2. Air Masses, Fronts and Storms Pages 670-673

3. An air mass is a large "bubble" of air that sits over an area a long time. It sits there long enough to take on the characteristics of the area.

5. General Characteristics Maritime will be moist Continental dry Maritime and continental are abbreviated with small letters Arctic will be cold Polar not quite as cold Tropical Warm

6. The names are a combination of letters that tell where they formed 1 st over land or sea Continental c Maritime m Small letters 2 nd approximate latitude Artic A Polar P Tropical T Capital letters

7. Continental Polar (cP) Source Formed over continent Near poles Characteristics Cold Dry

8. Put the letters together to form the name of an air mass What will the characteristics of these air masses be? mA cA cT cP

9. Maritime Tropical (mT) Source Near equator and over water Characteristics Warm Moist

10. Characteristics cP continental polar cold, dry, stable cT continental tropical hot, dry, stable air aloft--unstable surface air mP maritime polar cool, moist, and unstable mT maritime tropical warm, moist, usually unstable

11. OK, remember air masses? Continental polar and continental arctic produce cold dry winters Maritime polar and maritime arctic pick up moisture and bring cool moist weather Continental tropical - hot, dry Maritime tropical - warm, humid

12. Fronts Fronts form where air masses collide Cold front Cold air invades warm air Warm front Warm air invades cold air We find a variety of weather at fronts

13. Front types When the two masses meet, the differences in temperature, moisture and pressure can cause one air mass to override the other. They are named for the invader If cold air invades warm air--cold front If warm air moves into cold air--warm front If neither air mass is moving a stationary front If a cold front over takes a warm front it is an occluded front occluded front Fronts are accompanied by wind, clouds, rain, and storms.

14. Approaching cold front

15. Approaching warm front Less dense air gradually rises over the cold denser air Less obvious and more gradual than cold front Cirrus clouds Thicken into altocumulus and altostratus Sky turns gray Light to moderate rain or snow develops At front rain or snow turns to drizzle

16. Warm front animation source: http://www.learn-line.nrw.de/angebote/klima/medio/bilder/wfront.gif

17. Where they meet we find a variety of weather fronts

18. Atmospheric Lifting For clouds to form air must lift. 3 lifting mechanisms Convectional Orographic Frontal

19. I. Convectional Lifting Some areas of Earth's surface pick up heat better than others. As air warm it becomes less dense and rises. As it rises it cools and sinks

20. Circulatory motion is called convectional lifting If cooling occurs close to the air's saturation temperature, condensing moisture forms a cumulus cloud Form and dissipate over the same area

21. II. Orographic Lifting An air mass is lifted when it is pushed upward over an obstacle such as a mountain range

22. As rising air cools, if it is humid, it forms a cloud If stable air stratus If unstable air cumulus

23. As air moves down the other slope it warms The Name of the Wind is Chinook

24. The descending air is dry because the moisture was removed on the other side.

25. This is called the Rain Shadow Effect.Rain Shadow Effect Example: Eastern Colorado and western Kansas See figure 27.10

26. III. Frontal Lifting When fronts collide warm less dense air is forced up over cold dense air

27. Thunderstorm Formation As cold air pushes into warm moist air, the cold more dense air stays low and the warmer less dense air is pushed up rapidly This rapid upward movement forms thunderstorms

28. Fronts—dryline The source region for cT air is the desert Southwest, the high plains and Mexico with relation to the United States. The air has low dewpoints and warm to hot afternoon temperatures but with mild nighttime temperature. Skies are generally clear in cT air. This allows daytime heating during the day and radiational cooling at night. The cT air mass is most prevalent in summer; in the cool season it is not as discernible. Due to the buoyancy and elevation of cT air across North America, this air will advect into the mid-levels of the atmosphere once it moves out of its source region. This creates a cap of mild dry air. If this air advects over PBL mT air, the severe thunderstorm threat increases significantly. The boundary of cT is most noticeable with the creation of a dryline. A dryline separates mT air from cT air. Depending on the strength of the dryline, convergence along the dryline and the dynamics above the dryline, severe thunderstorms can form near a dryline boundary.

29. Hail Formation Hail stones form by accretion Small hail gets caught up in the updraft and goes to the top of the cloud where it receives another layer of ice This repeats until the hail cannot be supported by the updraft and it falls

30. Hail requires strong updrafts to form For the smallest hail to form, an updraft of around 36 to 54 km/h (24-34 mph) is required. golf-ball size (1 3/4 inch diameter) -- require updrafts of around 88 km/h (55 mph) to form. Softball-size hail involves updrafts exceeding 160 km/h (100 mph).

32. Record Hail Record hail fell in Coffeyville, KS diameter 14.4 cm (5.67 inches) Credit: NOAA Photo Library