areas.
Dynamic or Mechanical (due to motions of the atmosphere):
The dynamic causes are more complex and related to Earth’s rotation and broad patterns of circulation  Both the Subtropical High and Subpolar Low pressure regions are dynamically induced.

11. Mapping pressure distribution
Isobar Maps ► Pressure is mapped using isobars, every 4 mb
Pressure Gradient ► Winds blow from higher to lower pressure
Global Pressure Belts and Pressure Maps

12. C.J. Cox/ Air Pressure & Winds
Idealized World Pressure Belts
Geography Dept. (916)

13. Actual
Global Pressure
Maps –
Seasonal Variations

14. Wind and circulation patterns
Wind is the horizontal movement of air in response to differences in pressure
Winds are the means to balancing:
(a) the uneven distribution of pressure, and
(b) radiational heating and cooling
Winds also influence the rate and distribution of evaporation, and thus, precipitation
Basic Wind Patterns
-- induced by Pressure configurations
Linear -- Pressure Gradients and Winds
Circular -- Cyclones (L), Anticyclones (H) and Winds

15. Pressure Gradients and Winds

16. Cyclones (L), Anticyclones (H) and Winds
Movement of
Surface Winds
associated with
Cyclones and
Anticyclones
in the Northern
and Southern
Hemispheres

17. The Coriolis Effect Wind Modifiers:
Surface Friction (effective up to about 1000 m above the surface – reduces wind speed)
The Coriolis Effect and Wind – deflection due to rotation (Fig. 5.6)
The Coriolis Effect
Deflection of winds and ocean currents to the right in the northern hemisphere and to the left in the southern hemisphere
Caused by earth’s rotation below
Coriolis Effect

18. Geostrophic Wind
Upper-level winds in which the Coriolis effect and pressure gradient are balanced,
resulting in wind flowing parallel to the isobars.

19. Earth’s wind Systems – vertical layering
Upper Air Winds
Wind Belts
Surface Winds
Earth

20. Wind belts: Global surface wind systems
Idealized Model of Atmospheric Circulation
Trade Winds
ITCZ + Doldrums
Subtropical Highs Horse Latitudes
Westerlies
Polar Easterlies

21. Subglobal surface wind systems
Seasonal Winds --- The Monsoons

22. South AsiAn / INDIAN Monsoon

23. Movement of Pacific High and California Weather

24. LOCAL/Surface Winds
Land & Sea Breeze

25. Local / Surface Winds
Mountain & Valley Breeze

26. H L Santa Ana Winds Other Similar Local Winds: Chinook (Rockies)
Foehn (Alps) L

27. Chinook
Wind
Adiabatic Cooling
and Heating

28. Upper Air Winds

29. Ocean-atmosphere relationships
Ocean Currents – broad circulatory patterns (Gyres)
Clockwise in Northern H.
Counterclockwise in S.H.
Gyres do not cross the
equator
Upwelling
Oscillations

30. Major Ocean Currents

31. Anomalies – Pressure Oscillations
El Nino - Southern Oscillation (ENSO) and La Nina
These are linked atmosphere and oceanic phenomena of pressure and water temperature in equatorial Pacific.
Southern Oscillation refers to a periodic seesaw of atmospheric pressure in the tropical southern Pacific Ocean basin.
El Nino involves the weakening or reversal of the trade winds and the warming of surface water off the west coast of South America.
La Nina is often described as the opposite of El Nino; it is associated
with cooler than usual water off the west coast of South America.
Importantly, these events influence weather and climate patterns across the globe.
North Atlantic Oscillation (NAO)
Pacific Decadal Oscillation (PDO)
Arctic Oscillation (AO)

33. Thermal Infrared Satellite Images of
El Nino and La Nina episodes in the Tropical Pacific
El Nino La Nina