Air Masses and FrontsAir Masses and Fronts

Chapter 11Chapter 11Part 1

March 17 2009March 17, 2009

Air massesAir masses• Air mass = large body of air that has similar temperature

and humidity properties throughoutP ti f i h difi d th• Properties of air masses, however are modified as they move over areas outside where they originate

• Source regions = regions where air masses develop• Source regions = regions where air masses develop their characteristics– need large areas with similar characteristicneed large areas with similar characteristic

• Mid-latitudes not considered source regions for air masses as temperature, humidity vary considerablyp y y y

• Rather mid-latitudes are greatly affected by cold or warm, moist or dry air masses moving in

Cold air mass over eastern, central US,

Air Mass ClassificationAir Mass Classification• Air masses have 4 major classificationsAir masses have 4 major classifications• They are Polar (P) or Tropical (T), continental (c)

or maritime (m); extremely cold air masses are ( ); yArctic (A)

• Winds aloft may move air masses from their isource regions

• Cold air mass moving over warmer surface warmed form below instability may havewarmed form below- instability- may have clouds, showers

• Warm air mass moving over cold surface-Warm air mass moving over cold surfacestable- can have fog, stratiform coluds, pollution buildup

Air Mass ClassificationAir Mass Classification

Continental polar (cP) and Continental Arctic (cA) air masses

cP and cA air masses originate in northern Canada and• cP and cA air masses originate in northern Canada and Alaska and bring very cold air to much of the US at times in winter- following passage of cold fronts associated

ith id l tit d lwith mid-latitude cyclones• Long clear nights in winter allow for strong radiational

cooling – thus these air masses are very cold- also very g y ydry

• Due to lack of terrain, air easily moves south through the Great Plains- warms a bit but still dry so few cloudsGreat Plains- warms a bit, but still dry so few clouds

• Air mass stable- subsidence and warming aloft, cold air below- good for pollution buildup

• Western US less influenced by cP or CA air masses as coldest air usually stays east of continental divide

Air mass source regions and pathsg

2 very cold arctic outbreaks- including one setting records in Las Vegas

Lake effect snowLake effect snow• Lake effect snows- cP or cA air mass moves cold air over

relatively warm Great Lakes in early to mid winter• Air above lake warms and is saturated, cumulus clouds

f d i t bl i b i d iform and grow in unstable air- snow begins and is enhanced by lifting of air over hills and convergence at surface due to slowing of air from increased surface gfriction

• Lake effect snows greatest within first few kilometers of l k t di t 50 k i l dlake, extending up to 50 km inland

• Need reasonable travel distance (fetch) over water (80 km or so) for significant lake effect snowor so) for significant lake effect snow

• Can occur on lee side of other large lakes (e.g. Great Salt Lake) if land/lake temperature difference is large

Formation of Lake Effect SnowsFormation of Lake Effect Snows

• Cold, dry air crossing the lake gains moisture and warmth from the water

• The more buoyant air now rises, forming clouds that deposit large quantities ofclouds that deposit large quantities of snow on the lake’s leeward shores

Profiles of cP Air MassesProfiles of cP Air Masses• cA air masses reaching

south Texas and south Florida may cause largeFlorida may cause large damages to fruit and vegetable cropsg p

• Summertime excursions of cP air into the US are moderately cool and dry and give a break to aeras i th t US fin the eastern US from hot, humid weather

Maritime Polar (mp) air massesMaritime Polar (mp) air masses

Wi t ti P d A i th• Wintertime cP and cA air masses over northern Asia and the Arctic travels over the Pacific Ocean by the Aleutian Low and is modified byOcean by the Aleutian Low and is modified by added warmth and moisture- changes into maritime polar air mass (mP)

• Conditionally unstable- as it moves inland over mountains, orographic precipitation commonDries out as it moves over series of mountain• Dries out as it moves over series of mountain ranges in western US

• Can have chinook winds develop• Can have chinook winds develop• Brings moderate weather to east slope of

Rockies when replacing cP or cA airmassRockies when replacing cP or cA airmass

mPmP

Modification of mP airmass crossing western USModification of mP airmass crossing western US

mP airmass less common in northeastern US-gives cold damp drizzly weather to NE USgives cold, damp drizzly weather to NE USHatteras lows and Nor’easters may develop

Maritime Tropical (mT) air massesp ( )

• Wintertime source for mT airmasses is subtropical peast Pacific- warm, humid air coming from near Hawaiian Islands can produce very heavy rains and fl di l t t ( ti ll dflooding along west coast (sometimes called “Pineapple Express”)E t US T i f G lf f M i d• Eastern US mT air from Gulf of Mexico and Caribbean common in summer, less so in winterWi t T ll fi d t G lf C t Fl id• Winter – mT usually confined to Gulf Coast, Florida

• Briefly may move north, causing advection fog and l l d it ld dlow clouds as it moves over cold ground

• mT airmass pushed back to south from cold front b i i i P ibringing in cP airmass

mT airmass into California- heavy rain (Pineapple Express)

mT eastern US cP western USmT eastern US, cP western US

Summer mT EffectsSummer mT Effects

• mT airmass frequent over eastern US in summer- clockwise circulation around Bermuda High

• Summer mT airmass brings much moisture and gconditional instability- thunderstorms common

• Southwestern monsoon- warm moist air fromSouthwestern monsoon warm moist air from Gulf of Mexico or Gul of Mexico

Continental Tropical Air MassesContinental Tropical Air Masses

• cT source areas in northern Mexico and southwestern US during summerg

• Dry, hot, usually upper level high and subsidencesubsidence

• Sometimes moves into Great Plains

cT airmass into central, western US,

Air mass summaryAir mass summary

Ai i l b d f i ith f i l if• Air mass is large body of air with fairly uniform properties of temperature and humidityAi i t d t b fl t ith hi h• Air mass source regions tend to be flat with high pressure and light windsC ti t l i f l d iti• Continental air masses form over land, maritime over waterP l i f l i t i l i• Polar air masses from polar regions; tropical air masses from warm, tropical regionsP ld d A ld d T h t d• cP= cold, dry; cA=very cold, dry; cT=hot, dry;

mT=warm, moist; mP=cool, moist

FrontsFronts• Front = transition zone between 2 air masses of

different densities (temperatures) and often humidityy

• Frontal location often identified by: 1 Sharp temperature difference over short distance;1. Sharp temperature difference over short distance; 2. Change in moisture content (e.g. dew point

temperature);temperature); 3. Shift in wind direction; 4 Pressure and pressure changes;4. Pressure and pressure changes; 5. Clouds and precipitation patterns

• Lowest pressure at frontal boundary• Lowest pressure at frontal boundary

Example surface map showing pressure systems, f i i b i i ifronts, air masses, isobars, precipitation

Example cold f t d

Note kink in front and associated s rface eather

isobars and wind shift at front

surface weather

Pressure falling beforefalling before frontal passage, rising afterrising after

Cold FrontsCold Fronts• Cold, dry stable air replaces warm, moistCold, dry stable air replaces warm, moist

unstable airBl li ith bl t i l• Blue line with blue triangles

• Clouds of vertical development• Thunderstorms, squall lines

Vertical structure of cold frontVertical structure of cold front

Cold air wedges under warm air at front, causing warm air to rise ft d i h

Front is steep near the ground due to friction typical slope b t 1 50 t i l d b toften producing showers or

thunderstormsabout 1:50 typical speed about 25 knots