AIR MASSES FRONTS - Moore Public Schools

AIR MASSES&FRONTS

AIR MASSES

Air Masses An air mass is a defined as a large body ofair with very similar characteristics. Generally speaking, air masses aregenerally defined by temperature anddewpoints (moisture parameters.) Air masses are named based on the sourceregion of the air mass itself

Types of Air Masses Continental Polar, “cold and dry”– Originates closer to the Poles over landlocked regions. Continental Tropical, “warm and dry”– Originates closer to the Tropics over landlocked regions. Maritime Polar, “cold and damp”– Originates closer to the Poles over water. Maritime Tropical, “warm and humid”– Originates closer to the Tropics overwater. Arctic, “very cold”– Originates in the very cold land-lockedareas

Analyzing Air Masses An air mass is most easily identified bycomparing it to other air masses. Air masses can be modified with time, mostnotably by days of sunshine or lack thereof. Fronts are the dividing line between airmasses so understanding air masses, meansunderstanding where fronts are located.

FRONTS

Fronts4 different types:1. Cold front2. Warm front3. Stationary4. OccludedFronts are the boundaries between two airmasses.Fronts are the basic building blocks of weathersystems.Fronts occur where two large air masses collide atthe earth's surface.Each air mass has a different temperatureassociated with it.Fronts are caused by winds moving one air massaway from its birthplace.

Air Masses and FrontsFronts are classified as to whichtype of air mass (cold or warm) isreplacing the other.A cold front separates the leadingedge of a cold air mass displacinga warmer air mass.A warm front is the leading edgeof a warmer air mass replacing acolder air mass.If the front is essentially notmoving (i.e. the air masses arenot moving) it is called astationary front.

Types of Fronts Cold– Noted by cold air advancing anddisplacing warmer air that exists. Warm– Noted by cold air retreating froman area. Stationary– While differing air masses existalong a boundary, little movementis analyzed of the air masses. Occluded– A complicated process where thesurface low becomes completelysurrounded by cooler/cold air.Occlusion processes can be a “cooltype” or “cold type” (more later.)

Fronts1. Cold frontCold fronts occur when heavy cold airdisplaces lighter warm air, pushing it upward.Cumulus clouds form and usually grow intosnow storms.Temperatures drop anywhere from 5 to 15 .Winds become gusty and erratic.Rain, snow, sleet, and hail can occur with acold front.

Cold Front Marked on a map with a blue line and bluetriangles pointing towards the warm air. Associated with cumulus & cumulonimbusclouds ahead of the front in the warm air,producing showers and thunderstorms.

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Cold Front

Fronts2. Warm frontWarm fronts occur when warm air replacescold air by sliding over it.Altocumulus clouds form and may beassociated with rain, snow, or sleet.Temperatures may warm slightly.Winds are usually gentle with this kind offront.

Warm Front Marked on a map by a red line with red semicircles pointed towards the cool air (in thedirection the warm air is retreating to.) Generally associated with stratus type clouds,overcast skies, fog, and general rain or snow.

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Warm Front

Fronts3. StationaryStationary fronts occur when neither warm nor cold air advances.The two air masses reach a stalemate.These type of conditions can last for days, producing nothing butAltocumulus clouds.Temperatures remain stagnant and winds are gentle to nil.

Stationary Front Marked by alternating blue lines & blue triangles(pointed in the direction of the warmer air) andred lines & red semi-circles (pointed in thedirection of the cooler air) Usually noted as *quasi*-stationary as it is rarelyever completely stationary. It tends to meander abit.

Fronts4. OccludedCold occlusion: When a cold air mass follows a warm air mass, the coldair mass, which moves faster, eventually catches up the warm front.This then lifts the warm air (behind the warm front) off the ground, creatingan occluded front, where the two fronts are joined.Usually associated with rain or snow and cumulus clouds.Temperature fluctuations are small and winds are gentle.May indicate the end of a storm cycle

Occluded Front Marked by a purple line with alternating purpletriangles and purple semi-circles, all pointing inthe direction of the frontal movement. There are two general types of occlusions, cooltype and cold-type. Examples to follow.

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Occluded chool.com/images/occluded-front.gif

CYCLONES

Cyclones: Putting it all together There are two types of cyclones,tropical/warm core and extratropical/coldcore. Extratropical cyclones are characterized byhaving differing air masses frontalmovements where tropical cyclones do not. We will concern ourselves withextratropical cyclones in this presentation.

Cyclones: Putting it all together Students must begin to understand the fourdimensional view of a cyclone––––North-SouthEast-WestUp-DownTime

The cyclone

Cyclone: On satellite

Cyclone

Cyclone Radar & Satellite view of a cycloneOcclusionOcclusionWarm FrontWarm FrontCold FrontCold Front

Storm TracksPineapple Express SW Storm Track Pineapple express These storms can leave over 7” ofwater High snow accumulation at the highest ofelevations in the mountains High elevation freezing levels

Storm TracksWest-SW Storm Track Temperature and humidity islower than for the “pineappleexpress A common storm track A break between storms,sometimes a few hours,sometimes a full day. Cooler air moves in after eachdisturbance, Freezing level is 3000’ to5000’, when these systemsmove through.

Storm TracksWest-NW Storm TrackPrecipitation is short-lived,producing fairly rapid coolingThese systems drop light (lowSWE) snow in the mountainsFreezing level is 1500’ to nomore than 3000’ or so in winter.

Storm TracksNortherly Storm TrackCold air travels from the north, passes justlong enough over the ocean to pick upmoisture (but not long enough to warm theair and produce rain).Winds slide along, rather than across theranges, so more snow may fall in theflatlands than in the mountains.Freezing level is at ground level.

AIR MASSLIFTINGMECHANISMS

Types of Lifting Mechanisms1. Convective Uplifting2. Convergent Uplifting3. Orographic Uplifting4. Frontal Uplifting

Lifting MechanismsConvection andconvective liftingAir is heated by the Earth's surface. The air is heated unequally, areas ofwarmer air are formed amidst cooler air.Since warm air is lighter, it will tend to rise and this may lead to theformation of localized clouds and showers.

Convergent UpliftConvergent uplift occurs when airenters a center of low pressure. As airconverges into the center of a cycloneit is forced to rise off the surface. Asthe air rises it expands, cools, andwater vapor condenses. Convergentand convective uplift are the two mostimportant uplift mechanisms forcondensation in the tropics. Under theintense sun, surface heating causes themoist tropical air to rise. Convergenceof the trade winds in the Intertropicalconvergence zone creates copiousrainfall in the wet tropics as well.

Lifting MechanismsOrographic liftingOrographic lift takes place when a moving mass of air runs up against a mountain rangeand is forced upwards.This is the most powerful lifting mechanism and accounts for the majority of precipitationin the PacNW.Moist ocean air is lifted orographically and can cause precipitation without any associatedstorms or frontals systems.The warm and cold fronts that bring heavy snowfalls to the Cascades Mountains oftendissipate by the time they reach eastern Washington; little moisture remains and lesseramounts of snow fall at Mission Ridge

Orographic UpliftOrographic uplift is the forced ascent of airwhen it collides with a mountain. As airstrikes the windward side, it is uplifted andcooled. Windward slopes of mountains tendto be the rainy sides while the leeward sideis dry. Dry climates like steppes and desertare often found in the "rain shadow" of tallmountain systems that are orientedperpendicular to the flow of air.Arainshadow in the lee of the tradewinds asthey cross the mountainous northwestportion of the Big Isand of Hawaii (Figure7.10). Cloud formation and green vegetatonidentifies the windward, while the reddishbrown indicates the dry leeward side.

Rainshadow on the Big Island of Hawaii.(Source NASA EOS)

FRONTAL UPLIFTFrontal uplift occurs when greatlycontrasting air masses meet along aweather front. For instance, whenwarm air collides with cool air along awarm front, the warm air is forced torise up and over the cool air. As the airgently rises over the cool air,horizontally developed stratus-typeclouds form. If cold air collides withwarm air along a cold front, the moredense cold air can force the warmer airahead to rise rapidly creating verticallydeveloped cumulus-type clouds.

Fronts are the boundaries between two air masses. Fronts are the basic building blocks of weather systems. Fronts occur where two large air masses collide at the earth's surface. Each air mass has a different temperature associated with it. Fronts are caused by winds moving one air mass