1. Air Pressure and Wind
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2. WHAT IS PRESSURE?
Pressure is the amount of Force per unit area.

3. Example:
What pressure does a 50 kg person exert on the ground when standing on one foot that measures 20 cm x 8cm?
Pressure = weight/area

4. Answer:
What pressure does a 50 kg person exert on the ground when standing on one foot that measures 20 cm x 8cm?
50kg/ (20cmx8cm) = 0.31 kg/cm2
What if person is standing on both feet?

5. Answer:
What pressure does a 50 kg person exert on the ground when standing on one foot that measures 20 cm x 8cm?
50kg/ (20cmx8cm) = 0.31 kg/cm2
What if person is standing on both feet?
Half the pressure: kg/cm2

6. WHAT IS AIR PRESSURE?
Air Pressure is the weight of the atmosphere per unit area.
At the surface, the atmospheric pressure is about 1kg/cm2

7. Air Pressure is exerted in all directions

8. Atmospheric Pressure = Barometric Pressure

9. Measuring Air Pressure p523
Air pressure is measured with an instrument called a Barometer.
There are two main types of barometers

10. Measuring Air Pressure p523
Aneroid barometer measures the atmospheric pressure using the expansion of a metal.

11. Measuring Air Pressure p523
Mercury barometer measures the atmospheric pressure by adjusting the height of mercury inside a tube.

12. Air Pressure is measured in millibars p524
Millibar is a measure of the actual force of the atmosphere pushing down on a surface.
Standard pressure at sea level is millibars

13. Air Pressure drops with altitude
At higher elevations, there are fewer air molecules above a given surface than a similar surface at lower levels.
An altimeter is an instrument used by air craft to measure height above sea level.
Barometer reading drops about 1 cm for every 123 meter within the first few km of atmosphere.
Then pressure changes more slowly as air thins rapidly.
Half the weight of the atmosphere is within 5.5 km of surface of Earth.

14. Factors Affecting Air Pressure p525
Changing temperature: warm air is lighter than cold air because the molecules of warm air are farther apart.
So when warm air replaces an equal volume of cold air, the air pressure at the ground falls.
Conversely, the arrival of colder air higher up causes the pressure t ground to rise.

15. Factors affecting Pressure p525
Changing humidity: the more water vapor in the air the lighter it is.
When water vapor enters the atmosphere, it pushes out an equal volume of dry air.
Water vapor is lighter than the nitrogen and oxygen it pushes out. Therefore, humid air weighs less than dry air and exerts less pressure.

16. Pressure indicates weather
High pressure associated with “fair weather”
Low pressure associated with “rain”
Falling pressure associated with increasing cloudiness
Rising pressure associated with clearing conditions

17. Indicating Pressure
pressure reading in millibars (mb).

18. Isobars Isobars are lines that connect areas of equal air pressure.
Pressure Gradient is the spacing between the lines, indicate pressure change over a given distance.

19. What is the isobar interval on this map?

20. What is the isobar interval on this map?
4 millibars

21. Cyclones and Anticyclones
Isobars that decrease in value inward indicate a low-pressure center (L), or cyclone .
If the value increases toward center, this indicates a high-pressure center (H), or anticyclone.

22. Cyclones – add this to your notes
Cyclones - low pressure in center, spin inward
Effects: heavy rain, strong wind, storms, tornadoes. 
Anticyclone -  high pressure in center, spin outward.
air slowly circulates in a clockwise (northern hemisphere) or counterclockwise (southern hemisphere). 
Associated with calm, fine weather.

23. Anticyclones
a weather system with high atmospheric pressure at its center, around which air slowly circulates in a clockwise (northern hemisphere) or counterclockwise (southern hemisphere) direction. 
Anticyclones are associated with calm, fine weather.

24. What's the difference between cyclone and hurricane or typhoon?
the location where the storm occurs. 
Hurricanes, cyclones, and typhoons are all the same weather phenomenon; 
we just use different names for these storms in different  places. 
In the Atlantic and Northeast Pacific, the term “hurricane” is used.

25. Where are the areas of high and low pressures?
Is A an area of high pressure or Low? B? A B

26. Practice
Read pages
Answer 1-5 p526

27. Wind Direction and Isobars
Winds flow roughly parallel to the isobars.