1. Chapter 19: The Atmosphere in Motion
19.1 Air Pressure and Wind
What is Air Pressure?
The weight of the atmosphere as it pushes down upon Earth’s surface
Pressure is a force per unit area
Units: lbs/in2, millibars, hectopascals, inches of mercury
pressure decreases with height

2. air pressure is measured with a barometer
What is Air Pressure?
air pressure is measured with a barometer
mercury barometers use mercury in a glass tube; the mercury rises and falls

3. aneroid barometers use a metal capsule filled with air that expands and contracts with air pressure changes

4. Why Does Air Pressure Change?
As temperature increases, pressure decreases
Molecules are farther apart in warmer air; closer together in cooler air
As humidity increases, pressure decreases
Water vapor weighs less than oxygen or nitrogen gas; pushes out these gases and exerts less force

5. Why Does Air Pressure Change?
Isobars: lines of equal pressure on a weather map
pressure is reduced to sea-level (removes elevation differences) to track high and low pressure systems
High pressure area = general good weather (clear, cool)
Low pressure area = general bad weather (cloudy or precipitation, warmer)

6. Why Does Air Pressure Change?
Pressure Gradient: pressure change over distance
isobars spaced closer together = windy
isobars spaced far apart = little wind

7. Weather Map

8. What Makes the Wind Blow?
High pressure always moves toward low pressure to achieve balance
The greater the pressure difference, the stronger the winds
Ultimately caused by unequal heating of Earth’s surface

9. Measuring Wind
Wind vane: shows wind direction (a south wind comes from the south and moves toward the north)
Anemometer: measures wind speed

10. Review Questions
What is air pressure? How does air pressure vary with elevation?
Explain why humid air is lighter than dry air.
How would you be able to tell where the windiest areas are on a weather map that shows isobars?