1. Air Masses and Severe Weather

2. Air Mass
Air Mass is a large body of air where temperature and moisture content are similar throughout.
Moisture content and temperature are determined by the area over which the air mass is formed called source areas.

3. Air Mass Air Mass is designated by a two- letter symbol.
The first letter indicates moisture content.
The second letter represents temperature.
See chart

4. Cold Air Mass
There are three polar air masses that influence the weather in the United States.
Continental polar, Maritime polar over the North Pacific Ocean, Maritime polar over the North Atlantic ocean

5. Warm Air Masses
There are three warm air masses that influence the weather in the United States.
Maritime tropical that develops over the Gulf of Mexico, Maritime tropical that develops over the Pacific Ocean, Continental tropical

6. Fronts
Fronts are the boundary between air masses of different densities and usually different temperatures.
The four types of fronts are cold front, warm front, occluded front, stationary front.

7. Cold Front
Forms where cold air moves under warm air, which is less dense, and pushes the warm air up.
Cooler weather follows cold fronts.
Cold fronts bring thunderstorms, heavy rain or snow.

8. Warm Front Forms where warm air moves over cold, denser air.
Warm fronts bring drizzly rain and are followed by clear warm weather.

9. Occluded Front
Forms when a warm air mass is caught between two cooler air masses.
Occluded front has cool temperatures and large amounts of rain and snow.

10. Stationary Front Forms when a cold air mass meets a warm air mass.
A stationary front often brings many days of cloudy, wet weather.

11. Air Pressure and Weather
Cyclones and Anticyclones

12. Air Pressure and Weather
Cyclones
An area in the atmosphere that has lower pressure than the surrounding areas.
Air masses converge; winds spiral toward the center

13. Air Pressure and Weather
Anticyclones
Areas where air diverges or moves apart and sinks. Sinking air is denser than surrounding air.
Rotates in a direction opposite to Earth’s rotation.

14. Weather and Air Pressure
Cyclones form clouds and rain when the rising air cools.
Cyclones cause stormy weather.
Anticyclones cause dry, clear weather because as the air sinks it gets warmer and absorbs moisture.

15. Severe Weather

16. Thunderstorms
Thunderstorms are a usually brief, heavy storm that consists of rain, strong winds, lightning, and thunder.
Thunderstorms can occur along cold fronts, but only require warm and moist air near Earth’s surface and an unstable atmosphere.

17. Thunderstorms
An unstable atmosphere occurs when the surrounding air is colder than the rising air mass.
When the rising air reaches its dew point, the water vapor in the air condenses to form dark cumulonimbus clouds.

18. Lightning
Lightning is an electric discharge that takes place between two oppositely charged surfaces, such as between a cloud and the ground, between two clouds, or between two parts of the same cloud.

19. Thunder
Thunder is the sound caused by the rapid expansion of air along an electrical strike.
Balloon Demonstration

20. Severe Thunderstorms
Severe thunderstorms can produce one or more of the following conditions: high winds, hail, flash floods, and tornadoes.

21. Tornadoes
A destructive, rotating column of air that has very high wind speeds , is visible as a funnel-shaped cloud, and touches the ground.
1% of all thunderstorms have a tornado.
The average tornado has wind speeds between 120 and 180 km/h

22. Hurricanes
A severe storm that develops over tropical oceans and whose strong winds of more than 120 km/h spiral in toward the intensely low-pressure storm center.
At the center of the hurricane is the eye, which is a core of warm, relatively calm air with low pressure and light winds.
Hurricanes can vary in size from 160 to 1,500km and can travel thousands of kilometers.

23. Hurricanes
Hurricanes get their energy from the condensation of water vapor.
Moisture is added to the air through evaporation from warm water which eventually condenses and gives the hurricane more energy.

24. Severe Weather Safety Tornado Flood Hurricane Thunderstorm Safety
Watch- an alert that a tornado may happen
Warning- a tornado has been spotted
Go to a basement/cellar or a windowless room in the center of the building; outside lay in a deep ditch
Flood
Find a high place to wait out the flood; stay out of floodwaters
Hurricane
Have a disaster supply kit, evacuate if necessary, cover windows with plywood
Thunderstorm Safety
Stay away from water
If you are outside stay away from trees and crouch down