Physical Geography

Chapters 5 & 6:Precipitation, Air Masses,

Storms, and other fun stuff

Formation of Precipitation:The Bergeron Process

The Collision-coalescence process

Some Different Forms of Precipitation

• Rain– Drizzle vs. showers

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Some Different Forms of Precipitation

• Snow• Sleet• Glaze (ice storm)

Hail

The Formation of Hail

The Formation of Hail

Global Precipitation

Global Precipitation:The ITCZ Connection

Precipitation in the U.S.

What Is An Air Mass?A large parcel of air with

characteristics distinguishing it from surrounding air1000 mi (1600 km) across,

several miles deepConditions of temp., humidity,

stability consistent horizontally at any altitude

Moves as a coherent whole, not easily torn apart by local turbulence

Source region: Where an air mass originates

Source Regions Extensive, physically uniform surface area High or low latitude

Not found in the midlatitudes (too much atmospheric activity) High pressure zones are common source regions

(because air sinks, stays close to the ground, where it picks up surface characteristics)

Air Mass Movement & Modification

Once an air mass moves, it influences the regions it enters

It is also influenced by those regions, especially in its lower section, closest to the ground

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Buffalo, NY (Dec., 2001)--Nearly seven feet of lake effect snow fell in 5 days

Areas commonly affectedaround the Great Lakes

Lake-effect snow:cP air crossing warmer water

Air Mass ClassificationLatitude

A = arctic/antarcticP = polarT = tropicalE = equatorial

Surface Conditionsm = maritimec = continental

Major Air Mass Source Regions

(c)AmPcPmTcT(m)E

Air Masses of North America

So what happens when these air masses meet???

They start frontin’.

Frontal lifting

Movement of a Warm Front

Warm Front: Development

Movement of a Cold Front

Cold Front: Development

Comparison: Note the shapeof the frontal boundary

Stationary Front

Occluded Front

Fronts on a Weather Map

Putting it together:Note line A – A’

A cross section along line A – A’ (from the map on the previous slide)

Real-World Application:An Atlantic Storm

Life-cycle of a Midlatitude Cyclone

A Hypothetical Weather Map(note the alternating Highs and Lows…)

How do theUpper-level Winds Move?

Major Midlatitude Disturbances

Midlatitudes are the most dynamic weather regionWhere polar and tropical air masses meet and mix

Midlatitude cyclones(a.k.a. depressions, lows, wave cyclones)Large low pressure systems (1000+ miles across)

moving from west to east in the region of the Westerlies (35º to 70º N and S latitude)

Characteristic weather changes with the passage of a cold front: Sharp temp. drop as the front approaches As the front approaches, wind direction is southerly After the front passes, wind shifts to more northerly

(opposite for the Southern Hemisphere) Air pressure drops as the front approaches, rises

after it passes Clear skies, followed by clouds and precip. along

the edge of the front, then colder with clear skies again as the front passes

Mapping it out:

Midlatitude Anticyclones

High pressure systems moving west to eastNo frontsSubsidenceClear, dry weatherCold in winterMay stagnate, stalling other weather

systems behind them

Now on to the fun stuff!

Lightning

Lightning

Thunder

Tornadoes

Tornado formation

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Tropical Disturbances

Tropical Depression - winds up to 38 mphTropical Storm - winds 39 - 73 mphHurricane - winds 74+ mph

Hurricanes

Four different names for the same event:

HurricaneTyphoonCyclones, tropical cyclonesBaguios

Hurricane Origins Form in tropical and subtropical zones approx. 8 to 15 N or S

latitude Rarely form within 3 N or S of equator (no Coriolis force), rarely

cross it Tend to form in or just poleward of the ITCZ Tend to form in late summer and fall (warmest sea sfc. temps.) Storm’s low pressure cell feeds off warm sea sfc. temps. (up to

81F!) Gains energy from release of latent heat of condensation during

intense precipitation Always form over oceans Do not / rarely form in the south Atlantic or southeast Pacific

because the water is too cold and air pressure too high Storm intensity lessens as it gains latitude (into cooler waters) or

moves over land

Hurricanes

Hurricane Tracking

Pressure Signature of a Hurricane

Hurricane Structure

Hurricane Katrina making landfall

Storm Surge

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Storm surge damage in Galveston, TX from Hurricane Ike (Category 2)