Chapter 8 Air Masses Fronts and MiddleLatitude Cyclones

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

Air Mass Source Regions n n 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 • Because air sinks in high pressure systems, air stays in contact with the surface and acquires its temperature and moisture characteristics 2

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

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

c. P (Continental Polar) and c. A (Continental Arctic) Air Masses n n n Continental polar continental Arctic In the continental US, the coldest winter air is associated with c. A 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 5

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

m. P (Maritime polar) Air Masses m. P air often brings rain to the west coast of the US. Modification of m. P air mass by mountain ranges 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 7

Surface station 8

9

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

m. T (Maritime Tropical Air Masses) The ‘pineapple express’ on January 1, 1997 Atmospheric River: caused flooding and extensive damage (~$1. 5 billion) 11

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

What types of air mass would be responsible for the weather conditions listed below? choose the right answer: a) c. P, b) c. T, c) m. P, d) m. T Q 1: hot, muggy summer in the Midwest and East Q 2: cool breeze after a summer hot spell on the Central Plains Q 3: persistent cold, damp weather along the east coast Q 4: drought with high temperature over the Great Plains Q 5: record-breaking low temperature over U. S. Q 6: cool weather with showers over the Pacific Northwest Q 7: daily afternoon thunderstorm along the Gulf Coast 13

Front • 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. • cold front • warm front • stationary front • occluded front • dryline Air masses: m. P, c. P, m. T 14

Stationary Fronts n n n 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 15

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

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

18

Warm Fronts n n 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 19

20

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

Dryline: primary difference in dew-point T 22

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

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

Where Do Mid-Latitude Cyclones (i. e. , initial Low’s) Tend to Form? n n Both Lows and Highs move from west to east Highs also from north to south 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

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

Developing Mid-Latitude Cyclones and Anticyclones • 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

• 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 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

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

30

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: stationary front, b) warm front, c) cold front a) Q: describe the cloud change before passing, while passing, and after passing of a cold front. 31