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

Air massesAir masses FrontsFronts Middle-latitude cyclonesMiddle-latitude cyclones

Air Mass Source RegionsAir Mass Source Regions

air mass: an extremely large body of air whose properties of air mass: an extremely large body of air whose properties of temperature and humidity are fairly similar in any horizontal temperature and humidity are fairly similar in any horizontal direction at any altitudedirection at any altitude

source regions: regions dominated by surface high pressure source regions: regions dominated by surface high pressure over flat surfaceover flat surface

• Because air sinks in Because 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 characteristicsmoisture characteristics

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.

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

cP (Continental Polar) and cA cP (Continental Polar) and cA (Continental Arctic) Air (Continental Arctic) Air

MassesMasses Continental polarContinental polar continental Arcticcontinental Arctic

In the continental US, the coldest In the continental US, the coldest

winter air is associatedwinter air is associated with cA with cA air massesair 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

..

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,

mP (Maritime mP (Maritime polar) Air Massespolar) 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.

Q5: What are the small white clouds over the Pacific?

a) cirrocumulus, b) altocumulus, c) cumulus

Q6: What are the symbols in California?

A: see Appendix C

Modification of mP air mass by mountain ranges

mT (Maritime Tropical Air mT (Maritime Tropical Air Masses)Masses)

subtropical airsubtropical air Bermuda highBermuda high

• mT air brings hot, muggy mT air brings hot, muggy air to the eastern US in air to the eastern US in summer.summer.

The ‘pineapple express’ on January 1, 1997

High T on April 17, 1976

cT (Continental Tropical Air cT (Continental Tropical Air Masses)Masses)

Northern Mexico Northern Mexico

and southwestern U.S.and southwestern U.S. SummerSummer

Max T in July 2005

• A transition zone between two air masses of different temperature and/or humidity• This terminology was developed by Norwegian meteorologists This terminology was developed by Norwegian meteorologists shortly after World War I.shortly after World War I.

FrontFront

• cold front• warm front• stationary front• occluded front• dryline

Stationary FrontsStationary Fronts

Has essentially no Has essentially no movement, and wind is movement, and wind is usually parallel with usually parallel with the stationary front the stationary front

Large T and TLarge T and Tdd differences still existdifferences still exist

Light precipitation may Light precipitation may or may not appear on or may not appear on the cold air sidethe cold air side

Cold FrontsCold Fronts cold front: temperature, humidity, cold front: temperature, humidity,

wind direction differenceswind direction differences clouds and precipitationclouds and precipitation vertical cross section: vertical cross section:

slope of 1:50slope of 1:50

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

Warm FrontsWarm Fronts overrunning: slope of 1:300overrunning: slope of 1:300 Temperature, humidity, and wind direction changesTemperature, humidity, and wind direction changes

T and TT and Tdd differences not as large as differences not as large as

those for cold frontsthose for cold fronts Cloud and precipitation changesCloud and precipitation changes vertical cross sectionvertical cross section

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

Dryline: primary difference in dew-point T

Occluded Occluded FrontsFronts

cold occlusioncold occlusion warm occlusionwarm occlusion

• Occluded fronts have Occluded fronts have characteristics of both characteristics of both warm and cold fronts.warm and cold fronts.

Polar Front TheoryPolar Front Theory Stationary frontStationary front frontal wavefrontal wave open waveopen wave mature cyclonemature cyclone

Where Do Mid-Latitude Where Do Mid-Latitude Cyclones (i.e., initial Low’s) Cyclones (i.e., initial Low’s)

Tend to Form?Tend to Form? Both Lows and Highs moveBoth Lows and Highs move

from west to eastfrom west to east Highs also from north to southHighs also from north to south

Lows also from south to northLows 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 coldb) both are on the leeside of Rockies

Figure 4, p. 226

Northeasters (or nor’easters):develops or intensified off theeastern 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

Developing Mid-Latitude Developing Mid-Latitude Cyclones and AnticyclonesCyclones and Anticyclones

• convergence and divergence patterns aloft are extremelyconvergence and divergence patterns aloft are extremelyimportant to the development of mid-latitude cyclonesimportant to the development of mid-latitude cyclones

Q17: Why would the surface low be weakened if the low aloft Q17: Why would the surface low be weakened if the low aloft is right above it? A: because convergence will bring air is right above it? A: because convergence will bring air molecules to the column and increase surface pressuremolecules to the column and increase surface pressure

• In the figure, there is a divergence aloft above surface In the figure, there is a divergence aloft above surface Low pressure (and convergence); a convergence aloft Low pressure (and convergence); a convergence aloft above surface High; trough aloft is behind surface Low above surface High; trough aloft is behind surface Low but ahead of surface Highbut ahead of surface High

Q18: under what conditions Q18: under what conditions would surface Low be further would surface Low be further strengthened?strengthened?a) surface convergence is a) surface convergence is stronger than divergence aloftstronger than divergence aloftb) divergence aloft is strongerb) divergence aloft is strongerthan surface convergencethan surface convergence

Jet Streams and Developing Jet Streams and Developing Mid-Latitude CyclonesMid-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 supportupper-air support

• During World War II, the jet stream was used byDuring World War II, the jet stream was used bythe Japanese to carry balloon bombs across thethe Japanese to carry balloon bombs across thePacific Ocean to North America.Pacific Ocean to North America.

Fig. 8-30, p. 231