1. Chapter 8: Air Masses, Fronts, and Middle-Latitude Cyclones Air masses Air masses Fronts Fronts Middle-latitude cyclones Middle-latitude cyclones 1

2. Air Mass Source Regions air mass: an extremely large body of air whose properties of temperature and humidity are fairly similar in any horizontal direction at any given altitude air mass: an extremely large body of air whose properties of temperature and humidity are fairly similar in any horizontal direction at any given altitude source regions: regions dominated by surface high pressure over flat surface source regions: regions dominated by surface high pressure over flat surface Because air sinks inBecause air sinks in high pressure systems, high pressure systems, air stays in contact with air stays in contact with the surface and acquires the surface and acquires its temperature and its temperature and moisture characteristics moisture characteristics 2

3. Classification: Temperature and humidity Naming conventions continental: ‘dry’, maritime: ‘moist’ Q: Since Arctic is a sea, why can we use the term ‘continental Arctic air mass’? A: because Arctic is covered by sea ice in winter. Q: Why are midlatitudes not good source regions? 3

4. Q: which mP is warmer when reaching the U.S.? a) from the Pacific, b) from the Atlantic, c) the same 4

5. cP (Continental Polar) and cA (Continental Arctic) Air Masses Continental polar Continental polar continental Arctic continental Arctic In the continental US, the coldest winter air is associated with cA air masses In the continental US, the coldest winter air is associated with cA air masses (unfrozen) lake effect snows (unfrozen) lake effect snows. Tmin in Dec 1990 At the downwind side of the lake, additional lifting is provided by low hills and the convergence of air as it slows down over the rough terrain 5

6. . Q: What is the lake-effect snow and what is the mechanism? A: Lake-effect snows are snowstorms that form on the downwind side of a large lake. Cold, dry air crossing a lake gains moisture and warmth from the water. As the more buoyant air rises, clouds forms that deposit snow on the lake's lee shore. Q: Can we have lake-effect snow if the lake is frozen? a) yes, b) no, 6

7. mP (Maritime polar) Air Masses mP air often brings rain to mP air often brings rain to the west coast of the US. the west coast of the US. Q: What are the small white clouds over the Pacific? a)cirrocumulus, b) altocumulus, c) cumulus Q: What are the symbols in California? A: see Appendix C Modification of mP air mass by mountain ranges 7

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10. mT (Maritime Tropical Air Masses) subtropical air subtropical air Bermuda high Bermuda high mT air brings hot, muggy air to the eastern US in summer.mT air brings hot, muggy air to the eastern US in summer. Max and Min T on April 17, 1976 10

11. mT (Maritime Tropical Air Masses) The ‘pineapple express’ on January 1, 1997 Atmospheric River: caused flooding and extensive damage (~$1.5 billion) 11

12. cT (Continental Tropical Air Masses) Northern Mexico Northern Mexico and southwestern U.S. and southwestern U.S. Summer Summer Max T in 22 July 2005 12

13. What types of air mass would be responsible for the weather conditions listed below? choose the right answer: a) cP, b) cT, c) mP, d) mT Q1: hot, muggy summer in the Midwest and East Q2: cool breeze after a summer hot spell on the Central Plains Q3: persistent cold, damp weather along the east coast Q4: drought with high temperature over the Great Plains Q5: record-breaking low temperature over U.S. Q6: cool weather with showers over the Pacific Northwest Q7: daily afternoon thunderstorm along the Gulf Coast 13

14. A transition zone between two air masses of different temperature and/or humidity. It represents low pressure area. This terminology was developed by Norwegian meteorologists shortly after World War I. This terminology was developed by Norwegian meteorologists shortly after World War I. Front cold front warm front stationary front occluded front dryline Air masses: mP, cP, mT 14

15. Stationary Fronts Has essentially no movement, and wind is usually parallel with the stationary front Has essentially no movement, and wind is usually parallel with the stationary front Large T and T d differences still exist Large T and T d differences still exist Light precipitation may or may not appear on the cold air side Light precipitation may or may not appear on the cold air side 15

16. Cold Fronts cold front: temperature, humidity, wind direction differences cold front: temperature, humidity, wind direction differences clouds and precipitation clouds and precipitation vertical cross section: vertical cross section: slope of 1:50 slope of 1:50 16

17. Q: before the cold front approaches you, what do you see first? a) high clouds, b) rain shower, c) thunderstorm Q: Where is the surface minimum pressure located? a) before cold front, b) at cold front, c) behind cold front Q: Where do you expect snowfall? a) before cold front, b) at cold front, c) behind cold front 17

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19. Warm Fronts overrunning: slope of 1:300 overrunning: slope of 1:300 Temperature, humidity, and wind direction changes Temperature, humidity, and wind direction changes T and T d differences not as large as T and T d differences not as large as those for cold fronts those for cold fronts Cloud and precipitation changes Cloud and precipitation changes vertical cross section vertical cross section 19

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21. Q: which front slope is more steep? a) cold front, b) warm front Q: where is the warm area with small cumulus clouds most probably located? a) right behind a warm front, b) right behind a cold front Q: If the wind is northerly behind a cold front, the wind direction ahead of the cold front can not be a) westerly, b) southwesterly, c) easterly Q: if the wind is southerly behind a warm front, the wind direction ahead of the warm front can not be a) westerly, b) southeasterly, c) easterly 21

22. Dryline: primary difference in dew-point T 22

23. Occluded Fronts cold occlusion cold occlusion warm occlusion warm occlusion Occluded fronts have characteristics of both warm and cold fronts.Occluded fronts have characteristics of both warm and cold fronts. After occlusion, the front would die outAfter occlusion, the front would die out 23

24. Polar Front Theory Stationary front Stationary front frontal wave frontal wave open wave open wave mature cyclone mature cyclone Q: for the left figure, what is the corresponding stage based on the above figure? 24

25. Where Do Mid-Latitude Cyclones (i.e., initial Low’s) Tend to Form? Both Lows and Highs move Both Lows and Highs move from west to east from west to east Highs also from north to south Highs also from north to south Lows also from south to north Lows also from south to north Q: what is the reason for the Gulf of Alaska Low? a) permanent Aleutian low; b) water vapor from Pacific Q: what do Alberta Clipper and Colorado Low have in common? a) both are cold b) both are on the leeside of Rockies 25

26. Northeasters (or nor’easters): develops or intensified off the eastern seaboard of North America then move northeastward along the coast (not unlike a tropical cyclone) Q: what do the Gulf Low and Hatteras Low have in common? a) both have strong ocean currents; b) both are located between warm ocean and cold land 26

27. Developing Mid-Latitude Cyclones and Anticyclones convergence and divergence patterns aloft are extremely important to the development of mid-latitude cyclones convergence and divergence patterns aloft are extremely important to the development of mid-latitude cyclones Q: Why would the surface low be weakened if the low aloft is right above it? A: because convergence will bring air molecules to the column and increase surface pressure 27

28. In the figure, there is a divergence aloft above surface Low pressure (and convergence); a convergence aloft above surface High; trough aloft is behind surface Low but ahead of surface HighIn the figure, there is a divergence aloft above surface Low pressure (and convergence); a convergence aloft above surface High; trough aloft is behind surface Low but ahead of surface High Q: under what conditions would surface Low be further strengthened? a) surface convergence is stronger than divergence aloft b) divergence aloft is stronger than surface convergence 28

29. Jet Streams and Developing Mid- Latitude Cyclones jet stream (usually near tropopause of ~10 km) jet stream (usually near tropopause of ~10 km) jet streak: jet stream core with maximum wind jet streak: jet stream core with maximum wind upper-air support upper-air support 29

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31. Q: list four regions in North America where midlatitude cyclones tend to develop. Q: For surface low to intensify, the upper level trough should be located: a) west of surface low, b) above surface low, c) east of surface low Q: A mid-latitude cyclonic storm over eastern U.S. usually moves: a) northeastward, b) southeastward, c) southward Q: in winter, which front produce more violent weather: a) stationary front, b) warm front, c) cold front Q: describe the cloud change before passing, while passing, and after passing of a cold front. 31