1. Chapter 20.1
Air Masses and Weather

2. While You Read 20.1
What is an air mass and how does it typically gain its specific characteristics?
An air mass is a large body of air with similar characteristics throughout. It gains these characteristics from the air temperature and humidity in its place of origin. As it moves, it takes these characteristics with it, but may also change in response to new surroundings.

3. After You Read In the organizer below, record the characteristics of the five main types of air masses.
Place of origin
Direction of movement
Weather Impact
1. cA
Arctic Polar Regions
Southerly
Extreme cold
Little precipitation
2. mP
High latitude oceans
Southerly and toward U.S. center
Fog, clouds, precipitation
3. cP
Inland Alaska and Canada
Southeasterly
Cold, little precip, except lake effect snow
4. cT
Southern deserts
Northerly
Heat waves, drought
5. mT
Warm tropical oceans
Northerly and toward the U.S. center
Heat, humidity, thunderstorms

4. 20.2 Fronts and Lows While You Read
Define and explain the stage at which a low-pressure system produces the most intense storm.
A low-pressure system creates the most intense storm after about hours, when its warm front becomes occluded between cool air pushing northward in front of it and cold air pushing southward behind it.

5. In your science notebook, revise your definition of a front
In your science notebook, revise your definition of a front. Use information you find about each key term to help you expand your definition.
1.Warm front forms when warm air advances on cooler air; gradual slope as warm air rises slowly; large areas of precipitation, lasting several days.
2.Cold front forms when cold air advances on warmer air; steep slope as cold air sinks quickly, any precipitation Is brief; brings thunderstorms or cool breezes, depending on humidity in displaced warm air

6. 3. Occluded front forms when warm front is trapped between two cold fronts often causes cloudiness, precipitation.
4. Stationary front forms when front doesn’t move; warmer air rises within the front, causing precipitation; heavy storms due to stationary precipitation.

7. 20.3 Thunderstorms and Tornadoes: While You Read Thunderstorms develop:
A) in moist, stable air
B) in warming temps of afternoon
C) at frontal boundaries or when warm air hits other obstacles
This can cause
2. Lightning Tornadoes
Which is Which are
3. An electrical discharge when 5. Violently rotating columns
Positive and negative charges in a of air that touch the ground
Thundercloud collide and form a spark.

8. 20.3 After You Read
List and describe some ways that meteorologists predict tornadoes and warn people of the related danger.
Meteorologists use conventional radar to map precipitation in an area and Doppler radar to identify the wind directions within a storm. Doppler radar can identify the mesocyclone wind movements often associated with tornadoes. Tornado watches and warnings advise people of the possible tornado danger at any given time.

9. 20.4 Hurricanes and winter storms: While You Read
1. Mild atmospheric disturbance over tropical ocean
2. Humid air rises
3. Air cools and condenses, releasing heat
4. Cycle of air movement continues
5. Coriolis effect rotates air in the storm
Storm moves according to global wind patterns

10. 20.4 Hurricanes and winter storms: While You Read
7. a) storm surge
b) damaging winds
c) heavy rain
d) inland flooding
e) heavy surf
8. Storm weakens over cooler land or water

11. After You Read
List and describe the characteristics of a blizzard.
A blizzard is a special kind of mid-latitude low pressure system. It has winds over 56km/hour, temperatures below -7ºC, and falling or blowing snow that reduces visibility.

12. 20.5 Forecasting Weather While You Read
1. Visible and infrared pictures
2. Temperature, humidity, pressure
3. Surface observations
4. Station models
5. Surface weather map
6. Knowledge of local weather patterns

13. 20.5 After You Read
Explain why it is important that weather station models and surface maps be readable to meteorologists in any country.
In this way, meteorologists around the world can share data, and forecast farther ahead and provide necessary warnings of bad weather.