1. Air Pressure and Wind

2. Atmospheric pressure Force exerted by the weight of the air above
Decreases with increasing altitude
Units of measurement
Millibar (mb) – standard sea level pressure is mb
Inches of mercury – standard sea level pressure is inches of mercury

3. Atmospheric pressure Instruments for measuring Barometer
Mercury barometer
Aneroid barometer
Barograph (continuously records the air pressure)

4. A recording aneroid barometer
A mercury barometer
A recording aneroid barometer

5. Wind Horizontal movement of air Controls of wind
Out of areas of high pressure
Into areas of low pressure
Controls of wind
Pressure gradient force
Isobars – lines of equal air pressure
Pressure gradient – pressure change over distance

6. A weather map showing isobars and wind speed/direction

7. Wind
Controls of wind
Coriolis effect
Friction

8. Wind
Upper air winds
Generally blow parallel to isobars – called geostrophic winds
Jet stream
"River" of air
High altitude
High velocity ( ) kilometers per hour

9. The geostrophic wind

10. Comparison between upper-level winds and surface winds

11. Rotating Air Bodies Low Pressure Zone Formation
Warm air rises
Creates a low pressure zone
At the Earth’s surface, air “feeds” the low pressure zone, moves counterclockwise
High Pressure Zone Formation
Cool air sinks
Creates a high pressure zone
At the Earth’s surface, winds blow clockwise

13. Rotating Air Bodies Bends in the polar jet create troughs and ridges
Forms cyclones and anticyclones

14. Rotating Air Bodies Cyclones Low pressure zone in polar jet trough
Winds at surface flow counterclockwise towards the core
Air is updrafted and cooled
Forms clouds, rain and upper level outflow of air

15. Rotating Air Bodies Anticyclones
High pressure zone at ridge of polar jet
Air converges in upper atmosphere
Descends towards the ground
Flows outward at surface
Dry, windy conditions

16. Cyclones and Anticyclones
To view this animation, click “View” and then “Slide Show” on the top navigation bar.

17. Cyclonic and anticyclonic winds in the Northern Hemisphere

19. General atmospheric circulation
Underlying cause is unequal surface heating
On the rotating Earth there are three pairs of atmospheric cells that redistribute the heat

20. General atmospheric circulation
Idealized global circulation
Equatorial low pressure zone
Rising air
Abundant precipitation

21. General atmospheric circulation
Idealized global circulation
Subtropical high pressure zone
Subsiding, stable, dry air
Location of great deserts
Air traveling equatorward from the subtropical high produces the trade winds
Air traveling poleward from the subtropical high produces the westerly winds

22. General atmospheric circulation
Idealized global circulation
Subpolar low pressure zone
Warm and cool winds interact
Polar front – an area of storms

23. General atmospheric circulation
Idealized global circulation
Polar high pressure zone
Cold, subsiding air
Air spreads equatorward and produces polar easterly winds
Polar easterlies collide with the westerlies along the polar front

24. 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

25. Average surface pressure and associated winds for January

26. Average surface pressure and associated winds for July

27. General atmospheric circulation
Influence of continents
Monsoon
Seasonal change in wind direction
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

28. Circulation in the mid-latitudes
The zone of the westerlies
Complex
Air flow is interrupted by cyclones
Cells move west to east in the Northern Hemisphere
Create anticyclonic and cyclonic flow
Paths of the cyclones and anticyclones are associated with the upper-level airflow

29. Local winds Produced from temperature differences Small scale winds
Types
Land and sea breezes
Mountain and valley breezes
Chinook and Santa Ana winds

30. Illustration of a sea breeze and a land breeze

31. The Santa Ana Winds

32. Wind measurement Two basic measurements Direction Speed
Winds are labeled from where they originate
Direction indicated by either
Compass points (N, NE, etc.)
Scale of 0º to 360º
Prevailing wind comes more often from one direction
Speed
often measured with a cup anemometer

33. Wind measurement Changes in wind direction
Associated with locations of
Cyclones
Anticyclones
Often bring changes in
Temperature
Moisture conditions

34. El Niño

35. Normal conditions in the tropical Pacific Ocean
Surface winds move from east to west
From high pressure in S. America to low pressure in Australia
Drags water westward
Warm water pools in the western Pacific

36. Normal (Non-El Niño) conditions
Trade winds blow towards the west across the tropical Pacific. These winds pile up warm surface water in the west Pacific, so that the sea surface is about 1/2 meter higher at Indonesia than at Ecuador.
The sea surface temperature is about 8 degrees C higher in the west, with cool temperatures off South America, due to an upwelling of cold water from deeper levels. This cold water is nutrient-rich, supporting high levels of primary productivity, diverse marine ecosystems, and major fisheries. Rainfall is found in rising air over the warmest water, and the east Pacific is relatively dry. The observations at 110 W (left diagram of 110 W conditions) show that the cool water (below about 17 degrees C, the black band in these plots) is within 50m of the surface.

37. Normal conditions

38. Every 3 – 8 years, system reverses
Called the Southern Oscillation
Trade winds weaken or reverse
Warm water migrates from Australia to S. America
Arrives in time for Christmas – Corriente del Niño

39. During El Niño, the trade winds relax in the central and western Pacific leading to a depression of the thermocline in the eastern Pacific, and an elevation of the thermocline in the west. The observations at 110W show, for example, that during , the 17-degree isotherm dropped to about 150m depth. This reduced the efficiency of upwelling to cool the surface and cut off the supply of nutrient rich thermocline water to the euphotic zone. The result was a rise in sea surface temperature and a drastic decline in primary productivity, the latter of which adversely affected higher trophic levels of the food chain, including commercial fisheries in this region. The weakening of easterly tradewinds during El Niño is evident in this figure as well. Rainfall follows the warm water eastward, with associated flooding in Peru and drought in Indonesia and Australia. The eastward displacement of the atmospheric heat source overlaying the warmest water results in large changes in the global atmospheric circulation, which in turn force changes in weather in regions far removed from the tropical Pacific.

40. El Niño

41. What is El Niño?
Basically, it's a giant puddle (or pod) of heated water that sloshes across the Pacific Ocean
Similar to an iceberg
Bulge on the surface
Most of “pod” beneath the surface
Due to difference in density
National Geographic’s Model

42. ENSO - El Niño-Southern Oscillation
Typically lasts 1 year
May last up to 3
In multi-year events, first year not as affected
Affects both hemispheres

43. Recognizing an El Niño Sea Surface Temperatures (SST)
Normal: 6-8° C warmer in the western tropical Pacific than in the eastern tropical Pacific
Check SST to see if in “normal” range

45. La Niña Return to “normal” conditions from an El Niño strong Produces:
Strong currents
Powerful upwelling
Chilly and stormy conditions along S. American coast
Eastern Pacific cools rapidly, Western Pacific warms rapidly
Renewed Trade Wind activity spreads the cooler eastern Pacific waters westward

46. Global distribution of precipitation
Relatively complex pattern
Related to global wind and pressure patterns
High pressure regions
Subsiding air
Divergent winds
Dry conditions
e.g., Sahara and Kalahari deserts

47. Global distribution of precipitation
Related to global wind and pressure patterns
Low pressure regions
Ascending air
Converging winds
Ample precipitation
e.g., Amazon and Congo basins

48. Average annual precipitation in millimeters

49. Global distribution of precipitation
Related to distribution of land and water
Large landmasses in the middle latitudes often have less precipitation toward their centers
Mountain barriers also alter precipitation patterns
Windward slopes receive abundant rainfall from orographic lifting
Leeward slopes are usually deficient in moisture

50. ~ End ~