1. Unit 3: Meteorology Part 2
Air pressure and Winds

2. Air Pressure
REVIEW

3. Air Pressure
Air pressure = force that is the result of the mass and weight of a column of air pushing down on an area
Air in the atmosphere has pressure because air has mass.
The less air there is in a given volume, the less dense the air
Air at higher altitudes have less dense air, which means less pressure.

4. Measuring Air Pressure
Barometer: device that is used to measure air pressure
Two types of barometers
Aneroid
Measure the bulges in a thin metal disk that covers a chamber.
Mercury
Measures the height of mercury that is in a glass tube

5. Isobars and Air Pressure
Air pressure is an important factor that affects weather
Changes in air pressure help meteorologists predict how weather will change

6. Isobars and Air Pressure (cont.)
Air pressure readings from barometers are shown on weather maps with lines called isobars
Isobars = lines on a weather map that are drawn to connect areas that have the same air pressure

7. Dry Air vs. Wet Air Dry Air = 99% N and O Wet Air = 96% N and O
Mass = 1,200 g/m³
Wet Air = 96% N and O
Mass = 1,190 g/m³

8. Dry Air vs. Wet Air Dry air weighs more than humid (wet) air
When drier air moves into a region, air pressure will increase
Dry air has more mass (more molecules)
When wet air moves into a region, air pressure will decrease
Wet air has less mass (less molecules)

9. Low Pressure Warmer air As the warm air rises, it begins to cool
atoms are more spread out
Less pressure
As the warm air rises, it begins to cool
Capacity decreases
Water begins to condense
Energy is released because of the condensation
Clouds develop
Energy that is released powers storms

10. High Pressure Colder Air As the cold air sinks, it begins to warm
Atoms are more “packed” together
High pressure
As the cold air sinks, it begins to warm
Capacity increases
Water evaporates and stays as water vapor (gas form)
Clouds are unable to develop
Clear skies!

11. Low Pressure vs. High Pressure

12. Air Movement Up and down movement of air creates cloudy or clear skies
Low pressure (Rising air ) = cloudy
High Pressure (Sinking air) = clear

13. WIND

14. What is wind? Wind = when air moves from high pressure to low pressure
Two types of wind:
Global
Local

15. Global Winds
HIGH PRESSURE
Caused by differences in air pressure due to heating differences on Earth
Low pressure at the equator
High pressure at the poles
These differences cause global wind patterns that determine climate for each region
LOW PRESSURE

16. Global winds are named for the direction that they blow FROM.

17. Coriolis Effect
Air moves from high pressure (poles) to low pressure (equator)
So why doesn’t the wind move straight from the poles the to equator?
The Earth ROTATES!
Because the Earth rotates or spins, the winds seem to curve.
This is called the Coriolis Effect.

18. Jet Streams
POLAR EASTERLIES
Jet Stream = boundaries between the polar easterlies and the prevailing westerlies
PREVAILING
WESTERLIES

19. Jet Streams

20. Local Winds
Caused by a difference in air pressure at the surface of the Earth
Winds are created when air shifts from high pressure to low pressure
REMEMBER, global winds are caused by heating differences, not pressure differences.

21. Understanding Local Breezes
Which heats up faster and gets hotter?
Land or Water
Which cools down faster and gets colder?
During the day, when the sun is out, which would be warmer?
During the night, when the sun is down, which would be cooler?