1. Weather Patterns & Severe Storms
Chapter 20

2. 20.1 and 20.2

3. classified according to where they originate
air mass
a large body of air that has the same temperature and humidity throughout
classified according to where they originate
during the time the air mass is over an area, it determines the weather of that area

4. As an air mass moves away from where it formed and into a new area, it carries its temperature and moisture conditions with it.
Example: Frigid Canadian Air Mass

5. Classifying Air Masses
Source Region: the area over which an air mass gets its characteristic properties of temperature and moisture.
There are 4 source regions used to name air masses…

6. Maritime: humid air mass (m)
1. Originates over the ocean
Maritime: humid air mass (m)

7. Continental: dry air mass (c)
2. Originates over the land
Continental: dry air mass (c)

8. Polar: cool air mass (P)
3. Originates in cold air
Polar: cool air mass (P)

9. Tropical: warm air mass (T)
4. Originates in warm air
Tropical: warm air mass (T)

11. Maritime Tropical Atlantic (mT)
Which one affects us?
Maritime Tropical Atlantic (mT)
moves northward across eastern United States
brings mild & cloudy winters and hot & humid summers with hurricanes & thunderstorms

13. Fronts: boundary between two air masses having different temperatures and humidity
Types of Fronts

14. warm front warm air gradually rises & cools to produce clouds
Warm air moves into an area formerly covered by cooler air
warm air gradually rises & cools to produce clouds
brings hot, humid days and light-to-moderate precipitation over a large area

15. Warm Front Animation

17. cold front Cold, dense air moves into a region occupied by warmer air
Form and advance rapidly, forcefully lifting air
Brings heavy downpours and gusty winds (squalls) followed by clear cold air (high pressure)

18. Cold Front vs. Warm Front
Cold Front Animation
Cold Front vs. Warm Front

21. occluded front When an active cold front overtakes a warm front
Cold air wedges the warm front upwards
Complex weather results…may start with heavy rain and strong wind as warm front is lifted, then shift to long period of light rain

24. stationary front Surface position of the front does not move
Air on either side of a front moves parallel to the front
Gentle to moderate precipitation

27. always accompanied by lightning, thunder, and usually rain
Thunderstorms
small-area storms formed by the strong upward movement of warm, moist, unstable air
always accompanied by lightning, thunder, and usually rain
formed from cumulonimbus clouds

28. Observe an animation of a thunderstorm.

29. Two groups of thunderstorms:
Air-mass thunderstorm:
form in warm, moist air mass
occur in spring and summer- last less than 1 hour
single, widely scattered