1. Air Masses and Fronts
And precipitation

2. Precipitation
Precipitation is solid or liquid water that falls to Earth from clouds.
Four main types:
Rain, snow, sleet, and hail.

3. Precipitation Rain - most common form of precipitation.
Sleet forms when rain falls through a layer of freezing air.
Snow forms when temperatures are so cold that  water vapor changes directly to a crystalline solid.
Hail - balls or lumps of ice that fall from clouds. 
Water droplets are very tiny, smaller than the period at the end of a sentence. They combine with each other. Once reaching a certain size, they fall.

5. Air Masses p 543
 An air mass is a large volume of air with constant temperature and moisture content.
Their movement and interaction cause changes in weather.
Classified by water content and temperature.
These features depend on where the masses form.

6. Source Region p534
A source region is the area over which an air mass forms.
Eg. One source region is the Gulf of Mexico.
Air masses that form over this source region are wet and warm.

7. Naming Air Masses Air masses are abbreviated with two letters:
• 1st letter: describes water content 
• c – Continental (dry; forms over land) 
• m – Maritime (wet; forms over ocean) 

8. Naming Air Masses
2nd letter: describes temperature: T = Tropical P = Polar A = Arctic

10. Source Region A – Arctic (very cold) P – Polar (cold)
T – Tropical (warm) 
E – Equatorial (very warm) 
M – Monsoon
They can be combined in various ways… 
cP (Continental Polar) – dry, cold
mA (Maritme Arctic) – wet, very cold

11. Example cT stands for continental tropical
Air mass coming from tropical land areas.
In north America, these hot, dry air masses originate in the summer in the desert areas of southwestern United States.

12. Cold Air Masses in North America
Continental polar (cP) air masses form over northern Canada. They bring extremely cold winter weather. In the summer, cP air masses can bring cool, dry weather.
Maritime polar (mP) air masses form over the North Pacific Ocean. They are cool and very wet. They bring rain and snow to the Pacific Coast in winter. They bring fog in the summer.
Maritime polar air masses also form over the North Atlantic Ocean. They bring cool, cloudy weather and precipitation to New England.

13. Warm Air Masses
Maritime tropical (mT) air masses form over warm areas in the Pacific Ocean, the Gulf of Mexico, and the Atlantic Ocean.
They move across the East Coast and into the Midwest. In summer they bring heat, humidity, hurricanes, and thunderstorms to these areas.
Continental tropical (cT) air masses form over deserts and move northward. They bring clear, dry, hot weather in the summer

14. Fronts p 547
A Front is the area where two (or more) different air masses meet.
When air masses meet, the less dense air mass rises over the denser air mass.
Warm air is less dense than cold air.
Therefore, a warm air mass will generally rise above a cold air mass.
Less dense air rises, more dense goes below

15. Fronts
Type of front is determined by and named for which  air mass is advancing 
•Example: Cold front = cold air advancing 
Four types:
Less dense air rises, more dense goes below

16. Cold Fronts
A cold front forms when a cold air mass moves under a warm air mass.
The cold air pushes the less dense warm air up.
The warm air cools to its dew point and clouds form.
Cold fronts move quickly and bring heavy precipitation.
They cause abrupt changes in weather, including thunderstorms.

17. Cold Front
After a cold front has passed, the weather is usually cooler.
This is because a cold, dry air mass moves in behind the cold front.

18. Cold Fronts

19. Warm Fronts
Warm fronts form when a warm air mass moves over cold, denser air.
If the warm air mass is humid, precipitation will fall:
If it is dry, scattered clouds will form.
Followed by clear, warm weather

20. Warm Fronts
Warm fronts form where warm air moves over cold, denser air. If the warm air mass is humid, precipitation will fall: if it is dry, scattered clouds will form.

21. Occluded Front
An occluded front forms when a warm air mass is caught between two colder air masses. Cool temperatures and large amounts of rain and snow.

22. Occluded Front
Occluded Front  Forms  when a warm  air mass  is caught between two  colder  air masses. An  occluded front has cool  temperatures and large  amounts of rain  and s now.

23. Stationary Front
A Stationary Front forms when a cold air mass meet s a warm air mass, but neit her has the force to move t he other. Often brings many days of cloudy, and/or wet weather

24. Stationary Front
Sta>onary Front  Forms when a cold  air mass m eets a warm air mass, but  neither has the force t o move the other.  A sta*onary front o[en brings many  days of cloudy, and/or wet weather.

25. Temperature inversion
Normally air gets colder with height in the troposphere.
Temperature inversion occurs when a layer of cool air at the surface is covered by a layer of warmer air.
Form during clear, dry nights, when the ground and air cool rapidly.

26. Temperature Inversion

27. Effects of Temperature Inversions
Smoke and other pollution are trapped beneath the inversion - making sky hazy, sunsets red.
Inversions cause clouds just below the inversion to spread out and take on a flattened appearance.  
Temperature Inversions prevent thunderstorms from forming by keeping the warm, moist air from rising.

29. Practice Read pages 543-545 Answer page 546 #1-4 Read pages 547-549