1. Unit 2: Surface Processes and the Hydrosphere Lesson 4: Air Masses and Fronts ( Heath Earth Science – Pg. 542-555)

2. Today’s Objectives  Describe the function of the hydrologic cycle, including:  Identify the characteristics of the main kinds of air masses (eg., continental polar) and the resulting weather patterns

3. Air Masses  Before we begin, lets check out some videos!  Weather fronts song  Air masses video  So, what IS an air mass?  An air mass is a huge body of air that is characterized by a similar temperature and moisture throughout.  Usually 1500 kilometers or more across and perhaps several kilometers thick  Located in the lower troposphere

4. Formation of Air Masses  Air masses are so big, just two or three air masses can cover an entire continent  How do they form?  When a parcel of air maintains the same position for days or even weeks over a large, uniform surface, such as a cold ocean, or a warm area of land  If a body of air stays over the South China Sea for several days, it would become warm and moist  If a body of air stays over Heilongjiang province for several days, it would become cold and dry  Air masses originate in parts of the world where winds are light, such as in the polar and subtropical high pressure belts where air is sinking

5. Types of Air Masses  Maritime Tropical (mT)  Originates over tropical ocean areas  Warm and moist air  Continental Tropical (cT)  Originates over tropical land areas  Warm and dry air  Maritime Polar (mP)  Originates over cold ocean areas  Cold and moist air  Continental Polar (cP)  Originates over cold land areas  Cold and dry air  Continental Arctic (cA)  Originates over arctic regions  Very cold, very dry air

6. Weather in Air Masses  Air masses are so large, they may take several days to pass a given place, for example:  When mP air comes in from the northern ocean, it brings cool, humid weather  mT air masses bring mild, humid weather in winter, or hot, humid weather in summer  Air masses will change as they move southward or northward over different surfaces  Dry air masses become moist over moist ground  Moist air masses become dry over dry ground  The faster an air mass moves, the less it is affected by the surface it crosses, therefore, the faster moving air masses bring more extreme weather changes

7. Weather Fronts  The boundary where two or more air masses meet is called a front  Because different air masses have different temperatures and humidities, the front is where weather changes  An approaching front means a change in weather  The greater the difference between air masses, the greater the change in weather  On weather maps, fronts are drawn in regions of great change in temperature and wind direction

8. Weather Fronts  Weather fronts video  Quick Quiz!  What are the following weather fronts called, and what kind of weather is found at each one?

9. Kinds of Fronts  1) Warm Fronts  Warm air is pushing ahead, and displacing colder air  Warm air is less dense than cold air, so it slowly rises above the cold air  As it rises, the air cools, and the moisture condenses into clouds, and moderate precipitation  Cloud types: nimbostratus, altostratus, cirrostratus, cirrus  Weather: Increasing temperature, light winds and moderate precipitation

10. Warm Front

11. Kinds of Fronts  2) Cold Fronts  Cold air is pushing ahead and displacing warm air  Cold air is more dense than warm air, so it causes the warm air to rapidly rise above the cold air  Cold fronts move more quickly and are steeper than warm fronts, so they bring a more rapid, violent change in weather  Cloud types: cumulonimbus  Weather: Decreasing temperature and heavy precipitation and wind, often thunderstorms and/or hailstorms

12. Cold Front

13. Kinds of Fronts  3) Stationary Fronts  When air flow on both sides of front is neither toward the cold air mass or warm air mass, but almost parallel to the front, the front does not move across the surface  This is called a stationary front  Warm air may gently rise over cold air causing light precipitation  May eventually become a warm or cold front  Cloud types: stratus, altostratus, nimbostratus  Weather: Light precipitation, large temperature difference across the front

14. Stationary Front

15. Kinds of Fronts  4) Occluded Fronts  When a faster moving cold front overtakes a slower moving warm front, the result is an occluded front  The cold air wedges the warm front upward, a new front is created between the advancing cold air and the cool air that the warm front was gliding over  The wedge of warm air rises quickly and causes heavy rain initially, that gradually diminishes  Cloud types: cumulonimbus, nimbostratus, variable  Weather: highly variable, but usually moderate to high precipitation, cooling temperatures

16. Occluded Front

17. Cloud Types

18. Nimbus is Latin for rainstorm, so Cumulonimbus and Nimbostratus are precipitating cloud types

19. Homework  Topic Questions, pg. 546, #1-4  Topic Questions, pg. 549, #6-8  Topic Questions, pg. 551, #9-12  Review (matching), pg. 554-555

20. Group Project – Due October 12  You will be presenting your projects on Saturday  Remember – Your presentation should be between 10-15 minutes long  Each group member must have a speaking role in the presentation!