Chapter 8: Air Masses, Fronts, and Middle-Latitude Cyclones

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

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 altitude • source regions: regions dominated by surface high pressure over flat surface • Because air sinks in high pressure systems, air stays in contact with the surface and acquires its temperature and moisture characteristics

3. Classification: • Temperature and humidity • Naming conventions • continental: ‘dry’, maritime: ‘moist’ Q1: 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.

4. Q2: which mP is warmer when reaching the U.S.? a) from the Pacific, b) from the Atlantic, c) the same

5. cP (Continental Polar) and cA (Continental Arctic) Air Masses • Continental polar • continental Arctic In the continental US, the coldest winter air is associatedwith cA air masses • (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 .

6. Q3: 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. • Q4: Can we have lake-effect snow if the lake is frozen? • a) yes, b) no, .

7. mP (Maritime polar) Air Masses mP air often brings rain to the west coast of the US. • Q5: What are the small white clouds over the Pacific? • cirrocumulus, b) altocumulus, c) cumulus • Q6: What are the symbols in California? • A: see Appendix C Modification of mP air mass by mountain ranges

8. mT (Maritime Tropical Air Masses) • subtropical air • Bermuda high • mT air brings hot, muggy air to the eastern US in summer. The ‘pineapple express’ on January 1, 1997 High T on April 17, 1976

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

10. Front • A transition zone between two air masses of different temperature and/or humidity • This terminology was developed by Norwegian meteorologists shortly after World War I. • cold front • warm front • stationary front • occluded front • dryline

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

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

13. Q7: before the cold front approaches you, what do you see first? a) high clouds, b) rain shower, c) thunderstorm Q8: Where is the surface minimum pressure located? a) before cold front, b) at cold front, c) behind cold front Q9: Where do you expect snowfall? a) before cold front, b) at cold front, c) behind cold front

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

15. Q10: which front slope is more steep? • a) cold front, b) warm front • Q11: where is the warm area with small cumulus clouds most probably located? • a) right behind a warm front, b) right behind a cold front • Q12: 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 • Q13: 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

16. Dryline: primary difference in dew-point T

17. Occluded Fronts • cold occlusion • warm occlusion • Occluded fronts have characteristics of both warm and cold fronts.

18. Polar Front Theory • Stationary front • frontal wave • open wave • mature cyclone

19. Where Do Mid-Latitude Cyclones (i.e., initial Low’s) Tend to Form? • Both Lows and Highs move from west to east • Highs also from north to south Lows also from south to north Q14: what is the reason for the Gulf of Alaska Low? a) permanent Aleutian low; b) water vapor from Pacific Q15: what do Alberta Clipper and Colorado Low have in common? a) both are cold b) both are on the leeside of Rockies

20. 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) Q16: 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 Figure 4, p. 226

21. Developing Mid-Latitude Cyclones and Anticyclones • convergence and divergence patterns aloft are extremelyimportant to the development of mid-latitude cyclones Q17: 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

22. 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 High Q18: 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

23. Jet Streams and Developing Mid-Latitude Cyclones • jet stream (usually near tropopause of ~10 km) • jet streak: jet stream core with maximum wind • upper-air support • During World War II, the jet stream was used bythe Japanese to carry balloon bombs across thePacific Ocean to North America.

24. Fig. 8-30, p. 231