1. Air
Pressure
And
Wind
Chapter 19

2. Air
Pressure

3. Air pressure: the pressure exerted by the weight of the air above
Average air pressure at sea level
= 1 kg per cm2
= mass of a 50-passenger school bus!

4. Why aren’t we crushed under the weight?
Air pressure is exerted in all directions and the equal pressures balance each other out

5. Measuring
Air
Pressure

6. Barometer: instrument for measuring air pressure
Mercury Barometer: liquid mercury in a glass tube senses changing atmospheric pressure
Water Barometer

7. Air pressure increases
 mercury rises (“rising barometer”)
Air pressure decreases
mercury falls
(“falling barometer”)

8. Aneroid Barometer: small metal chamber with some air removed senses changes in pressure

9. Standard sea-level pressure
= 1 atmosphere
= inches of mercury
= millibars

10. Wind

11. Wind: Horizontally moving air due to differences in air pressure
Moves from areas of high pressure to areas of
low pressure
Caused by unequal heating of Earth’s surface
 Wind is powered by the sun!
3 factors combine to control wind:

12. Isobars: lines on a weather map showing equal air pressure
1. Pressure Differences – affects wind speed
The greater the pressure difference, the greater the wind speed.
Isobars: lines on a weather map showing equal air pressure

13. Pressure gradient: pressure changes expressed by isobars
Closely spaced isobars  steep pressure gradient & high winds
Widely spaced isobars  weak pressure gradient & light winds

14. 2. Coriolis Effect – affects wind direction
The deflection of wind due to Earth’s rotation

15. Deflects to the right (clockwise) in the Northern Hemisphere
Deflects to the left (counterclockwise) in the Southern Hemisphere

16. The stronger the wind speed, the greater the deflection
Strongest deflect at poles, and weakens towards equator, becoming nonexistent

17. 3. Friction – affects wind speed AND direction
Air above the ground moves at a right angle to isobars because pressure gradient and the Coriolis Effect are balanced
Jet streams: fast, high-altitude winds ( km/hr) flowing west-to-east
About km above the ground
Air on the ground is effected by friction from the terrain , so the wind speed slows down and wind direction changes because the Coriolis Effect is decreased.

18. http://www.weatherimages.org/data/imag192.html(current position)

19. Highs
&
Lows

20. Cyclone
Low pressure centers (“Lows”)
Anticyclone
High pressure centers (“Highs”)

21. Cyclone
Pressure decreases from the outer isobars toward the center isobars
Anticyclone
Pressure increases from the outer isobars toward the center isobars

22. Cyclone
Anticyclone
Winds blow counterclockwise around a low
Winds blow clockwise around a high

23. Cyclone
Rising air
 cloud formation & precipitation
 Bad weather in any season
Anticyclone
Sinking air
 clear skies & low humidity
 Good weather in any season

24. Cyclone
Air above cyclone diverges (spreads out)
Anticyclone
Air above
anticyclone converges
(comes together)