GEOG 203 502 Exam #2

What causes Horizontal pressure variations? 

1. Temperature

2. Air motion

Temperature:

• T inversely related to P \

  • High T (=hot) —> Low P

  • Low T (=cold) —> High P

—> Thermal Low or High 

Air Motion

• Rising air —> Low P at surface

• Sinking air —> High P at surface 

—> Dynamic Low or High 

Driving Forces in the atmosphere: 

1. Pressure Gradient Force 

2. Coriolis Force 

3. Friction Force 

Pressure Gradient Force

• Initially causes air to move from High —> Low pressure

• 90 degree angle to isobars 

Wide Isobar spacing

Light winds

Close isobar spacing

strong winds

Stronger pressure gradient

stronger wind speed

Pressure Gradient Force: HIGH

• Air diverges

• air sinks (fills in) 

Pressure Gradient Force: LOW

• air converges 

• air rises

Coriolis Force 

apparent deflection of moving objects 

Coriolis force is caused by what?

Earth’s Rotation (West —> East spin)

Coriolis force affects what?

Only the direction of movement

Deflection is to the _____ in the Northern Hem.

Right

Deflection is to the _____ in the Southern Hem.

Left

Coriolis Force is a function of:

• Latitude

• Wind Speed

Pressure Gradient Force + Coriolis Force = _________

Geostrophic Wind

—> only in middle & upper troposphere

Northern Hemisphere:

Anticyclonic geostrophic = 

Clockwise flow 

Northern Hemisphere:

Cyclonic geostrophic = 

counterclockwise flow 

Friction

• Occurs below ~1000m (3300 ft) altitude 

• Decreases Wind speed

  • disrupts Coriolis and PGF balance 

—> Causes wind to cross at an angle to isobars 

Pressure Gradient Force + Coriolis + Friction

• Friction slows wind — > decreases Coriolis —> wind crosses isobars

  • wind flow converges into low 

  • wind flow diverges out of high 

Two main winds

1. Geostrophic Wind

2. Surface Wind

Geostrophic Wind

no friction (mid/upper troposphere)

Surface Wind

friction (on Earth’s Surface)

Equatorial Low Pressure

• Found between ~10 deg. N to 10 deg. S

• A lot of energy 

  • Constant high sun angle

  • Consistent day length (always 12 hours)

  • Surface air heats up

—> Air Converges and rises (= clouds & rain!)

Intertropical Convergence Zone (ITCZ) =?

Equatorial Low Pressure

ITCZ Northern Hemisphere Summer —> maximum north altitude

25 degrees

ITCZ Southern Hemisphere Summer —> maximum South altitude

20 degrees

Easterly Trade Winds

• 25 N to 25 S

  • Northeast Trades 

  • Southeast Trades 

Hadley Cells

Trade Winds converge & air rises at ITCZ

• Air travels poleward in upper troposphere 

—> Cools & therefore sinks back to surface near ~30 deg. latitude 

Subtropical Highs

• Near 30 deg. N/S latitude

• High pressure, therefore: 

  • hot, dry (low humidity), descending air 

  • clear, warm, calm weather 

• H migrates 5-10 deg. latitude seasonally 

Westerlies

• Diverging winds north of Hadley cell 

• 30 deg to 60 deg. N/S

• cold and dry

Subpolar Low Pressure

• 60 deg. N/S

• Cool, moist air, clouds

• Seasonal variability —> Strongest in winter 

Polar High Pressure

• 90 deg. N/S

• Persistent in S.H., seasonal in N.H.

• Cold/dry air 

Jet Streams:

• Polar Front jet streams 

• Subtropical jet stream 

Polar front jet stream

• Latitude: 30-70 deg.

• Altitude: 7,600-10,700 m

Subtropical jet stream

• Latitude: 20-50 deg.

• Altitude: 10,000-16,000 m

How much of Earth’s water is fresh?

2.78%

Humidity

Water vapor content of air

Measures of humidity

1. Absolute 

2. Relative

3. Specific

4. Dewpoint

Absolute Humidity

mass of water vapor per volume of air

—> If volume changes, humidity changes even if the water vapor stays the same

Relative Humidity

((actual water vapor) / (water vapor capacity at T)) x 100

—> relative to temperature

Maximum Water Vapor capacity depends on ______

Temperature

Specific Humidity

mass of water vapor per total mass of air

—> not affected by changes in temperature, volume, or pressure

Saturation occurs when

• RH = 100%

• Rate of evaporation = condensation —> equilibrium 

• Air contains the maximum amount of water vapor possible 

Temperature at which air is saturated:

Dewpoint Temperature

Atmospheric Stability

tendency of the atmosphere to resist upward motion and instead stay in place (or maybe even sink)

Parcel

A body of air with homogeneous temperature & humidity characteristics, analog for the atmosphere

Parcel is stable if:

resists upward movement, and/or sinks back to its starting point

Parcel is unstable if:

rises & continues to rise freely, on its own

For parcel stability you need to know the _______ _______ the parcel and environment ______ the parcel

Temperature inside,

outside 

Temperature change inside the parcel occurs via what process

Adiabatic

Adiabatic = ?

Warming or cooling rate inside an air parcel

Temperature change occurs from ______ process only, without a loss or gain of energy from the surrounding environment 

internal 

Adiabatic cooling

As air parcel rises, it expands due to lower air pressure —> therefore cools

Adiabatic heating

As air parcel sinks, it compresses due to higher air pressure —> therefore heats

Lapse Rates

1. Dry 

2. Moist 

Dry (unsaturated) parcel —> DAR

• rises and cools at 10 deg. C/1000m 

• sinks and warms at 10 deg. C/1000m

Moist (saturated) parcel —> MAR

• rises and cools at 6 deg. C/1000m 

• Sinks and warms at 10 deg. C/1000m 

Stability Guidelines: Need to know

1. Temperature inside of parcel 

2. Temperature of surrounding environment 

Temperature inside of parcel

always use either DAR if unsaturated, or MAR if saturated

Temperature of surrounding environment

always varies … use the given Environmental Lapse Rate, ELR

If Temperature of parcel < Temperature of environment

• parcel is cooler/denser

• parcel does not rise 

—> STABLE

If temperature is parcel > temperature of environment

• Parcel is warmer/less dense 

• Parcel rises & cools adiabatically

—> UNSTABLE

ELR > DAR & MAR

Unstable

MAR < ELR < DAR

Conditionally unstable

ELR < MAR & DAR

Stable

As air rises and T decreases

Relative Humidity increases

Parcel may switch from DAR to MAR as it rises, if ________ . The switch is ________

Relative Humidity = 100%,

Lifting condensation level

Clouds

mases of tiny moisture droplets

Clouds are classified according to:

1. Form 

2. Altitude 

• Vertically developed: spanning multiple heights 

Cloud form: flat

Stratus (aka stratiform)

Cloud form: puffy

cumulus (aka cumuliform)

Cloud form: wispy

cirrus (aka cirroform)

Cloud Altitude: LOW

< 2,000 m (different prefixes)

Cloud Altitude: MIDDLE

2,000-6,000 m (“alto” prefix)

Cloud altitude: HIGH

6,000-13,000 m (“cirr” prefix)

High Cloud: Cirrus

Mares’ tails, wispy, feathery, hairlike, delicate fibers, streaks, or plumes

High cloud: Cirrostratus

Veil of fused sheets of ice crystals, milky, with sun and moon halos

High cloud: Cirrocumulus

Dappled, mackerel sky, small white flakes, tufts, in lines or groups, sometimes ripples

Middle cloud: Altocumulus

patches of cotton balls, dappled, arranged in lines or groups, rippling waves

Middle cloud: Altostratus

Thin to thick, no halos, sun’s outline just visible, gray day

Low cloud: Stratocumulus

Soft, gray, globular masses in lines, groups, or waves, heavy rolls, irregular over cast patterns

Low cloud: Stratus

Uniform, featureless, gray, like high fog

Low clouds: Nimbostratus

gray, dark, low, with drizzling rain

Vertically developed: Cumulus

Sharply outlined, puffy, billowy, flat-based, swelling tops, fair weather

Vertically developed: Cumulonimbus

Dense, heavy, massive, dark thunderstorms, hard showers, explosive top, great vertical development 

Fog

clouds that are in contact with the ground

• restrict visibility to less than 1 km (otherwise —> mist) 

Two types of fog:

Advection

• Evaporation

• Upslope fog

• Valley fog

Radiation

Advection fog

Air moving from place to another, where conditions lead to saturation

Evaporation Fog

AKA “steam fog” — cold air on top of warm and moist surface

Radiation fog (and evaporation fog)

Surface air cools off to dewpoint, usually over moist ground

Atmospheric lifting mechanisms

1. Convergent lifting 

2. Convectional lifting 

3. Orthographic lifting 

4. Frontal lifting 

Cold Fronts

• quickly forces warm air up

• 400 km wide (250 mi)

Warm Fronts

• gradually moves up and over cold air

• 1000 km (600 mi) 

Air masses

• Like gigantic air parcels

• A homogeneous body of air that has taken on the temp and moist characteristics of its source region

  • Source region = large, flat, homogenous

Good Source region for air masses 

• Oceans 

• Deserts

• plains

• tropics

• artic 

Bad source region for air masses

• mountains 

• midlatitudes

Cyclone

area of low pressure with converging and rising air

Cyclones develop in what stages?

1. Cyclogenesis

2. Open Stage 

3. Occulated Stage 

4. Dissolving Stage