AIR MASSES • Large bodies of air

• SOURCE REGIONS – areas where air masses originate

– Uniform in composition– Light surface winds– Dominated by high surface

pressure– The longer the air mass

remains over a region, the more likely it will acquire properties of the region

• Temperature and moisture

AIR MASSES • Similar horizontal properties

– Temperature– Moisture content– Lapse rates

• These characteristics are carried with it as air mass moves to other areas

• Challenge of weather forecasting – To predict behavior of air masses– Where it will go and how it will change

AIR MASS CLASSIFICATION • Bergeron classification – 1920s

• Consists of 2 letters ab– Source region (a)– Thermal property (b)

• Air mass properties gradually change as it travels

• Acquire characteristics of invading regions– Thermal properties – Stability

• Whether it is heated or cooled from below

• Nomenclature only applies to recent history of air mass

AIR MASS CLASSIFICATION

SOURCE REGIONS

• Maritime (m) – originates over oceans or large bodies of water

• Continental (c) – originates over land

• m or c indicate influence of surface on air mass characteristics (water and land)

THERMAL TYPES

• Tropical (T) – from low latitudes

• Polar (P) – from mid-high latitudes

• Arctic (A) – from high latitudes (> 65°N)

• P and T suggest importance of latitude of source regions

AIR MASS PROPERTIES OVER NORTH AMERICA

WINTERTIME cP

• Source – Central Canada and Siberia

• Frozen surface – ice and snow

• Intense radiation cooling, lack of insolation heating

• Extremely cold, stable, and dry

• Clouds are non-existent

AIR MASS PROPERTIES OVER NORTH AMERICA

SUMMERTIME cP

• Source – central Canada

• Ample warming of surface through insolation

• Melts snow and permafrost

• Cool, dry and sometimes unstable due to insolation heating of lower layers

AIR MASS PROPERTIES OVER NORTH AMERICA

SUMMERTIME cT

• Source – Northern Mexico and extreme SW deserts of US

• Hot, dry and unstable

AIR MASS PROPERTIES OVER NORTH AMERICA

WINTERTIME mP

• Source – open oceans of high latitudes – Gulf of Alaska and North Atlantic

• cP air mass from Syberia becomes mP air mass as it moves over ocean

– Lower layers modified by warmer water surface

• Cool, moist, unstable in surface layers

• Cool, dry aloft

AIR MASS PROPERTIES OVER NORTH AMERICA

SUMMERTIME mP

• Source – open oceans in high latitudes – Gulf of Alaska and North Atlantic

• Cool and moist in lower layers and cool and dry aloft

• Overall temperature higher than in winter

• Instability in lower layers

AIR MASS PROPERTIES OVER NORTH AMERICA

WINTERTIME mT

• Source – over open ocean near 30N

• Great semi permanent subtropical high pressure centers and in SW Caribbean

• Warm, moist and very unstable

AIR MASS PROPERTIES OVER NORTH AMERICA

SUMMERTIME mT

• Source – semi permanent high centers near 15N

• Very warm, moist and unstable

AIR MASS MODIFICATION • As air masses migrate, changes in their properties exist – AIR MASS

MODIFICIATION

• As air masses move from source regions they carry with them physical characteristics of the region – INITIAL CONDITIONS

• Most variations within air masses are found in vertical distribution

• Air masses are modified by the climatic characteristics of the regions over which they move

• Types of modification: – Lapse rate modifications

• subsidence, uplift,

– Moisture modifications– Thermal modifications

AIR MASS MODIFICATION

cP MODIFICATIONS

• WINTER – moving from land to water (lakes or ocean)

• Increased instability

• Surfaces in their paths are warmer than those of their source regions

• Warming from below increases instability

AIR MASS MODIFICATION cP MODIFICATIONS

• LAKE EFFECT SNOW – as cold dry air mass moves over warmer water (winter), instability is enhanced

• Produces snow belts on eastern shores of Great lakes

AIR MASS MODIFICATION cP MODIFICATIONS

• SUMMER

• cP air moves from land to water, becomes more stable

• Decrease of lapse rate

• Haze, fog and low stratus clouds appear

AIR MASS MODIFICATION mP MODIFICATIONS

WINTER• Becomes more unstable in lower layers as it reaches coast• Showers, squalls occur as air ascends over coastal mountains• Becomes dry and stable as it moves inland due to subsidence

SUMMER – increases instability

AIR MASS MODIFICATION mT MODIFICATIONS

WINTER• Moisture rich• Releases in precipitation

as arrives at coast• Stability increases inland

SUMMER• From Gulf of Mexico• Penetrates as far as

Canada flowing along the east side of Rocky Mountains

• Instability increases as it moves inland

FRONTS • Transition zone between two

air masses of different densities

– Temperature– Humidity

• FRONTAL SURFACE (ZONE) – upward extension of a front

• Types of fronts– Stationary– Cold– Warm– occluded

• Frontolysis – the weakening or dissipation of a front– Decreased temperature contrast between two air masses

• Frontogenesis – a formation, strengthening or generation of a front– Increased contrast of temperature conditions between two air masses

CRITERIA OF FRONT LOCATION• Contrasting conditions in the

following exist on either side of a frontal system

• Surface temperature• Air moisture content• Winds

– Speed and direction

• Cloud types & precipitation• Sea level pressure and its

tendencies

STATIONARY FRONTS• Fronts that have no movement

• Designation – alternating red half-circles and blue triangles

• Obstacles prevent front from progressing– Mountain range

• Drawn along a line that separates two air masses

STATIONARY FRONTS• Example:

– Cold cP air from Canada against Rocky Mountains in US

– cP air to the north

– mP air to the west of mountains

– Cold air unable to cross barrier

– No westward movement

STATIONARY FRONTS• Weather

– Winds blow parallel to front but in opposite direction on either side

– Clear to partly cloudy

– Cold to the east

– Warm to the west

– Little or no precipitation

• Both air masses are dry

• Stationary front can become warm front or cold front

COLD FRONTS• Transition zone where a cold

air mass advances and replaces a warm air mass

• Leading edge of cold air– cP replacing mT– cP due south, mT due north

• Designation – solid blue triangles oriented towards the direction in which front is moving

• Rapid movement – up to 50 km/hr

COLD FRONTS• Cold dense air wedges under warm air forcing it upwards

– Sharp slope at front’s edge

– Steepness due to friction which slows the airflow near the ground

– Slope = 1:50 (vertical to horizontal distance from front’s edge to middle of frontal system)

COLD FRONTS• The faster the front, the steeper the slope• Movement of the front causes most of the weather associated with its

passage• Cold fronts are associated with dramatic shifts in weather

COLD FRONTS• Affected local meteorological conditions as front passes over:

– Temperature

– Seal level pressure tendency

– Wind direction

– Cloud cover

– Dew point

– visibility

COLD FRONTS• Position of the leading edge of advancing portion of cold air mass

FACTORS THAT DETERMINE COLD FRONTAL WEATHER

• There is no average cold front• Weather associated with cold fronts may vary from minor wind

shifts to severe thunderstorm activity• Weather is determined by

– The nature of warm air that is being lifted• Moist versus dry – precipitation production• Stability of air mass - uplift

– Degree of lift to which the warm air is subjected by the advancing cold air wedge

• Speed of the cold front and the steepness of frontal surface• Mechanical uplift possible, precipitation

COLD FRONTS

• with slow-moving cold front, clouds and precipitation usually cover a broad area behind the front

e.g. if rising air is dry and stable scattered clouds are all that form – no precipitation

e.g. during the winter, a series of cold polar outbreaks may travel across the US so quickly that warm air is unable to develop ahead of the front.

frigid arctic air usually replaces cold polar air, and a drop in temperature is the only indication that a cold front has moved through the area

COLD FRONTS• SURFACE WEATHER CHANGES ASSOCIATED WITH COLD

FRONTS (cP replacing mT)

1. Surface winds• Change direction when cold front passes over

• Cold air moves from W-NW and is undercutting a flow of warm air from S-SW

• Winds S-SW ahead of cold front and W-NW behind cold front

2. Temperature– Arrival of cold front will result in a decrease in temperature (warm to

cold)

– After passage, temperature keeps decreasing as the cold air begins to modify surface conditions

COLD FRONTS• SURFACE WEATHER CHANGES ASSOCIATED

WITH COLD FRONTS (cP replacing mT)

3. Moisture content– Onset of cold air results in drop in dew point

4. Cloud and precipitation– Ahead – cirrus and cirro-stratus clouds due to stronger upper

level winds that push uplifted air at the front’s edge ahead– While passing – towering cumulus clouds (lots of moisture in

warm air and lots of precipitation, hail and thunder)– After passing – as warm air is drying out, showers decrease

in intensity and skies clear

COLD FRONTS• SURFACE WEATHER CHANGES ASSOCIATED WITH COLD

FRONTS (cP replacing mT)5. Visibility

– Usually improves after a cold frontal passage• Air behind front is unstable as a result of passing over a warm

surface– Vertical motion will carry pollution aloft

• Tropical air accumulates considerable smoke from industrial areas during its movement north

• Polar and arctic air masses are relatively free of pollution

6. Pressure– Because front lies in a trough, approach will be accompanied by

decrease in pressure– A marked rise will be observed as trough passes

WARM FRONTS

• Transition zone between a retreating cold air mass and advancing warm air mass

• Designation – red semi-circles pointing in the direction where the warm air is advancing

• Warm and moist mT replacing dry cold mP

WARM FRONTS• Frontal changes are less abrupt than cold air frontal passages

– Long spells of cold weather do not come to a rapid end

• Weather at the warm front portion of a frontal system is more extensive than at the cold front portion

– Cloud system & precipitation cover extensive areas

WARM FRONTS• Cold air lies as a wedge under warm air

– As warm front approaches depth of cold air decreases

• Advancing warm air overruns the retreating wedge– Forces air to rise – creates frontal inversion

– Air expands and cools • Extensive cloud on top of cold air

WARM FRONTS• Warm fronts are slow

– About half that of average cold front

• Average slope of 1:300– More gentle than cold front

WARM FRONTSWeather associated with warm

Fronts

• Weather patterns associated with warm fronts depend on:

– Moisture content of warm air mass

• precipitation

– Stability of the warm air mass

• uplift

– Degree of overrunning

• Uplift and precipitation

WARM FRONTSWeather associated with warm fronts

• If overrunning air is dry and stable, only high and middle clouds will form

– no precipitation

• If overrunning air is moist and unstable, heavy showers can develop as thunderstorms become embedded in the cloud mass

• Arrival of warm front produces wind shifts, warmer temperatures and overall improvement of weather conditions

OCCLUDED FRONTS• OCCLUSION – frontal system

that forms when a cold front overtakes a warm front

• Designation – alternating purple triangles and half-circles that point towards the direction to which the front progresses

• Development – rapidly approaching cold front catches up to slow-moving warm front and overtakes it

OCCLUDED FRONTS

WARM OCCLUSION• When the air behind the occluded front is

warmer than the air ahead– cP air overrun by mP from Pacific ocean in NW

USA

• As cold front overtakes warm front, the milder and lighter air behind the cold front is unable to lift the colder and heavier air off the ground

• Surface weather associated with this occlusion is similar to that of warm front

OCCLUDED FRONTSCOLD OCCLUSION

• When the air behind the occluded front is cooler than the air ahead– Most common to Pacific coast states– cP overrun by cA

OCCLUDED FRONTSCOLD OCCLUSION WEATHER

• Cold front rapidly approaches warm front

• Warm air rides up the cool air in front of the warm front

• As cold air overtakes warm front, the warm front and the warm air mass are lifted off the ground

• As front approaches weather is similar to warm fronts

• Frontal passage brings weather similar to that of cold fronts