1. Chapter 20 Weather 20.1Air Masses and Weather 20.2Fronts and Lows
20.3Thunderstorms and Tornadoes
20.4Hurricanes and Winter Storms
20.5Forecasting Weather

2. 20.1Air Masses and Weather
Objectives
What are the factors that determine the characteristics of an air mass?
How are types of air masses the same? Different?

3. Introduction
Meteorology: the study of processes that govern Earth’s atmosphere, helps make weather predictions possible
Meteorologist: a scientist that studies meteorology
Ranges from making weather forecasts to studying how tornadoes develop

4. Origin of an Air Mass
Air mass: a large body of air in the lower troposphere that has similar characteristics throughout
Temperature and humidity are nearly uniform

5. Origin of an Air Mass Cont’d
Temperature and humidity depend on where the air mass comes from
North = cold
South = warm

6. Origin of an Air Mass Cont’d
Moisture content of an air mass also depends on the underlying surface
Stays over land for a long time = dry
Stays over water + absorbs water vapor = humid

7. Origin of an Air Mass Cont’d
Air masses take the temperature and humidity of its place of origin
Characteristics may change as it travels
Affects the weather of the area it enters and gradually heats up as it moves over a warmer surface (cools down over a colder surface)

8. Types of Air Masses Air masses are classified by where they originate
Temperature: arctic, polar, or tropical region
Humidity: land (continental) or sea (maritime)

9. Continental Arctic (cA)
Originate in the arctic regions
Capable of causing extreme cold waves in the regions they enter
Very dry

10. Continental Polar (cP)
Originates over the inland regions of Alaska and Canada
Somewhat warmer than cA
Usually cold and dry
Sometimes can create precipitation
Lake-effect snow: air passes over the Great Lakes in the late fall when the water is still warm, the cold, dry air picks up moisture from the lakes, then deposits the heavy snow

11. Maritime Polar (mP) Originate over the ocean in high latitudes
Cold and damp
Not usually as cold as cP b/c oceans are warmer than land
Cools to dew point
Fog, clouds, or precipitation

12. Maritime Tropical (mT)
Originates over a warm tropical ocean
Warm and humid
Air from the Bahamas and the Gulf of Mexico moves clockwise around the high pressure over the Atlantic Ocean
Brings heat and humidity to Midwestern and Eastern US
Thunderstorms develop during the heat of the day
Humid air is heated from below and rises

13. Continental Tropical (cT)
Originates over deserts
Hot and dry
Often begins as a maritime air mass but becomes dry as it passes over mountains
Makes tremendous heat waves during summer
Doesn’t bring clouds or thunderstorms
A drought occurs if cT air advances into an agricultural region

14. 20.1 Exit Ticket