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Mid-latitude weatherMid-latitude weather

Prof. Jeff GawrychProf. Jeff Gawrych

De Anza CollegeDe Anza College

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Introduction Here in the mid-latitudes, day-to-day weather changes are Here in the mid-latitudes, day-to-day weather changes are

closely linked to moving, synoptic-scale disturbancesclosely linked to moving, synoptic-scale disturbances These are the high and low pressure systems These are the high and low pressure systems

meteorologists often speak of.meteorologists often speak of. Bay Area is at ~ 37º NBay Area is at ~ 37º N These systems are steered by upper-levels winds (jet These systems are steered by upper-levels winds (jet

streams)streams) Using observations (such as weather balloons), we can Using observations (such as weather balloons), we can

track their development and decaytrack their development and decay Unfortunately, coverage is sparse.Unfortunately, coverage is sparse.

No two systems are exactly alikeNo two systems are exactly alike

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Atmospheric Scales of MotionScaleScale Time ScaleTime Scale Distance ScaleDistance Scale ExamplesExamples

MacroscaleMacroscale -Planetary-Planetary Weeks to yearsWeeks to years Westerlies, Westerlies,

trade windstrade winds

-Synoptic-Synoptic Days to weeksDays to weeks Cyclones, anticyclones Cyclones, anticyclones and hurricanesand hurricanes

MesoscaleMesoscale Minutes to daysMinutes to days Land-sea breeze,Land-sea breeze,thunderstorms and thunderstorms and

tornadoestornadoes

MicroscaleMicroscale Seconds to minutesSeconds to minutes Turbulence, dust Turbulence, dust devils and gustsdevils and gusts

500 – 25,000 miles500 – 25,000 miles

50 – 3,000 miles50 – 3,000 miles

1 – 50 miles1 – 50 miles

< 1 mile< 1 mile

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Upper-level flow

Between 500 - 250 mb, or 18,000-36,000 ftBetween 500 - 250 mb, or 18,000-36,000 ft In this portion of the atmosphere, we can most In this portion of the atmosphere, we can most

easily see these traveling synoptic events such as:easily see these traveling synoptic events such as: Cyclones (low pressure systems) Cyclones (low pressure systems) Anticyclones (high pressure systems)Anticyclones (high pressure systems)

Surface features such as mountains, cities, Surface features such as mountains, cities, oceans/land can be misleadingoceans/land can be misleading

Ben Franklin noted how rain bands do not Ben Franklin noted how rain bands do not always travel in the same direction as the large always travel in the same direction as the large storm system storm system

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Cylonic flowCylonic flow Anticylonic flowAnticylonic flow

Low and High pressure systems

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Upper air patterns Aloft, the pressure field normally does not consist of closed Aloft, the pressure field normally does not consist of closed

high and low pressure contours, it simply flows:high and low pressure contours, it simply flows: In a general west to east directionIn a general west to east direction In a wavelike patternIn a wavelike pattern

At upper levels, the pressure field is normally given as the At upper levels, the pressure field is normally given as the geopotential heightgeopotential height..

The geopotential height: The geopotential height: is the height of a particular pressure level.is the height of a particular pressure level.

Consider a 500mb geopotential map. Consider a 500mb geopotential map. In this map, the height lines (~5000m) are In this map, the height lines (~5000m) are

physically the same as the surface pressure.physically the same as the surface pressure.

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Trough

TroughTrough

Ridge

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Surface flow At the surface, friction is more important than at At the surface, friction is more important than at

upper levels, upper levels, thus geostrophic balance is not validthus geostrophic balance is not valid

The result, winds not exactly parallel to isobars, The result, winds not exactly parallel to isobars, rather angled:rather angled: Into low pressure systemsInto low pressure systems Away from high pressure systemsAway from high pressure systems

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Friction slows down wind, which decreases coriolis force:Wind now aimed more in direction of PGF.

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Why do low-pressure systems cause rain? Aloft: flow is counter clockwise in nh and

flow is geostrophic Surface: flow is ~cc, but NOT geostrophic, Instead flow is inward towards low causing

convergence Leads to clouds/rain

convergence divergence image

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Why do high-pressure systems cause clear conditions? Aloft: flow is clockwise in nh and geostrophic Surface: flow is ~clockwise but NOT geostrophic Instead flow is outward from high causing divergence Leads to sinking motion (subsidence) and clear skies What happens to air that sinks???

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Air Masses A general description of the atmosphere over a A general description of the atmosphere over a

certain area.certain area. Tells temperature and moisture profile.Tells temperature and moisture profile. ExamplesExamples

cT: continental tropical (warm and moist)cT: continental tropical (warm and moist)

cP: continental polar (cold and dry)cP: continental polar (cold and dry)

mT: marine tropical ( warm and moist)mT: marine tropical ( warm and moist)

mP: marine polar (cold and moist)mP: marine polar (cold and moist)

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Air masses Type of air mass over a region tells what weather Type of air mass over a region tells what weather

may be like. may be like. Name of air masses tells you where it originatedName of air masses tells you where it originated

Air masses originate in specific areas and can Air masses originate in specific areas and can dominate the climate.dominate the climate. Remember: climate is what you expect, weather Remember: climate is what you expect, weather

is what you get.is what you get.

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Fronts Separate different air masses of different Separate different air masses of different

densities. Cold air is more dense than warm densities. Cold air is more dense than warm airaircold air sinks, warm air rises. So when cold air sinks, warm air rises. So when a cold front passes, it forces lifting.a cold front passes, it forces lifting.

Cold fronts bring cold air into regionCold fronts bring cold air into region Warm fronts bring warm air into regionWarm fronts bring warm air into region

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Cold Front

Warm front

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Temperature advection Wind direction can be good indicator of temperature change.Wind direction can be good indicator of temperature change.

N. Hem:N. Hem:

Wind from the north generally indicates cool air approaching. (cold advection)

Wind from the south generally indicated warm air approaching. (warm advection)