1. What Causes Our Daily Weather To Change? Change in our weather is a result of a change in air masses.

2. What Is An Air Mass?  An air mass is a large body of air in the lower troposphere that has similar characteristics throughout.  An air mass can be several thousand kilometers in diameter and several kilometers high  The temperature and humidity are nearly uniform throughout  The temperature and humidity depend on where the air mass originates.

3. Where Do Air Masses Originate?  Air masses originate in source regions.  Source regions have flat, uniform composition and light winds.

4. How Are Air Masses Classified?  The temperature of each air mass depends on whether the air mass originates in an arctic, polar, or tropical source region.  The humidity depends on whether the air mass originates over land ( continental ) or over water ( maritime ).  Air masses are classified by temperature and humidity.

5. Air Mass Classification Air MassSymbol Temperature & Humidity Where Air Mass Forms Area of the Country that is Impacted Southwest and Southeast U.S. Northeast and Northwest U.S. Midwest to Eastern U.S. Southwest U.S. Northern U.S. in winter Maritime Tropical Continental Tropical Maritime Polar Continental Polar Continental Arctic mT mP cA cT cP Warm and Wet Very Cold and Dry Cold and Wet Warm and Dry Cold and Dry Low latitudes over water High latitudes over water High latitudes over land Low latitudes over land High latitudes over land

6. mP – West Coast Tends to be unstable Heavy rains as cool moist air flows over mountains along west coast mP is modified by time it reaches interior of US, though is milder that cP (how???)

7. mP – East Coast Not as common as west coast mP Colder than west coast mP Usually brought onshore by high pressure to the north of us and/or low pressure to the south moving up the coast.

8. Maritime Tropical Air Mass West CoastEast Coast Brings showers and thunderstorms Summer - Brings showers and thunderstorms Winter – snow or heavy rain

9. Continental Tropical Air Mass – Summer only

10. Continental Polar Air Mass

11. Types of Air Masses

12. Global Source Regions For Air Masses

13. Stability of an Airmass  Changes to the stability of an air mass can result from temperature differences between an air mass and the surface  The stability of an air mass may be shown using a third letter:  "k“ = un-stable or "w“ = stable

14. Air Mass Source Region – Practice Quiz Continental Arctic Continental Polar Maritime Polar Maritime Polar Continental Tropical Maritime Polar Maritime Tropical Maritime Tropical 1. 2. 3. 4. 6. 8. 7. 5.

15. Weather Patterns Weather patterns are caused by the movement of air masses and what happens when different air masses collide. Front – boundary between two air masses. Air masses meet but do not mix. The boundary line between the offensive and defensive lines would be a front. Front Great Air Mass Poor Air Mass

16. NameFormationCloudsWeatherSymbol Wind, heavy rain Thunderstorms Lightning Colder air after front passes Clouds then gentle, steady rain arriving before the front Steady rain for days along front if stationary too long Strong winds and heavy rains Cold Altocumulus Cumulonimbus Warm Stationary Occluded Air masses stop moving Cirrus Cirrostratus Altostratus Nimbostratus Cirrus Cirrostratus Altostratus Nimbostratus Cumulus Cumulonimbus

17. Fronts  Cold Front: boundary between advancing cold air mass & a warmer air mass it is displacing  Rising warm air usually produces precipitation if wet  Air becomes colder after front passes

18. Fronts  Warm Front: boundary between advancing hot air mass & a colder air mass it is displacing  1 st clouds days in advance, then RAIN  Air becomes warmer after front passes

19. Fronts  Occluded Front: when cold front ‘catches up’ to a warm front, producing clouds & precipitation

20. Fronts  Stationary front: when a front stops moving forward, producing clouds & precipitation – causes floods if stationary too long

21. The Life Cycle Of A Cyclone (Low Pressure System) The United States often experiences weather due to the boundary between the cold air masses of the Polar Easterlies and the warm air masses of the Prevailing Westerlies. This arrangement results in a weather conditions called a Traveling Cyclone. The formation of a cyclone is called "cyclogenesis". Cyclones form in areas of low pressure.

22. The Life Cycle Of A Cyclone (Low Pressure System) The process begins when two air masses of different densities and temperatures move in opposite directions along a polar front resulting in a large temperature gradient.

23. The Life Cycle Of A Cyclone (Low Pressure System) Cyclones are the result of instability forming along the polar front, resulting in a wave developing between the cold and warm air masses. The fronts are separated, and the warm front begins to move north as the cold front begins to move south

24. The Life Cycle Of A Cyclone (Low Pressure System) The pressure at the center of the cyclone continues to get lower as the cyclone continues to form. Clouds ahead of the warm front begin to precipitate over a large area of surface, and clouds along the cold front begin to precipitate over a narrow stretch [along the cold front]. As the pressure is lowered, wind speeds increase.

25. The Life Cycle Of A Cyclone ( Low Pressure System) The cold front continues to move eastward, speeding to overtake and undercut the warm front. This is called occlusion. It is at this point which the cyclone is at its peak. The area of the warm sector is decreasing.

26. The Life Cycle Of A Cyclone (Low Pressure System) By the time the cold front completes occlusion with the warm front, the air in the warm sector is lifted upward as the cold air replaces it at the surface. The system is stabilizing.

27. Weather Forecasting  Satellites  Radiosondes  Surface observations

28. Station Model

29. Surface Weather Map

30. Locating a Front  Wind direction changes  Temperature changes sharply  Dew Point changes sharply

31. Forecasting  Computer models take current data & plug it into equations to predict weather  Meteorologists take computer models & tweak them to fit their experience with local conditions

32. Forecasting Trend Method: using past movement of a front & precipitation to predict future movement

33. Practice reading a weather map