1. Understanding Wind

2. Review: What is air pressure?
Air pressure: the force of the weight or air on a surface
exerted in all directions

3. How is air pressure measured?
Barometer: a device used to measure air pressure
when air pressure ↑, mercury in the tube ↓
when air pressure ↓, the mercury in the tube ↑
pressure is measured in millibars (mb)
(Sea-level pressure
= mb)
Mercury
Aneroid

4. High v. low air pressure
High air pressure = more downward force, air sinks, clears the air of clouds!
Low air pressure = less downward force, air rises, causes clouds, rain, bad weather
High pressure sky
Low pressure sky

5. What causes wind? The sun is the ultimate source for most wind
Creates areas of high and low pressure

6. Mr. Fetch's Earth Science Class
Air flows from areas of higher pressure to areas of lower pressure. This causes air to move, creating WIND!
H L
YouTube:
Indoor
Skydiving
Mr. Fetch's Earth Science Class
Mr. Fetch’s Earth Science Classroom

7. What affects wind? Three factors combine to control wind:
pressure differences (pressure gradient)
the Coriolis effect
friction

8. How do pressure differences affect wind?
Differences in pressure = wind
The greater the difference in pressure, the greater the wind speed (= stronger wind)

9. What are isobars?
Isobars: lines on a map that connect places of equal air pressure

11. What is a pressure gradient?
Pressure gradient: the spacing between isobars
(remember that wind moves from higher pressure to lower pressure)
Closely spaced isobars = steep pressure gradient and strong winds
Widely spaced isobars = weak pressure gradient and light winds

12. Light Winds
(SLOW)
Strong Winds
(FAST)

13. What is the Coriolis effect?
The Coriolis effect describes how Earth’s rotation deflects all free-moving objects or fluids, including wind and water
Air does not move in a straight line because of Earth’s rotation.
Coriolis Effect

15. How does Coriolis effect change wind?
Changes only wind direction, not wind speed
stronger wind = greater deflection
strongest at poles, weakens toward the equator

16. YouTube: Coriolis

17. Coriolis Effect
Northern Hemisphere
Southern Hemisphere

18. How does friction affect wind?
“Bumpy” surfaces such as hills and mountains increase friction, which can slow wind and change its direction
If friction ↑, then wind speed ↓
If friction ↓, then wind speed ↑

20. What are jet streams?
Jet streams: fast-moving rivers of air that travel kilometers per hour ( mph) from west to east
They are above the “friction layer” and tend to blow parallel to isobars

21. Air pressure
& Jet streams

22. What type of weather would you expect?
What is a cyclone?
Cyclone = a center of low pressure
Pressure decreases from the outer isobars towards the center
Moves counterclockwise in Northern Hemisphere
What type of weather would you expect?

23. What type of weather would you expect?
What is an anticyclone?
Anticyclone = center of high pressure
pressure increases from the outer isobars towards the center
Clockwise in Northern Hemisphere
What type of weather would you expect?