Principles of Physical Geography Atmospheric …geomatic.disat.unimib.it/home/valter/didattica/scienzeterra-2011/...Atmospheric Moisture and Precipitation GY Principles of Physical

Atmospheric Moisture and Precipitation

Principles of Physical Geography


AimsTo understand the relationship between temperature and humidityTo define evapotranspirationTo understand the concept of stability in the atmosphereTo understand why it rainsTo understand convection

Objectives• To identify the utilization of energy during the various phase change processes of

water.• To describe the saturation process and how the amount of atmospheric water vapour

at saturation depends upon temperature.• To understand the processes of evaporation and transpiration• To explain how the water vapour content of air is quantified and define the following

moisture parameters: dewpoint temperature, wet bulb temperature, vapour pressure, mixing ratio, relative humidity

• To explain the temperature dependency of relative humidity. • To define dry and wet adiabatic temperature changes.• To explain the concept of stability and its application to the atmosphere• To explain absolute stability and absolute and conditional instability of an air column• To explain convection, orographic lifting, frontal wedging and convergence

GY


Outline

Introduction• Moisture and atmospheric stability• Air water vapour content• Relevance of moisture for the hydrologic cycle• Importance for local surface weather and climate, role in tropical storms

Water’s changes of state• Evaporation, condensation• Transpiration, evapotranspiration• Energy supply for evapotranspiration• Latent heat

Humidity• Vapour pressure• Saturation• Mixing ratio and absolute humidity• Dewpoint

Relative humidity• Relative humidity• Changing water content and temperature

Adiabatic temperature changes• Ideal Gas Law• Dry adiabatic rate• Wet adiabatic rate

Lifting processes and precipitation• Orographic lifting• Convective lifting• Frontal wedging• Convergence

Convection

GY


Main topicsInteractions of water vapour with atmospheric stabilityEvapotranspirationRelationship between temperature and humidityConcept of stability in the atmosphereLifting and precipitationConvection

OutlineIntroduction• Moisture and atmospheric stability• Air water vapour content• Relevance of moisture for the hydrologic cycle• Importance for local weather and climate, role in tropical storms

Water’s changes of state• Evaporation, condensation, transpiration, evapotranspiration• Energy supply for evapotranspiration• Latent heat

Humidity• Dewpoint• Saturation• Mixing ratio and absolute humidity• Vapour pressure

Relative humidity• Relative humidity• Changing water content and temperature

Adiabatic temperature changes• Ideal Gas Law• Dry adiabatic rate• Wet adiabatic rate

Lifting processes and precipitation

Convection

GY


BulletsIntroduction• Moisture <-> atmospheric stability?• What is stability anyway?• Water vapour - only small fraction of the atmosphere 0<v<5% !• Takes energy to evaporate water - as water vapour condensates, what happens with

the heat being released in the atmosphere?• Why care?• Very important: precipitation, groundwater recharge, cooling, moisturizing• Latent/sensible heat -> cooling of surface• Released energy - used to drive other processes? Warm some areas? One way how

the energy from the sun drives the processes one earth and allows for warming the higher latitudes

• Example for moisture/stability/energy content: tropical convection and Hurricanes• Thus: for several reasons important to understand the water cycle and to measure

water vapour content of air, that is air moisture, humidity• Importance of energy transformations and temperature measurementsWater’s changes of state• Evaporation - requires energy - latent heat of vaporization, calorie• Change of state• During evaporation some fast molecules escape to the air• Transpiration: water loss from plants• Evapotranspiration: sum of evaporation and transpiration• Evaporation is a cooling process since energy is used to evaporate the water and not

to heat the water or the air.• Heat/energy used for evaporation is now contained in the water vapour• Latent/sensible heat -> forest/open• Latent heat is released again during condensation• Melting, requires energy, too, the latent heat of fusion, released again during freezing• Frost protection by sprinkling water onto fruit crops: heat content of water, latent heat

of fusion is released as the water freezes -> as long as there is liquid water on the fruit, their temperature will not fall below 0oC as the freezing water releases heat

Humidity• Humidity - general term for the amount of water vapour in the air• Water vapour pressure - partial pressure of the water vapour in the atmosphere• Saturation of air with water vapour <-> after some time an equilibrium between evap-

oration and condensation over a water surface in closed container• Saturation vapour pressure - pressure exerted by air saturated with water• Saturation water vapour pressure is temperature dependent as at higher tempera-

tures more water evaporates <-> more molecules have higher energy• Absolute humidity - mass of water vapour in a given volume of air• Mixing ratio - mass of water vapour in a unit mass of dry air• Difficult to determineRelative humidity• Relative humidity - ratio of the air’s actual water vapour content compared with the

amount of water vapour required for saturation at the same temperature

GY


• Temperature dependent -> can be changed by changing water content or by chang-ing temperature

• Increases in water vapour content lead to increases to relative humidity (at the same temperature) until condensation occurs - at saturation.

• Increases in temperature decrease relative humidity because at higher temperatures air can hold more water

• Decreases in temperature increase relative humidity because at higher temperatures air can hold less water - at some temperature condensation might occur removing water from the air and relative humidity remains at 100%

• 1. diurnal cycle due to temperature cycle, 2. air movement horizontally or, 3. vertically• Relative humidity <-> mixing ratio• Dew point: temperature at which a parcel of air would be saturated• Human discomfort: sweating dissipates heat by evaporation of perspiration (latent

heat of vaporization!) - not efficient if surrounding air is very moist and can thus not take up easily more moisture -> heat stress -> eventually fatal due to failing protein functions. Very dangerous in enclosed spaces - cars!

Adiabatic temperature changes• Ideal gas law• Dry adiabatic changes: when air is allowed to expand it cools, when it is compressed

it warms - no heat is added nor subtracted• If air is lifted: dry adiabatic rate: 10oC per 1000m• Applies only to vertically moving unsaturated air• If air cools sufficiently condensation will occur and release heat, thus slowing the rate

of cooling• Wet adiabatic rate • Wet adiabatic rate depends on the amount of moisture in the air: 5oC for air with high

to 9oC for air with low moisture content• Wet adiabatic rate applies above the level, where condensation occurs, the lifting

condensation level -> rain• Cooling is faster at the dry rate than at the wet rate• Importance of moisture and temperature measurements to estimate lifting and thus

precipitation and energy release• Linkage of air movement and energy exchange

Lifting processes and precipitation• Lifting processes are important as they can force air up to a level where the temper-

ate is below it’s dew point and thus producing clouds and rain• Convective lifting• Orographic lifting: air forced upward by air flow over elevated terrain ->rain shadow

deserts• Frontal wedging: air forced upward by air flow over denser air; often warmer over

colder air.• Convergence

Links• http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/cld/home.rxml• http://cwx.prenhall.com/bookbind/pubbooks/aguado2/chapter4/deluxe.html

GY

At

GY

WMost important for climate:

•Energy transfer

•Air density

•Air stability

•Circulation

•Precipitation

•Transpiration

ation

pour

Today

mospheric Moisture and Precipitation

k

ater’s changes of state

Heat energy absorbed

Heat energy released

Liquid

Melting

Freezing

Evaporation

Sublim

CondensationIce

DepositionVa

Freezing

Source: Ahrens, C.D., 1994. Meteorology

At

GY

Sfo

Fu

Aeth(h

SLuph


k

praying with water used r frost protection

reezing releases the energy sed for melting

s long as the water releases nough heat while it freezes e fruit will not be damaged eld at 0oC)

ource:tgens, F.K. and E.J. Tarbuck, 1998. The Atmos-ere

At

GY

AW

• cules •• y the type of gas• emperature and pres-

ture, T, and pressure, nge distance between thus the density)

weights of O2 (red), N2 are about 32, 28, and

1 either an oxygen or a n air in a volume and th ensity of dry air.


k

ir density and water vapour concentrationet air is less dense than dry air

There is a lot of empty space between gas moleThey neither attract nor repel each other muchDistances between molecules are not affected bThe number of molecules in container at same tsure independent of the type of gas.

Same temperaP! (T, P do chamolecules! And

The molecular (blue) and H2O

8, respectively. Substituting a water molecule foritrogen molecule, will decrease the total mass ofus the density of the wet air is less than the d

lighterheavier

At

GY

W

E

• tent heat)

•

Im ion

S


ater’s changes of state

ffects:

Change energy content or temperature of air (la

Change density (and so the weight!) of air

pacts on temperature, pressure and circulat

ources of water vapour?

At

GY

ETp

E and transpiration• e •

Efs

Efs


k

vapotranspirationranspiration: loss of water vapour from lants by evaporation through stomata

vapotranspiration is the sum of all evaporationwater flux from earth’s surface to the atmospherdriving the upward water transport in plants

vaporation rom leaf urface

vaporation rom water urface

Soil evaporation

Transpiration

At

Gk

HH r vapour in the air

• iven volume of air

• s of dry air

•

•

S surface in closed con-ta ndensation will form

DT formation would start)


umidityumidity - general term for the amount of wate

Absolute humidity - mass of water vapour in a g

Mixing ratio - mass of water vapour in a unit mas

Relative humidity

Water vapour pressure

aturation of air with water vapour <-> Over wateriner an equilibrium between evaporation and co

ew Point Temperature (Dew Point)emperature at which air would be saturated (dew

At

GY

S t of air Vted

Sbu

Cfo . in thunderstorms, in

rature (oC)


aturation water vapour conten

ery strongly mperature ependent

ource: Lutgens, F.K. and E.J. Tar-ck, 1998. The Atmosphere

rucial relationship r understanding the power of convection, e.g particular in the tropics -> hurricans

T [oC]H2O vapor

[g/kg]5 3.510 720 14

Tempe

Wat

er v

apor

(g/k

g)

At

GY

RR ive to the amount of w re

V

•

• water vapour content


k

elative humidityatio of the air’s actual water vapour content relatater vapour at saturation at the same temperatu

aries with:

the amount of water vapour in the air

temperature, as temperature changes saturation

At

GY

R

Source: Lut-gens, F.K. and E.J. Tar-buck, 1998. The Atmos-phere


elative humidity at constant temperature

At

Gk

R

Source: Lut-gens, F.K. and E.J. Tar-buck, 1998. The Atmos-phere


elative humidity at different temperatures

Dew point!

At

GY


1

At

GY

W• by the water vapour

P


ater vapour pressureWater vapour pressure - partial pressure exertedin the atmosphere

ressure?

At

G

TRs

DM

A<

in

fu

ir molecules

Air density

High


he Ideal Gas Law elationship between pres-ure, volume and temperature

ensityass per volume [kg/m3]

mount of matter per volume -> heavy/light

low atmosphere high density

rther up lower density

A

Airpressure

Low

At

GY

P

th

th

th

th


k

ressure

e lower in the atmosphere,

e more air above,

e larger the weight of air above,

e larger the air pressure

At

GY

T

e


1

he Ideal Gas Lawp - pressureV - volumeT - temperaturK - a constant

p VT

-------- K=

At

GY

Ws•

•

•

20oC

30oC

ated

ensation e rate

Moist air

O vapour


ater vapor pressure and aturationWater vapour pressure - partial pressure exerted by the water vapour in the atmosphere

Saturation vapour pressure - pressure exerted by air saturated with water

Saturation water vapour pressure is temperature dependent as at higher tem-peratures more water evap-orates <-> more molecules have higher energy

20oC

20oC

Satur

Evaporation and condoccurring at the sam

Dry air

Pres-sure gauge

H2

At

GY

L tion

ifting

Converg-ing winds


1003 - Principles of Physical Geography, Lecture 14, Jörg Kaduk

ifting processes and precipita

(b) Orographic l

(c) Fontral wedging (d) Convergence

(a) Convective lifting

Cold air

Warm air

condensa-tion level

Converg-ing winds

At

GY

C•

•

••

•

•• point was reached•• further water vapour

adiabatic lapse rate


1k

onvective lifting & cloud formationAir at surface becomes less dense (Warming from surface (Ideal Gas Law)) or increasing water vapour content)If air is less dense than air above, air tends to rise (instability)Air risesAir expands due to decreasing pressure (Ideal Gas Law)Air cools (Ideal Gas Law) - Cooling rate (no condensation): Dry adiabatic lapse rateAir can rise further - depending on surroundingsCondensation might occur due to cooling, if dewCondensation releases heatair warms, expands, density drops, air rises, cools,condenses,... Cooling rate with condensation: Wet

At

GY

S• any processes

•

• to atmosphere

• ation

• ly on temperature

• volume

• the density of near apour concentration


ummaryChanges of state of water vapour interact with m

Wet air less dense than dry air

Evapotranspiration - total water flux from surface

Drives plant water uptake, source for all precipit

Saturation water vapour content depends strong

Ideal gas law relating temperature, pressure and

Atmospheric stability modified through reducing surface air due to warming or increasing water v

Documents

Principles of Physical Geography Atmospheric …geomatic.disat.unimib.it/home/valter/didattica/scienzeterra-2011/...Atmospheric Moisture and Precipitation GY Principles of Physical