Chapter 12 Fronts & Air Masses - Storm.colorado.edu

Chapter 12Fronts & Air MassesChapter overview: Anticyclones or highs Air Masseso Classificationo Source regionso Air masses of North America Frontso Stationary frontso Cold frontso Warm frontso Fronts and the jet streamo Frontogenesiso Drylineso Occluded frontso Upper-tropospheric fronts

Anticyclones or HighsHigh pressure centers(highs) are identifiedas relative maxima inpressure (on constantheight maps) or height(on constant pressuremaps).A ridge is an extendedarea of relatively highpressure (height).What causes high pressure to occur at the surface?The mechanisms that lead to the formation of high pressure include:- the global circulation- monsoons / seasonal temperature contrasts- transient weather systems (Rossby waves)- small scale features (thunderstorms, mountains)Why are highs observedto tilt west with height?Winds and advection around surface highWhere is cold (warm) air advection occurring relative to the surface highshown above?How does the height of upper level pressure surfaces change as the airbelow warms or cools?

Role of upper level high (ridge) in creating convergenceHow does the gradient wind speed compare to the geostrophic wind speedin a ridge (trough)?As air exits a ridge and moves towards a trough it slows down andconvergence occurs.As air exits a trough and moves towards a ridge it speeds up anddivergence occurs.Convergence occurs ahead of upper level ridges and divergence occursahead of upper level troughs.Air MassesAir mass: A widespread bodyof air whose temperature andhumidity (and otheratmospheric properties) arefairly similar in any horizontaldirection at a given altitudeWhy are meteorologistsinterested in air masses?

Air Mass ClassificationAir masses are classified basedon their temperature andhumidity.The four most common airmasses to impact weather inNorth America are:continental polar (cP)maritime polar (mP)continental tropical (cT)maritime tropical (mT)In the winter continental Arctic(cA) air masses can also impactweather in the United States.What temperature and moisturecharacterize these air masses?Air Mass Source RegionsWhat features of a source region allow for the formation of air masses?

Transient weather patterns will cause air masses to move out of theirsource region.What happens to an air mass once it moves out of its source region?- modification due to surface fluxes- modification due to flow over mountainsAir Masses of North AmericaContinental Polar (cP) and Continental Arctic (cA) Air MassesWhere do cP and cA air masses form?What are the defining characteristics of cP and cA air masses?What type of weather is experienced in the mid-latitudes when a cP or cAair mass moves into this region?How do cP air masses differ between winter and summer?How do mountains alter the impact of cP and cA air masses?

Maritime Polar (mP) Air MassesWhere do mP air masses form?What are the definingcharacteristics of mP airmasses?What type of weather isexperienced in the midlatitudes when an mP air massmoves into this region?How do mountains alter the impact of mP air masses?Maritime Tropical (mT) Air MassesWhere do mT airmasses form?What are the definingcharacteristics of mT airmasses?What type of weather isexperienced in the midlatitudes when an mT airmass moves into thisregion?

What is the impactof mT air massesoriginating in theGulf of Mexico?What large-scalepressure feature isoften responsiblefor bringing mT airmasses into thecentral andeastern UnitedStates?Continental Tropical (cT) Air MassesWhere do cT air massesform?What are the definingcharacteristics of cT airmasses?What type of weather isexperienced in the midlatitudes when a cT air massmoves into this region?

FrontsFront: A boundary (or transition zone) between two air masses of differentdensityFronts are characterized by:- large horizontal temperature gradients- large horizontal moisture gradients- strong horizontal wind gradients- relative minimum in pressure- clouds and precipitation- kinks in isopleths (isobars, isotherms) on weather mapsNotice that the fronts shown above separate different air masses and thatthe fronts are located in areas of relatively low pressure.What types of fronts are present on the weather map above?

The following symbols are used to indicate fronts on weather maps.Fronts move in the direction the frontal symbols point.In order to locate a front look for: Change in temperatureChange in absolute humidityShift in wind directionPressure and pressure changesCloud and precipitation patternsStationary FrontsStationary front: A front with essentially no movement.What direction are the surface (upper level) winds relative to a stationaryfront?What type of weather is associated with a stationary front?

Cold FrontsCold front: A front where cold (polar or Arctic) air is advancing andreplacing warm (tropical) airHow will the weather change asa cold front approaches andthen passes a given location? How do the clouds andprecipitation change? How does thetemperature change? How does the dew pointtemperature change? How do the windschange? How does the pressurechange?Where do clouds and precipitation form relative to a cold front?

Why do clouds and precipitation form along fronts?What determines how steep a frontal surface will be?What impact does this have on the distribution of clouds and precipitationassociated with the front?The actual weather observed when a cold front passes varies from front tofront and can also differ regionally.Backdoor cold front: A cold front that moves into an area from the east ornortheastCold air damming: Cold air that gets trapped near the surface by mountains

Warm FrontsWarm front: A front where warm (tropical) air is advancing and replacingcold (polar or Arctic) airHow will the weather changeas a warm front approachesand then passes a givenlocation? How do the clouds andprecipitation change? How does thetemperature change? How does the dew pointtemperature change? How do the windschange? How does the pressurechange?How does the speed of movement of a warm front compare to that of a coldfront?Why does the speed differ between these types of fronts?

How does the slope of a warm front compare to the slope of a cold front?Where do clouds form relative to a warm front?Overrunning: Warm, less dense air rising over cold airWhy does a frontal inversion exist ahead of the warm front?Would you expect a frontal inversion to be associated with a cold front?How does the wind direction change with height ahead of a warm front?

Fronts and the Jet StreamThe large temperaturegradient associatedwith fronts indicatesthat there is a largethermal wind effectnear fronts.As a result the polarjet stream is locatedabove fronts.FrontogenesisFrontogenesis: A strengthening of a front as the temperature contrastacross the front increasesFrontolysis: Weakening or dissipation of a front as the temperature contrastacross the front lessensWhat processes lead to frontogenesis (frontolysis)?- kinematics- thermodynamics- dynamics

DrylinesDryline: A boundary that separates moist (maritime) and dry (continental)airHow does theweather changeacross a dryline? How do theclouds andprecipitationchange? How does thetemperaturechange? How does thedew pointtemperaturechange? How do thewinds change?Where in the United States are drylines most often observed?Occluded FrontsOccluded front: A front that forms when a cold front catches up to andovertakes a warm frontWhat causes a cold front to catch up to a warm front?

Cold occlusion: An occludedfront with colder air behind theoccluded frontHow does the weather changeas a cold occluded frontapproaches and passes a givenlocation? How do the clouds andprecipitation change? How does the temperaturechange? How does the dew pointtemperature change? How do the winds change? How does the pressurechange?

Warm occlusion: An occludedfront with colder air ahead of theoccluded frontHow does the weather changeas a warm occluded frontapproaches and passes a givenlocation? How do the clouds andprecipitation change? How does the temperaturechange? How does the dew pointtemperature change? How do the winds change? How does the pressurechange? What are the differencesbetween a cold and warmoccluded front? How does the position ofthe upper level front differ? How does the temperaturediffer across these types offronts?

Upper-Tropospheric FrontsUpper-tropospheric (or upper level) front: A front that is present aloft thatmay or may not extend all the way to the surfaceWhat is the relationship between upper air fronts, the tropopause, and polarjet stream?Where is air rising (sinking) relative to the upper air front and jet stream?

o Air masses of North America Fronts o Stationary fronts o Cold fronts o Warm fronts o Fronts and the jet stream o Frontogenesis o Drylines o Occluded fronts . - transient weather systems (Rossby waves) - small scale f