1. How Does Air Move?

2. Air Masses
Air mass – huge body of air with uniform temperature and humidity. Can cover thousands of square kilometers.
Air masses take on the characteristics of the earth’s surface where they form (cold, hot, wet, dry)

3. Air masses are named after their source regions.
Arctic: extremely cold
Polar: cool or cold
Tropical: warm
Continental: forms over land (dry)
Maritime: forms over oceans (wet)

4. What kind of air mass would form over:
Canada?
Continental polar
The north Pacific or Atlantic?
Maritime polar
The Mojave Desert?
Continental tropical
The Gulf of Mexico?
Maritime tropical
The Arctic Circle?
arctic
The boundary between air masses is called a front.

5. Weather Fronts Warm front: warmer air replaces colder air
Warm air mass overtakes a slower cold air mass.
Warm air mass rises over the cold air mass, forming clouds and long, steady rain.

6. Cold fronts: colder air replaces warmer air
A fast-moving cold front can force warm air to rise quickly, producing cumulonimbus clouds and thunderstorms.
Squall line: line of violent thunderstorms along a cold front

7. Wind Remember, cold air has higher pressure than warm air.
The pressure differences
between air masses cause
wind.

8. Global Winds Six huge convection currents form in the air above Earth.
The sun heats earth unevenly. At the Equator, warm air rises then “slides downhill” to the poles.
The spin of the earth causes the air to move diagonally.
This creates predictable wind patterns across the globe.

9. Storms

10. Thunderstorms
On warm summer days, convection causes cumulonimbus clouds to form, often resulting in thunderstorms.
Marked by cold downdrafts, heavy rain, hail, and lightning.

11. Lightning
Electrical discharge between clouds or between a cloud and the ground
Can be 50,000 degrees F
Heat causes air around it to expand, then quickly collapse, creating a sound we call thunder.

12. How lightning forms
The top of a thunderhead has a positive charge, and bottom has negative charge.
Negative charges drop to ground to form a stepped leader (path of ionized gas)
Positive charges from the ground connect with the stepped leader. This completes a circuit and creates a lightning bolt!

13. Cloud to Cloud Lightning
The same process also creates cloud to cloud lightning.
Instead of connecting with positive charges on the ground, the stepped leader connects with positive charge within the same cloud or in another cloud.

15. Tornadoes

16. Tornado Facts
A tornado is a narrow funnel extending from a cumulonimbus cloud.
Texas averages more than any other state at about 100 per year
Wind speeds up to 320 mph
Base can be up to 3 miles across
Rated based on estimated wind speed (F0-F5)

17. How a tornado forms and moves
Most tornadoes occur between 3:00 and 9:00 pm from March to May.
Tornadoes seldom move in a straight line and are invisible until they pick up dirt and debris.
Tornado watch: Conditions are right for a tornado to form.
Tornado warning: A tornado has been spotted in your area. Take shelter immediately.

19. Hurricanes

20. Hurricanes are giant cyclonic windstorms that form over tropical or subtropical oceans.
Can be up to 600 miles across
Winds mph
High winds, high tides, and heavy rains

21. Hurricane Formation
Hurricanes form over warm tropical oceans (water must be at least 80 degrees F) between 10 and 15 degrees north or south of the equator.
Form June through October

22. How hurricanes form
Hot, moist air rises near the doldrums and cools adiabatically, forming an area of low pressure.
Cooler air rushes in, which causes the wind to flow in a spiral. (counterclockwise in NH, clockwise in SH)
Fueled by more rising hot, moist air, the storm builds on itself and escalates. When wind reaches 74 mph, it is classified as a hurricane.

23. Anatomy of a hurricane

24. How hurricanes move
Trade winds blow hurricanes to the west. The Coriolis force deflects their path to the right in the NH.
Friction and absence of warm water cause them to slow down then dissipate once they hit land.

25. Hurricane Effects
Far out at sea, high winds cause a storm swell – high, fast-moving waves. These hit first, along with heavy rain.
Storm surge – water pushed ahead of the storm. The surge of high water can be 35 feet tall!
Since the storm is a spiral, winds can come from any direction, depending on your location.
In the eye, air is calm, and the sky is blue.
Then winds start again in the opposite direction, along with more rain.

27. Hurricane Safety: