Warm Up 4/1/08Warm Up 4/1/081) Which type of air mass originates in northern

Canada?a. mT c. mPb. cP d. cT

2) Which air masses have the greatest effect on weather conditions in much of the United States?a. cP and mT c. mP and cTb. mP and cP d. mT and cT

3) Which air mass often originates as a different air mass in Siberia and is subsequently altered?a. mT c. mPb. cP d. cT

Answers: 1) b. 2) a. 3) c.Answers: 1) b. 2) a. 3) c.

FrontsFronts

Chapter 20, Section 2Chapter 20, Section 2

Formation of FrontsFormation of FrontsWhen two air masses meet, they form a When two air masses meet, they form a frontfront, , which is a boundary that separates two air masseswhich is a boundary that separates two air masses

Fronts can form between any two contrasting air Fronts can form between any two contrasting air massesmasses

In contrast to the vast sizes of air masses, fronts In contrast to the vast sizes of air masses, fronts are narrow (~15-200 km wide)are narrow (~15-200 km wide)

In the ideal case, the front would move in the same In the ideal case, the front would move in the same direction with warmer air overlying cooler airdirection with warmer air overlying cooler air

However, the distribution of pressure across a front However, the distribution of pressure across a front causes one air mass to move faster than the other, causes one air mass to move faster than the other, causing it to advance into the slow air masscausing it to advance into the slow air mass

Warm FrontsWarm FrontsA A warm frontwarm front forms when warm air moves into forms when warm air moves into an area formerly covered by cooler airan area formerly covered by cooler airThe surface position of a warm front is shown by The surface position of a warm front is shown by a red line with semi-circles that point toward the a red line with semi-circles that point toward the cooler aircooler airThe first sign of the approaching warm front is The first sign of the approaching warm front is the appearance of cirrus cloudsthe appearance of cirrus cloudsBecause of their slow rate of movement and Because of their slow rate of movement and very low slope, warm fronts usually produce very low slope, warm fronts usually produce light-to-moderate precipitation over a large area light-to-moderate precipitation over a large area for an extended periodfor an extended periodA gradual increase in temperature occurs with A gradual increase in temperature occurs with the passage of a warm front, as does a wind the passage of a warm front, as does a wind shift from the east to the southeastshift from the east to the southeast

Warm FrontWarm Front

Concept CheckConcept Check

What causes a warm front to form?What causes a warm front to form?

When warm air moves into an area When warm air moves into an area formerly covered by cooler air, a warm frot formerly covered by cooler air, a warm frot forms.forms.

Cold FrontsCold FrontsA A cold frontcold front forms when cold, dense air moves into forms when cold, dense air moves into a region occupied by warmer aira region occupied by warmer airOn a weather map, the surface position of a cold On a weather map, the surface position of a cold front is shown by a blue line edged with blue front is shown by a blue line edged with blue triangles that point toward the warmer air masstriangles that point toward the warmer air massAs a cold front moves in, it becomes steeper and As a cold front moves in, it becomes steeper and advance more rapidly than warm frontsadvance more rapidly than warm frontsThe forceful lifting of air along a cold front can lead The forceful lifting of air along a cold front can lead to heavy downpours and gusty windsto heavy downpours and gusty windsAs a cold front approaches, towering clouds can As a cold front approaches, towering clouds can often be seen in the distanceoften be seen in the distanceThe weather behind a cold front is dominated by a The weather behind a cold front is dominated by a cold air mass, so weather clears shortly after the cold air mass, so weather clears shortly after the front has passedfront has passed

Cold FrontCold Front

Concept CheckConcept Check

How are cold fronts different from warm How are cold fronts different from warm fronts?fronts?

Cold fronts are much steeper than warm Cold fronts are much steeper than warm fronts and advance more rapidly than fronts and advance more rapidly than warm fronts do, causing more violent warm fronts do, causing more violent weather to form.weather to form.

Stationary FrontsStationary Fronts

Occasionally, the flow of air on either side of a Occasionally, the flow of air on either side of a front is neither toward the cold air mass nor front is neither toward the cold air mass nor toward the warm air mass, but almost parallel to toward the warm air mass, but almost parallel to the line of the frontthe line of the frontThe surface position of the front does not move, The surface position of the front does not move, and a and a stationary frontstationary front forms formsOn a weather map, stationary fronts are shown On a weather map, stationary fronts are shown by blue triangles on one side of the front and red by blue triangles on one side of the front and red semicircles on the othersemicircles on the otherSometimes, gentle to moderate precipitation Sometimes, gentle to moderate precipitation occurs along a stationary frontoccurs along a stationary front

Occluded FrontsOccluded Fronts

When an active cold front overtakes a When an active cold front overtakes a warm front, an warm front, an occluded frontoccluded front forms forms

The weather associated with an occluded The weather associated with an occluded front is complex: most precipitation is front is complex: most precipitation is associated with the warm air’s being associated with the warm air’s being forced upwardforced upward

Remember, fronts, like all aspects of Remember, fronts, like all aspects of nature, do not always behave as we nature, do not always behave as we expect them toexpect them to

Formation of an Formation of an Occluded FrontOccluded Front

Middle-Latitude CyclonesMiddle-Latitude CyclonesThe main weather producers in the U.S. are The main weather producers in the U.S. are middle-latitude cyclones, denoted by the letter Lmiddle-latitude cyclones, denoted by the letter LMiddle-Latitude Cyclones –Middle-Latitude Cyclones – large centers of large centers of low pressure that generally travel from west to low pressure that generally travel from west to east and cause stormy weathereast and cause stormy weatherThe air in these weather systems moves in a The air in these weather systems moves in a counterclockwise direction and in toward the counterclockwise direction and in toward the center of the lowcenter of the lowMost middle-latitude cyclones have a cold front, Most middle-latitude cyclones have a cold front, and frequently a warm front, extending from the and frequently a warm front, extending from the central areacentral areaForceful lifting causes the formation of clouds Forceful lifting causes the formation of clouds that drop abundant precipitationthat drop abundant precipitation

Middle-Middle-Latitude Latitude Cyclone Cyclone ModelModel

Cloud Cloud Patterns Patterns

Associated Associated With With

Middle-Middle-Latitude Latitude

CyclonesCyclones

Concept CheckConcept Check

How do middle-latitude cyclones form and How do middle-latitude cyclones form and develop?develop?

The first stage is the development of a front. The first stage is the development of a front. Over time, the front takes on a wave shape. Over time, the front takes on a wave shape. Changes in air flow and pressure result in a Changes in air flow and pressure result in a counterclockwise flow of air. The cold front counterclockwise flow of air. The cold front eventually closes in on the warm front to eventually closes in on the warm front to produce an occluded front. As the cold front produce an occluded front. As the cold front lifts, an occluded forms. Eventually, the cyclone lifts, an occluded forms. Eventually, the cyclone weakens.weakens.

The Role of Airflow AloftThe Role of Airflow AloftAirflow aloft plays an important role in maintaining Airflow aloft plays an important role in maintaining cyclonic and anticyclonic circulationcyclonic and anticyclonic circulationThe rotating wind systems are actually generated The rotating wind systems are actually generated by upper-level flowby upper-level flowSurface convergence must be offset by outflow Surface convergence must be offset by outflow somewhere higher in the atmospheresomewhere higher in the atmosphereAs long as the spreading out of air high up is equal As long as the spreading out of air high up is equal to or greater than the surface inflow, the low-to or greater than the surface inflow, the low-pressure system can be sustainedpressure system can be sustainedMore often than not, air high up in the atmosphere More often than not, air high up in the atmosphere fuels a middle-latitude cyclonefuels a middle-latitude cycloneIn an anticyclone, air spreading out at the surface is In an anticyclone, air spreading out at the surface is balanced by air coming together from high upbalanced by air coming together from high up

Role of Airflow Aloft in Cyclonic ActivityRole of Airflow Aloft in Cyclonic Activity

AssignmentAssignment

Read Chapter 20, Section 2 (pg. 564-570)Read Chapter 20, Section 2 (pg. 564-570)

Do Chapter 20 Assessment #1-37 (pg. 583-Do Chapter 20 Assessment #1-37 (pg. 583-584)584)

Print Out “Investigation 20A – Analyzing Print Out “Investigation 20A – Analyzing Severe Weather Data” Severe Weather Data”