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) 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 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) Moisture content Onset of cold air results in drop in dew point 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) 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 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 Advancing warm air overruns the retreating wedge 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 Average slope of 1:300 Warm fronts are slow About half that of average cold front Average slope of 1:300 More gentle than cold front

WARM FRONTS Weather 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 FRONTS Weather 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 FRONTS COLD 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 FRONTS COLD 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