Chapter 8: Air Masses and Fronts

IntroductionIntroduction

• Air masses have uniform temperature and humidity

characteristics

– They affect vast areas

• Fronts are boundaries between unlike air masses

– Fronts are spatially limited

– They are inherently linked to mid-latitude cyclones

– Source regions: The areas of globe where air

masses form are called source regions.

Surface conditions transfer to overlying air to

form air mass.

Formation of Air Masses

(1) air gains temperature and humidity characteristics of the surface.

(2) Topographically uniform areas.

It will take a few days for an air mass to take on

the characteristics of the underlying surface. So,

air masses are usually formed in low and high latitude,

since middle latitude is too variable. It requires large

regions and uniform topography to form air masses.

•

(3) It requires days for temp/moisture

imprinting to form air masses.

(4) air masses classified by temp/moisture

characteristics of source region

– moisture: continental (dry) v. maritime (marine) – c or m

– temp: tropical (warm), polar (cold), arctic (very cold) – T, P or A

• Once formed, air masses migrate within the general circulation

• Upon movement, air masses displace residual air over locations thus changing temperature and humidity characteristics

• Further, the air masses themselves

moderate from surface influences

North American air masses and

air mass source regions

Continental Polar (cP)

• Winter: originate over high-latitude land masses (northern Canada, Siberia)

• Low solar angle, short days, high albedo � cooling of over-lying air �inversions and highly stable conditions

• very cold and dry � limited cloud formation, bright and sunny

• Summer: warmer with higher moisture content � fair weather cumulus develop

Continental Arctic (cA)

• extremely cold and dry conditions: low temperature � limited vapor content

• boundary between cA and cP � Arctic Front

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

• Migrations of cP air � induce colder, drier

conditions over affected areas

• As air migrates heat and moisture capacity

increases � stability decreases

• ~ to cP but warmer and higher moisture content

• forms over N. Pacific/Atlantic

• e.g. cP flowing out of Asia, over warm Japan current � adds heat/moisture

• west coast of the US � mP air affects

regions during winter

• process is different along east Coast �

cyclonic flow: winds approach coast

from the northeast (nor’easters)

� cold, heavy snowfall

Maritime Polar (mP) Air Masses

• Summer phenomena – hot, low-latitude areas � SW US, northern Mexico• little available moisture, high temp � hot, dry air masses• steep lapse rates, unstable conditions due to intense surface heating �limited cloud formation

• thunderstorms may occur: • advection• orographic lifting

Continental Tropical (cT) Air Masses

• Form over low latitude oceans: Gulf of Mexico, tropical Atlantic

• very warm, humid � unstable

• affects southeastern US

• Migration inland �heating of air mass from ground surface increases lapse rates � increases instability � intense ppt (thundershowers) .

• moisture content: reduced in northward

direction � Miami v. Chicago

Maritime Tropical (mT) Air Masses

• separate air masses � leads to changes in temperature and humidity

as one air mass is replaced by another

• changes in temp � lead to uplift and ppt

• four types of fronts:

cold � cold advancing on warm

warm � warm advancing on cold

stationary � air masses not advancing

occluded � does not separate tropical

from polar/arctic, boundary btw two

polar air masses

Fronts

• cold air displaces warm air

• results in heavy precipitation events and rapid temperature drops

• Extreme precipitation � extensive vertical lifting

• warm air ahead of the front � forced aloft � cumulonimbus clouds

Frontal development about a

low pressure system

Cold Fronts

The vertical displacement of air along a cold front boundary

Course: Introduction to Atmospheric sciences(ATOC210) by GyuWon LEE

Types of frontsIdentification of fronts

1. Sharp temperature changes

2. Change in the air’s moisture

3. Shifts in wind direction

4. Pressure and pressure change

5. Clouds and precipitation patterns

Consequence: cloud and precipitation patternsCold front

-Showers with gusty winds are prevalent when the warmer air is moist and unstable

- Anvil with Ci (Cirrus), Cs (Cirrostratus). -Move faster (15 ~25knots) and are steeply sloped (~ 1: 50) -Ac: Altocumulus (mid-clouds); Cb: Cumulonimbus.

The sharp cold front boundary is evident

on both satellite pictures and radar composites

Consequence: cloud and precipitation patternsCold front: frontogenesis (�frontolysis)

• occurs when warm air displaces colder air

• overrunning – warm air gently rides over cold, dense air

• leads to gradual progression of cloud types � stratus, nimbostratus, altostratus, cirrostratus, cirrus

• cirrus is seen first, clouds continue to thicken and lower

• zone of contact is less steep than cold fronts � greater horizontal extent

• longer, less intense periods of rain (uplift not as dramatic)

• ppt falls through cold air mass � frontal fog, or sleet/freezing rain

Warm Front

Identification of fronts

Warm front

- Advancing warm, moist, subtropical (mT) air replaces the retreating cold maritime polar air. - Precipitation well in advance of the front’s surface boundary.

1. Sharp temperature changes

2. Change in the air’s moisture

3. Shifts in wind direction

4. Pressure and pressure change

5. Clouds and precipitation patterns

Consequence: cloud and precipitation patternsWarm front

- Wide spread rain ahead of the sfc front (“overrunning”)- Ci, Cs, As, Ns - The wind veers with altitude-Frontal inversion in the region of the upper level front - Move slower (10knots) and are gently sloped (~ 1: 200)

• contact zones sometimes are stalled � relatively ‘fixed’ in position

• relationship btw the air masses ~, do not move as quickly

• subjective in terms of speed � difficult to establish contact zone precisely

• Fronts may slowly migrate and warmer air is displaced above colder

Stationary Fronts

• Occluded front (occlusion): closure � cold air mass cuts off warm air from ground

• separate cold/warm air masses BUT at surface air masses merge

• cold-type v. warm-type occlusion

– cold-type occlusion: eastern half of the continent � cP air meets mP

– warm-type occlusion: western edges of continent � mP advances on cP

Occluded Fronts

Occlusion

sequence

Some occlusions form when the surface low

elongates and moves away from the junction of

the cold and warm fronts

Some occlusions occur when the intersection

of the cold and warm fronts slides along

the warm front

Alternative Mechanisms: Occluded Fronts

Consequence:

cloud and precipitation patterns

Cold-type occluded front

The faster moving cold front catches up to the slower moving warm front, and forces the warm air mass to rise off the ground.

Consequence:

cloud and precipitation patterns

Warm-type occluded front

The faster moving cold front overtakes the slower moving warm front. The lighter air behind the cold front rises up and over the denser air ahead of the warm front.

• Fronts based on temp and density differences of air masses

• Humidity (moisture content) affects density � humid air < dense (H20 lower molecular weight than N2 and O2)

• Dryline: boundary btw humid and dry air.

A dryline over Texas

Drylines