physical geography by alan arbogast chapter 7

© 2007, John Wiley and Sons, Inc.

Physical GeographyPhysical Geographyby Alan Arbogastby Alan Arbogast

Chapter 7Chapter 7Atmospheric Moisture and

PrecipitationLawrence McGlinnDepartment of GeographyState University of New York - New Paltz

© 2007, John Wiley and Sons, Inc.

Physical Properties of WaterPhysical Properties of Water• H2O molecule – O side (-) – H side (+)

• Hydrogen bonding – (+) bonded to (-)

• Liquid – flexible bond

• Ice – rigid hexagonal bond

• Surface tension – water molecules hold together

• Capillary action – upward movement of water through soil and plants

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Thermal Properties of WaterThermal Properties of Water

• Water absorbs and releases latent heat, hidden energy stored in molecular bonds

• Heat absorbed when hydrogen bonds loosened or broken – melting & evaporation

• Heat released when hydrogen bonds strengthened – freezing & condensation

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Three States of WaterThree States of Water

GAS

LIQUIDSOLID

Depos

ition

Heat R

eleas

edSu

blimati

onHea

t Abs

orbe

d

VaporizationCondensation

Heat Released

Heat Absorbed

Heat Released

Heat Absorbed

Melting

Freezing

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HydrosphereHydrosphere

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Hydrologic CycleHydrologic Cycle• Model illustrating how water is stored and moves

from one reservoirs on Earth

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HumidityHumidity• Concentration of water vapor in the air

• 3 types:

• Maximum Humidity

• Specific Humidity

• Relative Humidity

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Maximum HumidityMaximum Humidity

• Max amt of water vapor a body of air can hold

• Depends on air temperature

• Warmer air can hold more water vapor

• Saturation – air with max amt of water vapor is saturated, can hold no more

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Saturation CurveSaturation Curve

Maximum Humidity risesdramatically with rising temperature

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Specific HumiditySpecific Humidity

• The measurable amt of water vapor in a mass of air

• units g/kg (grams water vapor/kg air)

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Relative HumidityRelative Humidity• Ratio of specific to maximum humidity – how

close the air is to saturated• RH (%) = (SH/MH) X 100• Cooling an unsaturated body of air raises its

relative humidity• Cool body of air to point of saturation –

100% RH - this is Dew Point Temperature

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Relative HumidityRelative Humidity

Saturation

Water Vapor

Water Vapor

Water Vapor

Air Temperature Air

TemperatureAir

Temperature

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Daily Pattern of HumidityDaily Pattern of Humidity•Specific humidity constant•As air warms, its water vapor capacity increases•RH falls•In evening, temp & vapor capacity will fall•RH will rise•At 100% RH, dew forms

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Dew-Point TemperatureDew-Point Temperature• Dew-point temperature not really a

temperature, but a measure of moisture content

• When air temperature tries to decrease below the dew point, surplus water vapor is removed from the air by condensation

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Dew Point TemperatureDew Point TemperatureThink about a glass of ice water or your windshield in the morning

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Humidity ExamplesHumidity Examples

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EvaporationEvaporation• Evaporation - Liquid water to water vapor• Transpiration – water flows thru leaf pores in

plants to atmosphere• Evapotranspiration – combination of evaporation

and transpiration• Evapotranspiration rate depends on :

• Net radiation – higher in sunshine• Air temp – higher in warmer temps• RH – higher in lower humidity

© 2007, John Wiley and Sons, Inc.

Physical GeographyPhysical Geographyby Alan Arbogastby Alan Arbogast

Chapter 7Chapter 7Atmospheric Moisture and

PrecipitationLawrence McGlinnDepartment of GeographyState University of New York - New Paltz

© 2007, John Wiley and Sons, Inc.

Adiabatic ProcessesAdiabatic Processes• Rising air expands due to reduced pressure• Thus, rising air cools• Falling air compresses due to greater pressure• Thus, falling air warms• Bouyancy caused initially by differences in (near)

surface temperature• Less dense, warmer air rises, more dense, colder air

sinks, after which…• Ascending or descending air will undergo changes

in temperature with no exchange of heat. This is an adiabatic process.

© 2007, John Wiley and Sons, Inc.

© 2007, John Wiley and Sons, Inc.

Adiabatic Lapse RatesAdiabatic Lapse Rates• Near surface, air usually unsaturated (< 100%

RH)

• Unsaturated rising air (< 100% RH) cools at DRY Adiabatic Lapse Rate (10ºC/1000m elevation)

• Cooling air may reach dew point temp (100% RH) – condensation begins – heat is released

• Rising air =100% RH cools at WET Adiabatic Lapse Rate 5ºC/1000m elevation – less due to heat released by condensation

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Adiabatic CoolingAdiabatic Cooling

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Adiabatic Processes  Adiabatic Processes  • Dry adiabatic rate (DAR)

• Also called the Dry Adiabatic Lapse Rate (DALR)

• 10 C°/ 1000 m• 5.5 F°/ 1000 ft

• Lifting Condensation Level (LCL) is reached, then…

• Moist adiabatic rate (MAR)• Also called the Wet Adiabatic Lapse Rate

(WALR)• 6 C°/ 1000 m• 3.3 F°/ 1000 ft

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© 2007, John Wiley and Sons, Inc.

© 2007, John Wiley and Sons, Inc.

Atmospheric Stability  Atmospheric Stability  • Stable and unstable atmospheric conditions

• Involves a parcel of air and its surrounding environment in the atmosphere

• Stable atmosphere:• A parcel of air is discouraged from rising• Kind of weather normally associated?

• Unstable atmosphere:• A parcel of air is encouraged to rise• Kind of weather normally associated?

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  Examples of StabilityExamples of Stability

Figure 7.20

  Unstable AtmosphereUnstable AtmosphereParcel of air is encouraged to riseParcel of air is encouraged to rise

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  Examples of StabilityExamples of Stability

Figure 7.20

  Stable AtmosphereStable AtmosphereParcel of air is discouraged from risingParcel of air is discouraged from rising

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Atmospheric Stability  Atmospheric Stability  

• For example:• We measure and find the ELR to be 12 Cº/ 1000 m• We know the DAR is 10 Cº/ 1000 m.• We know the MAR is 6 Cº/ 1000 m.• If ELR (12) > DAR (10) > MAR (6) then?• If ELR > DAR > MAR = UNSTABLE

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Distribution of Water VaporDistribution of Water Vapor

January

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Distribution of Water VaporDistribution of Water Vapor

July

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PrecipitationPrecipitation

• Forms within clouds from either water droplets or ice crystals

• When droplet or crystal is heavy enough, it falls to earth as precipitation

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Condensation NucleiCondensation Nuclei

• Pure water droplets are uncommon• Homogeneous nucleation

• Hygroscopic aerosols• Dust, salt, pollution, ash

• Heterogeneous nucleation

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Moisture DropletsMoisture Droplets

Figure 7.20

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Precipitation TypesPrecipitation Types• Rain – large, unfrozen water droplets• Snow – ice crystals that do not melt before

they hit ground• Sleet – rain that freezes before hitting ground• Freezing Rain – rain that freezes on impact

with ground• Hail – ice crystals that are repeatedly drawn up

into a violent thunderstorm, growing each time

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Rain and SnowRain and Snow

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Cloud Formation & ClassificationCloud Formation & Classification

• Clouds – visible masses of suspended, minute water droplets or ice crystals

• Two conditions for cloud formation :• Air must be saturated• Small airborne particles of dust,

Condensation Nuclei, must be present

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FogFog• Fog forms when surface air is saturated• How it forms :

• Radiation fog – cool or cold air is trapped at the surface – Temperature Inversion, in deep valleys or over snowy/icy surfaces

• Advection fog – warm air flows over a cooler surface – air cools to saturation

• Sea fog – cool marine air contacts colder ocean water – Calif coast

• Evaporation fog – dole air moves over warmer water body

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Fog TypesFog TypesRadiation fog at Blue Mts

Natl Park, AustraliaSea fog across the Golden Gate, San Francisco, CA

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Evaporation FogEvaporation Fog

Figure 7.26

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Cloud ClassificationCloud Classification• Categories of clouds:

• Cirrus – thin, wispy, made of ice crystals; highest altitude

• Altus – middle altitude clouds

• Stratus – layer-like gray sheets that cover most or all of sky; lowest altitude

• Cumulus – individual, puffy clouds with a flat, horizontal base; any altitude

• Nimbo- or -nimbus → precipitation

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Cloud Types and Identification  Cloud Types and Identification  

12

3

5

6 78

9

104

2

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Cumulonimbus DevelopmentCumulonimbus Development