Chap. 9 Air Masses And Fronts

Chap. 9Air masses and fronts

Air mass Definition: Large body of air with relatively uniformtemperature and “moisture” (e.g., dewpointtemperature) characteristics Formation – air remains over a source region for asufficiently long time to acquire specificcharacteristics that depend on the surface andother atmospheric processes––––Low wind conditionsUniform surface properties (e.g., snow, water)Anticyclonic conditions (High Pressure)Semi-permanent low pressure regions Source regions – examples to two pure air masses– Continental polar air masses (from Canada)– Maritime tropical air mass (from Gulf of Mexico)

Source regionsThe areas where airmasses form are calledsource regions.

More on definitions Main descriptors:– Arctic – bitter cold– Polar – cold (not as cold as arctic)– Tropical – warm to hot Secondary descriptors– Continental – origin over a land mass– Maritime – origin over large body of water; air hasgreater water vapor content Classification (based on T and Td)–––––cP (polar continental)A (Arctic)mT (tropical maritime)cT (tropical continental)mP (polar maritime)

Air mass structure Horizontal extent: 1000 km Vertical extent (depth): lower troposphere,2-5 km AGL

Fronts Defined as a boundary between two airmasses. The boundary zone can be wide ornarrow, depending on the type of front– Warm fronts are typically wide, 10-100 km– Cold fronts can be very narrow down to 1km in width (1-10 km is typical)– Example: A cold front occurs when the coldair advances and displaces (lifts upwards) thewarmer air ahead of it

Focus 8.1: Frontal symbolsStudy ”“Frontgenesis”“Frontolysis”Also: Arctic front (basically a strong cold front)

Frontsn A weather front is a boundary separating two air massesn Types: cold front, warm front, occluded front, stationaryfront, dry line, squall line

What is an atmospheric front? A front is a transition zone between two air masses of differentdensities. The density contrast results from:– Difference in temperature;– Difference in humidity. The frontal zone (surface) is theupward extension of the front. Sometimes the frontal zones can be very sharp. The intensity of the weather along the front depends on the contrastof the air mass properties. The type of front depends on both the direction in which the air massis moving and the characteristics of the air mass.

Types of Fronts Cold front: cold, dry stable air isreplacing warm, moist unstable air. Warm front: warm, moist unstableair is replacing cold dry stable air. Stationary front: boundary betweenthe two air masses is not moving. Occluded front: when a cold frontcatches up with a warm front The symbols on a map are in thedirection of the air mass motion.

Weather Map Shown: surface-pressure systems, air masses, fronts,isobars, winds and air flow (large arrows) Green-shaded area: precipitation

Fig. 8.3. Example of a cold front moving southeastward across the central US. The coldair is lifting warm moist conditionally unstable air, creating a ling of thunderstorms alongthe front. This cold front would have a structure similar to that in Fig. 8.2A. Note thatthe cold air north of the warm front has a component toward the north, so that the coldair is retreating northward with time. Gray shading denotes cloud cover.

What are the Signsof a Passing Front? Signs– Sharp temperature changes over a relativelyshort distance.– Changes in the air’s moisture content (asindicated by changes in the dew point).– Shifts in wind direction– Pressure and pressure changes.– Clouds and precipitation patterns. The location of the front is not always veryobvious! Even meteorologists sometimesdisagree

Cold Fronts A cold front is a mass of cold air advancing towards warm air. Typically associated with heavy precipitation, rain or snow, combinedwith rapid temperature drops. Since friction decreases with height, winds move faster at higheraltitude. Then the surface of cold front becomes more steeperthrough time, leading to a narrow belt of precipitation. Moving speed 0-30mph

Fig. 8.2Cross section through three coldfronts.In A, the air ahead of the front isconditionally unstable and formsthunderstorms (or a squall line)when lifted by the front.In B, stable, moist air is lifted alongthe frontal surface.In C, dry air is lifted (but not to thelifting condensation level), soclouds do not form.In all cases, the cold air isadvancing into warm air.

Cold Fronts Cold front- a front in which cold air isreplacing warm air at the surface.Notice the difference in– Temperature– Dew point– Wind direction– PressureAssociated with low pressure centers (lowpressure troughs): follow the dashed lineThe pressure is minimum as the frontpasses (first decreases as the frontapproaches and then increases behind thefront)

Cold Front: cloud and precipitation patterns The warm, moist air ahead ofthe front is forced upward andcondenses– Cirrus clouds well ahead ofthe front– Strong thunderstorms withheavy showers and gustywinds along and ahead ofthe front: squall lines– Broad area of cumulusclouds immediately behindthe front (although fastmoving fronts may be mostlyclear behind the front).

Warm Fronts Warm fronts are warm air moving towards cold air. This overrunning process produces large amounts of warm, moist airover cooler, drier air. Shallow stratus clouds dominate and bring light precipitation toaffected regions. Stable regions above the warmer air create verticallylimited clouds and light precipitation. Frontal fogs may occur as rainevaporates in the colder air near the surface. Moving speed about 12 mph

Warm Fronts Warm front - in which warm airreplaces cooler air at thesurfaceNotice the difference in– Temperature– Dew point– Wind direction– PressureNotice the presence ofprecipitation well ahead of thefront

Slope of warm Fronts Friction decreases withheight, so winds movefaster at higher altitude This causes the surface ofthe front to become lesssteep through time. Thenclouds will be spread to awider region.

Warm Fronts: cloud and precipitation patterns Although they can trigger thunderstorms, warm fronts are morelikely to be associated with large regions of stratus clouds andlight to moderate continuous rain. Warm fronts are usually preceded by cirrus first, thenaltostratus or altocumulus, then stratus and possibly fog. At the warm front, gradual transition. Behind the warm front, skies are relatively clear.

Warm air advances over the cool/cold air; the slope is generally shallowerFig. 8.4. Cross section through two warm fronts. In A, the air south of the front isconditionally unstable and forms showers and thunderstorms as it rises over the coolair. In B, the rising moist air south of the front is stable, so the clouds are stratifiedand showers are absent (but rain bands may exist).

Stationary Fronts Stationary fronts do not move. They do not advance.They are two unlike air masses side by side. They may slowly migrate and warmer air is displacedabove colder.From EnvironmentCanada

Stationary Front Stationary front- a front which does not move or barely moves. Stationary fronts behave like warm fronts, but are more quiescent. Many times the winds on both sides of a stationary front are parallelto the front and have opposite direction. Typically stationary fronts form when polar air masses are modifiedsignificantly so as to lose their character (e.g., cold fronts whichstall). Typically there is no strong precipitation associated with stationaryfronts (why? – no big contrast in the air mass properties, no airuplifting and condensation).

Stationary Fronts Stationary fronts do not move. They do not advance.They are two unlike air masses side by side. They may slowly migrate and warmer air is displacedabove colder.From EnvironmentCanada

Stationary fronts:The boundary is stationary, notadvancingOpposing flows (at low levels) arenearly balanced in each side of thefrontFig. 8.5. Maps showing two examplesof stationary fronts and associatedclouds (gray shading). Note that inboth examples, the cold air on thenorth side of the front is movingparallel to the front. In A, clouds formon the warm side of the front andprecipitation would fall in this region.In B, clouds form on the cold side ofthe front and any precipitation wouldfall out on the cold side.

Fig. 8.6Vertical cross sectionsthrough two stationaryfronts.Cold air flow parallel to thefront in each case.The warm air isconditionally unstable ineach case.In panel A, whichcorresponds to the crosssection shown in Fig. 8.5A,the warm air is lifted at theleading edge of cool air.In panel B, warm air flowsover the cold air and formsless vigorous showers and/or thunderstorms.

Fig. 8.7. Development of an occluded front. See text caption

Occluded Fronts Occluded fronts occur when two fronts meet, the warm airmass between them is displaced aloft. This typically occurs when a cold front meets a warm front asit circulates the low pressure center of a mid-latitude cyclone. The cold and warm fronts curve naturally poleward into thepoint of occlusion, which is also known as the triple point.

Formation ofOccludedFronts

Occluded fronts. Cold fronts move faster than warm fronts.They can catch up and overtake theirrelated warm front. When they do, anoccluded front is formed.Cold occlusion: very cold air behind, notso cold air ahead of, the warm frontThe upper warm front follows the surfaceoccluded frontCold occlusion

Different types of occluded fronts A cold-type occlusion usuallyoccurs in the eastern half of thecontinent where a cold frontassociated with continental/Polarair meets a warm front withmaritime/Polar air ahead. A warm-type occlusion is typicalof the western edges of continentswhere the cold front, associatedwith maritime/Polar air, migrates toan area that is occupied bycontinental/Polar air.

Warm Occlusion Very cold air ahead of, not so cold airbehind, the warm frontThe cooler air from the cold front cannotlift the very cold air ahead, rides“piggyback”The warm front aloft precedes thesurface occluded front

Fig. 9.7 (cont.)This of each sequence in terms of a variation in north-south location at a giventime, or a temporal evolution at a (nearly) fixed location. For the latter, timewould advance upwards in each column above.

Drylines Drylines are boundaries between lighter humid air and denserdry air. Air masses with similar temperatures but strong humiditygradients will act as fronts. They frequently occur throughout the Great Plains, and are afavored location for thunderstorm development.

Dry linesFig. 9.8. A dry line isdefined as a boundaryseparating warm dry air tothe west, from cooler butmoister air to the east.The dry line often servesas a focusing mechanismfor severe storms over theGreat Plains, especiallyover Texas, Oklahoma,and Kansas.Dry lines are rarelyanalyzed over Alabama.

Dry Line Found just east of theRocky Mountains– Formed from warm, dryair on leeward side

Dry Line

Upper-level fronts(cold fronts)Fig. 9.9. Surfaceconditions and 500 mbflow patterns associatedwith an upper-level coldfront.Such fronts often have atrailing cold front at thesurface.Upper level fronts caninitiate thunderstorms, butforecasting them isdifficult since the frontallocation cannot bedetermined from surfacedata.Many unknownsText, p. 146

Fig. 8.10. Analysis of sea-level pressure and surface temperature at 1200 UTC, 11 Dec2000. Note the effects of cold air damming on the east side of the Rocky Mountains,and also along the east side of the Appalachian Mountains over NC, SC, and GA.

Frontolysis & Frontogenesis Frontolysis– What kind of temperature gradient isassociated with frontolysis? Frontogenesis– What kind of temperature gradient isassociated with frontogenesis?

Cold Air Damming Cold air istrappedtopographically– Usually alongeastern slopes– Why? Circulationaround the highpressure Mesoscale event

Cold Air Damming in US Cold air is confined along the easterncoastal plain of Appalachian Mountains– Usually occurs during winter and early spring– Ice storm