1. The Atmosphere in Motion Chapter 18

2. Atmospheric pressure
Force exerted by the weight of the air above in all directions
Weight of the air at sea level (1 atm)
14.7 lbs per in2, or
1 kg per cm2
Decreases with increasing altitude
Your body was built to withstand 1 atm
Units of measurement
Millibar (mb)—standard sea level pressure is mb

3. Instruments for measuring air pressure
Units of measurement
More common then mb is inches of mercury—Standard sea level pressure is in of mercury
Instruments for measuring air pressure
Barometer
Mercury barometer
Invented by Torricelli in 1643
Uses a glass tube filled with mercury

4. A mercury barometer

5. Instruments for measuring
Barometer
Aneroid barometer
“Without liquid”
Uses an expanding chamber
Barograph - continuously records the air pressure

6. Aneroid barometer

7. Aneroid barograph

8. Factors affecting wind
Wind is horizontal movement of air
Out of areas of high pressure
Into areas of low pressure
Unequal heating of the Earth causes these pressure difference
Controls of wind
Pressure gradient force (PGF)
Isobars—Lines of equal air pressure
Wind moves at right angles to the isobars
Pressure gradient—Pressure change over distance that different isobars indicate

9. Isobars on a weather map

10. Controls of wind Coriolis effect
Apparent deflection in the wind direction due to Earth’s rotation
Deflection is the right in the Northern Hemisphere and to the left in the Southern Hemisphere (like ocean currents, not water going down a drain)
The stronger the wind, the larger the deflection (does not affect wind speed)
Strongest at the poles and weakens towards the equator where it is nonexistent

11. The Coriolis effect

12. Controls of wind Friction
Only important near the surface (below 2,000 ft)
Acts to slow the air’s movement (lowers Coriolis effect)
Alters wind direction
Roughness of terrain determines the angle of airflow across the isobars

13. Upper air winds
Lack of friction with the Earth’s surface allows them to blow fast = higher Coriolis effect
Generally blow parallel to isobars— called geostrophic winds
Jet stream
“River” of air
High altitude
High velocity (75 to 150 mph)

14. The geostrophic wind

15. Surface and upper-level winds

16. Highs and lows Cyclone A center of low pressure
Pressure decreases toward the center
Winds associated with a cyclone
In the Northern Hemisphere
Inward (convergence)
Counterclockwise
In the Southern Hemisphere
Clockwise
Associated with rising air
Often bring clouds and precipitation

18. Anticyclone A center of high pressure
Pressure increases toward the center
Winds associated with an anticyclone
In the Northern Hemisphere
Outward (divergence)
Clockwise
In the Southern Hemisphere
Counterclockwise
Associated with subsiding air
Usually bring “fair” weather

19. Surface cyclones and anticyclones

20. Cyclonic and anticyclonic winds
mygeoscienceplace.com animation

21. General atmospheric circulation
Underlying cause is unequal surface heating
Tropical regions = more solar radiation received than lost; polar regions = less solar radiation received than lost
On a non-rotating Earth there is one atmospheric cell that redistributes the heat
Upper level air flows poleward
Surface air flows equatorward

23. Idealized global circulation
On the rotating Earth there are three pairs of atmospheric cells that redistribute the heat
Idealized global circulation
Equatorial low pressure zone
Rising air
Abundant precipitation
Reaches to degrees latitude

24. Idealized global circulation
Subtropical high pressure zone
Subsiding, stable, dry air
Near 30 degrees latitude
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

25. Idealized global circulation
Subpolar low-pressure zone
Warm and cool winds interact
Polar front—An area of storms
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

26. Idealized global circulation

27. Influence of continents
Seasonal temperature differences disrupt the
Global pressure patterns
Global wind patterns
Influence is most obvious in the Northern Hemisphere
Monsoon
Seasonal change in wind direction
Example, Asia: winter = cold = subsiding air = high pressure system = dry wind direction off land

28. Average pressure and winds for January

29. Average pressure and winds for July

30. The Westerlies Complex pattern in the midlatitudes (30-60 degrees)
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

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

32. Illustration of a sea breeze and a land breeze

33. Illustration of a valley breeze and a mountain breeze

34. Wind measurement Two basic measurements Direction Speed
Winds are labeled from where they originate (e.g., north wind—blows from the north toward the south)
Instrument for measuring wind direction is the wind vane

35. Speed—Often measured with a cup anemometer
Direction
Direction indicated by either
Compass points (N, NE, etc.)
Scale of 0 degrees to 360 degrees
Prevailing wind comes more often from one direction
Speed—Often measured with a cup anemometer

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

38. Questions?