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Topic 13

Weather

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Basic Questions

1. Why does Buffalo get lake-effect snow?

2. What are the air masses surrounding NA?

3. What causes air mass instability (lifting)?

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Weather: Topics

Air Masses

Atmospheric Lifting Mechanisms

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General Atmospher ic Circulation

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Air Masses

Distinct volume of air having specific

Temperature

HumidityStability

Moisture and temperature

characteristics of itssource region

Alberta Clipper

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Air Masses Affecting NAMoisture

mfor maritime (wet)

cfor continental (dry)

Temperature (Latitude)

Afor artic

Pfor polar

Tfor tropicalEfor equatorial

AAfor Antarctic

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NA Air MassesWinter

4. Maritime tropical (MA)

Stable, conditionally unstableSubtropical high-pressure cells

Pacific more stable, drier

2. Continental Polar( CP)

NH continentality

Cold, stable, clear skies, high P

Lifts moist, warm air in its path condensation

1.Continental Arctic (CA)

Stable, very cold and very dry

3. Maritime Polar (MP)

Cool, moist, unstable air

Subpolar low-pressure cells

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NA Air MassesSummer

1. Mariritme Tropica(MP) Move northward, onto land

2. CP: Move northward, off

shore

3. CT : Hot, dry air moves into southwest

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Lake-effect Snow

Below freezing cP and cA air absorbs heat and moisture

over Great Lakes

On shore, air becomes chilled heavy snow (200 in.)

Restricted to ~50 to 100 km inland

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Lake-effect Snow

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Atmospher ic L if ting

For air masses to cool adiabatically and form

precipitation, they must lift and rise

Four mechanismsConvergent Lifting

Convectional Lifting

Orographic Lifting

Frontal Lifting

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Atmospher ic L if ting Mechanisms (1)

Trade Winds and ITCZ

Dew point condestion: cloud shape flat bottom

Convergent lifting

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Atmospher ic L if ting Mechanisms (2)

Air warms over

ground surface

Floridaboth convergent

and convectional lifting

Convectional ( local) Lifting

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Atmospher ic Lif ting Mechanisms (3a)

Air lifted, cools adiabatically;

rain shadow

Orographic Lifting: adiabatic heating

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Sierra Nevada

Mt. Range

Cools adiabatically

Heats adiabatically

Rain shadow, WA

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Atmospher ic Lif ting Mechanisms (4a)

.: leading edge

of advancing airmass

A place of

atmosphericdiscontinuity, line

of conflict

between two airmasses

Frontal lifting

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Atmospher ic Lif ting Mechanisms (4a)

Leading edge ofCold air mass: Cold front

Warm air mass: Warm front

Frontal lifting

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Atmospher ic Lif ting Mechanisms (4b)

Steep face due to density anduniform characteristics

Warm air is lifted, coolsadiabatically, and becomesunstable

.: a wind shift, Tdrop,lower P, cumulonimbusclouds, and heavy precipitation

An advance of cp or mp toward mt

A fast advancing cold front can cause violent lifting

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Atmospher ic Lif ting Mechanisms (4c)

Warm air pushes the coolerunderlying air into a wedgea temperature inversion

mTair gently lifted, leadingto cloud development

Leading edge of mt air unable to lift cooler air mass

Larger spatial space than cold air mass.

Warm fronts form a progression of clouds-not as

energetic as an advancing cold front.

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Basic Questions

1. Why does Buffalo get lake-effect snow? cP

air mass migrating across lake2. What are the air masses surrounding NA?

cP, mP, and mT

3. What causes air masses instability (lifting)?Convergence, convection, orography, fronts.