Chapter 6Chapter 6

Atmospheric MoistureAtmospheric Moistureand Precipitationand Precipitation

The HydrosphereThe HydrosphereHydrosphereHydrosphere –– water in the earthwater in the earth--atmosphere systematmosphere system

Oceans and Salt LakesOceans and Salt Lakes 97.6%97.6%Ice Caps and GlaciersIce Caps and Glaciers 1.9%1.9%

(Not available for humans)(Not available for humans) 99.5%99.5%Subsurface Water (soil, groundwater)Subsurface Water (soil, groundwater) 0.5%0.5%Surface Water (rivers, freshwater lakes)Surface Water (rivers, freshwater lakes) 0.02%0.02%Atmosphere Atmosphere 0.0001%0.0001%

If all land were flat, the oceans would cover it to a depth of If all land were flat, the oceans would cover it to a depth of 3 km3 kmIf all atmospheric water were precipitated, it would cover If all atmospheric water were precipitated, it would cover the earth to a depth of 25 mm (1 inch)the earth to a depth of 25 mm (1 inch)Holt, MO Holt, MO ---- 12 inches of rain in 42 minutes!12 inches of rain in 42 minutes!

The HydrosphereThe Hydrosphere

Over 70% of the planet is covered by waterOver 70% of the planet is covered by water

Water is unique in that it can simultaneously exist in Water is unique in that it can simultaneously exist in all three states (solid, liquid, gas) at the same all three states (solid, liquid, gas) at the same temperaturetemperature

Water is able to shift between states very easilyWater is able to shift between states very easily

Hydrologic cycleHydrologic cycle defines the movement of water defines the movement of water among the great global reservoirsamong the great global reservoirs

Positive balance over land = more precipitation that Positive balance over land = more precipitation that evaporationevaporationNegative balance over water = more Negative balance over water = more evaporationevaporation that that precipitationprecipitation

EvaporationEvaporation

molecules escape liquid water as water vapormolecules escape liquid water as water vaporWater vapor increases in air as surface water Water vapor increases in air as surface water evaporatesevaporatesCondensationCondensation begins and water returns to the begins and water returns to the surfacesurface

Water vapor molecules become liquidWater vapor molecules become liquid

Evaporation =Evaporation =CondensationCondensation

Saturation

No EvaporationNo EvaporationOr Condensation

Evaporation >>Evaporation >>CondensationOr Condensation Condensation

Atmospheric HumidityAtmospheric Humidity

HumidityHumidity –– the amount of water vapor in the airthe amount of water vapor in the air

When saturated, warm air contains more water vapor When saturated, warm air contains more water vapor than cold airthan cold air

Thus, we erroneously say that warm air can “hold” Thus, we erroneously say that warm air can “hold” more water vapor than cold airmore water vapor than cold air

So, we would expect deserts near the poles due to So, we would expect deserts near the poles due to the cold air temperaturesthe cold air temperatures

AtmosphericAtmosphericHumidityHumidity

PHYSICALLY IMPOSSIBLE!PHYSICALLY IMPOSSIBLE!

Atmospheric HumidityAtmospheric Humidity

Relative HumidityRelative Humidity –– the amount of water vapor in the amount of water vapor in the air relative to the amount at saturationthe air relative to the amount at saturation

If air is saturated, we say that it has a relative If air is saturated, we say that it has a relative humidity of 100%humidity of 100%

A relative humidity of 50% implies that the air has A relative humidity of 50% implies that the air has only half as much water vapor as it would have when only half as much water vapor as it would have when saturatedsaturated

Atmospheric HumidityAtmospheric HumiditySince at saturation, warm air contains more water Since at saturation, warm air contains more water vapor than cold air, relative humidity is inversely vapor than cold air, relative humidity is inversely related to air temperaturerelated to air temperature

As air temperature rises and the amount of moisture As air temperature rises and the amount of moisture in the air remains the same, the relative humidity in the air remains the same, the relative humidity decreasesdecreases

As air temperature falls and the amount of moisture As air temperature falls and the amount of moisture in the air remains the same, the relative humidity in the air remains the same, the relative humidity increasesincreases

DAILY CYCLES IN RELATIVE HUMIDITYDAILY CYCLES IN RELATIVE HUMIDITY

Atmospheric HumidityAtmospheric Humidity

Dew PointDew Point ---- the temperature the air must reach for the temperature the air must reach for the air to become saturated. If you lower the air the air to become saturated. If you lower the air temperature until the relative humidity reaches temperature until the relative humidity reaches 100%, you have reached the dew point. Lowering 100%, you have reached the dew point. Lowering the air temperature below the dew point will result the air temperature below the dew point will result in condensation.in condensation.

AtmosphericAtmosphericHumidityHumidity

PHYSICALLY IMPOSSIBLE!PHYSICALLY IMPOSSIBLE!

Atmospheric HumidityAtmospheric Humidity

Relative humidity is strongly dependent on the Relative humidity is strongly dependent on the air temperature; the dew point is a more air temperature; the dew point is a more conservative measure of the moisture content of conservative measure of the moisture content of the air.the air.

Therefore, dew point is a better way to represent Therefore, dew point is a better way to represent the moisture content of the air.the moisture content of the air.

Atmospheric HumidityAtmospheric Humidity

Relative humidity is inversely related to air Relative humidity is inversely related to air temperature; the dew point is nottemperature; the dew point is not

The highest value of relative humidity usually occurs The highest value of relative humidity usually occurs just after sunrisejust after sunrise

The lowest value of relative humidity usually occurs The lowest value of relative humidity usually occurs in the midin the mid--afternoonafternoon

Air Temperature and HeightAir Temperature and Height

Air Temperature and HeightAir Temperature and Height

In the troposphere, air temperature decreases In the troposphere, air temperature decreases with heightwith height

Rising air expands due to the decreasing Rising air expands due to the decreasing atmospheric pressure and cools as it expandsatmospheric pressure and cools as it expands

Similarly, descending air will warm as it Similarly, descending air will warm as it compresses due to the increasing atmospheric compresses due to the increasing atmospheric pressurepressure

Air Temperature and HeightAir Temperature and Height

The process by which changes in air temperature The process by which changes in air temperature occur solely as a result of changes in atmospheric occur solely as a result of changes in atmospheric pressure is called an pressure is called an adiabatic processadiabatic process

When air is heated or cooled adiabatically and no When air is heated or cooled adiabatically and no condensation occurs, air temperature will follow the condensation occurs, air temperature will follow the drydry adiabatic lapse rateadiabatic lapse rate which equals about 10°C which equals about 10°C per kilometer (or about 5.5°F per 1000 feet)per kilometer (or about 5.5°F per 1000 feet)

Air Temperature and HeightAir Temperature and HeightHowever, rising air will cool and it is possible that it However, rising air will cool and it is possible that it will reach the dew point temperature (a relative will reach the dew point temperature (a relative humidity of 100%)humidity of 100%)

At that point, or the At that point, or the lifting condensation levellifting condensation level, , moisture will condense from the air and the air moisture will condense from the air and the air temperature will follow the temperature will follow the wet adiabatic lapse ratewet adiabatic lapse ratewhich ranges from between 4°C and 9°C per which ranges from between 4°C and 9°C per kilometerkilometer

The wet adiabatic lapse rate will be lower for warmer The wet adiabatic lapse rate will be lower for warmer air since there is more moisture to condenseair since there is more moisture to condense

Air Temperature and HeightAir Temperature and Height

Consider air flowing over a mountain that is 4 Consider air flowing over a mountain that is 4 kilometers high (13,123 feet)kilometers high (13,123 feet)

For simplicity, we will assume the wet adiabatic For simplicity, we will assume the wet adiabatic lapse rate is 6°C per kilometerlapse rate is 6°C per kilometer

At sea level on the windward side of the At sea level on the windward side of the mountain, the air temperature is 30°C and the mountain, the air temperature is 30°C and the dew point is 10°C dew point is 10°C

WindwardLeeward

Air Temperature and HeightAir Temperature and Height

This demonstrates the This demonstrates the rainshadow effectrainshadow effect of of mountain rangesmountain ranges

Precipitation occurs on the windward side Precipitation occurs on the windward side

Warmer and drier conditions are found on the Warmer and drier conditions are found on the leeward side of the mountain range (leeward side of the mountain range (e.g.e.g., , Washington, Oregon, California)Washington, Oregon, California)

Downslope winds can be warm and dry Downslope winds can be warm and dry –– sometimes sometimes called called Chinook WindsChinook Winds

PHYSICALLY IMPOSSIBLE!PHYSICALLY IMPOSSIBLE!

AtmosphericAtmosphericHumidityHumidity

PrecipitationPrecipitation

To get precipitation, you need:To get precipitation, you need:

Moisture in the AtmosphereMoisture in the Atmosphere

Mechanism to Release the MoistureMechanism to Release the MoistureEvery Mechanism Causes Air to Cool Below the Every Mechanism Causes Air to Cool Below the Dew Point TemperatureDew Point Temperature

AtmosphericAtmosphericHumidityHumidity

PHYSICALLY IMPOSSIBLE!PHYSICALLY IMPOSSIBLE!

To get precipitation, you need:To get precipitation, you need:

Moisture in the AtmosphereMoisture in the Atmosphere

Mechanism to Release the MoistureMechanism to Release the MoistureEvery Mechanism Causes Air to Cool Below the Every Mechanism Causes Air to Cool Below the Dew Point TemperatureDew Point TemperatureThe Easiest Mechanism is to Force Air to RiseThe Easiest Mechanism is to Force Air to RiseThere are 4 Basic Mechanisms for Precipitation There are 4 Basic Mechanisms for Precipitation

and 1 for Condensationand 1 for Condensation

To get precipitation, you need:To get precipitation, you need:

1)1) Moisture in the AtmosphereMoisture in the AtmosphereClouds are the source of precipitationClouds are the source of precipitation

In warm clouds, liquid droplets condense and In warm clouds, liquid droplets condense and collidecollide

When the droplets become large enough, they When the droplets become large enough, they fall as rainfall as rain

Precipitation Causing MechanismsPrecipitation Causing Mechanisms

Precipitation Causing MechanismsPrecipitation Causing Mechanisms

Orographic PrecipitationOrographic Precipitation

1) Orographic Precipitation1) Orographic Precipitation

Warm moist air is forced over a mountain Warm moist air is forced over a mountain barrierbarrier

It cools adiabaticallyIt cools adiabatically

At the LCL condensation (and often At the LCL condensation (and often precipitation occursprecipitation occurs

Precipitation Causing MechanismsPrecipitation Causing Mechanisms

Orographic PrecipitationOrographic PrecipitationConvective PrecipitationConvective Precipitation

2) Convective Precipitation2) Convective Precipitation

Convection = upwards motion of heat air Convection = upwards motion of heat air (convection cells)(convection cells)

Caused by uneven surface heatingCaused by uneven surface heating

Parcel of warmer, less dense air risesParcel of warmer, less dense air rises

As the parcel rises, it coolsAs the parcel rises, it cools

2) Convective Precipitation2) Convective Precipitation

If the air is quite moist, the release of latent heat If the air is quite moist, the release of latent heat will ensure that the parcel of air remains warmer will ensure that the parcel of air remains warmer than the surrounding airthan the surrounding air

If conditions remain favorable, the rising thermal If conditions remain favorable, the rising thermal will grow into a thunderstormwill grow into a thunderstorm

Precipitation Causing MechanismsPrecipitation Causing Mechanisms

Orographic PrecipitationOrographic PrecipitationConvective PrecipitationConvective PrecipitationCyclonic PrecipitationCyclonic Precipitation

3) Cyclonic Precipitation3) Cyclonic Precipitation

Pressure differences cause air motion (wind)Pressure differences cause air motion (wind)

A low pressure cell at the surface causes A low pressure cell at the surface causes convergenceconvergence

Low level convergence causes upward vertical Low level convergence causes upward vertical motion, and therefore precipitationmotion, and therefore precipitation

Precipitation Causing MechanismsPrecipitation Causing Mechanisms

Orographic PrecipitationOrographic PrecipitationConvective PrecipitationConvective PrecipitationCyclonic PrecipitationCyclonic PrecipitationFrontal PrecipitationFrontal Precipitation

4) Frontal Precipitation4) Frontal Precipitation

Frontal uplift Frontal uplift –– air can be forced upward through air can be forced upward through the movement of air massesthe movement of air masses

Air massesAir masses = a large body of air, with a set of = a large body of air, with a set of relatively uniform temperature and relatively uniform temperature and moisturemoisturepropertiesproperties

4) Frontal Precipitation4) Frontal Precipitation

When a colder, more dense air mass advances on When a colder, more dense air mass advances on a warmer, less dense air mass (a cold front), it is a warmer, less dense air mass (a cold front), it is forced to rise over the cold air massforced to rise over the cold air mass

Precipitation Causing MechanismsPrecipitation Causing Mechanisms

Orographic PrecipitationOrographic PrecipitationConvective PrecipitationConvective PrecipitationCyclonic PrecipitationCyclonic PrecipitationFrontal PrecipitationFrontal Precipitation

Atmospheric Lifting MechanismsAtmospheric Lifting Mechanisms

CloudsClouds

Clouds and Cloud FormationClouds and Cloud Formation

Clouds are instrumental to the Earth’s energy and Clouds are instrumental to the Earth’s energy and moisture balancesmoisture balances

Most clouds form as air parcels are lifted and Most clouds form as air parcels are lifted and cooled to saturationcooled to saturation

Different lifting mechanisms exist to lift air and form Different lifting mechanisms exist to lift air and form clouds through condensationclouds through condensation

In the 1700s, a British Pharmacist developed a In the 1700s, a British Pharmacist developed a cloud classification scheme that we still use todaycloud classification scheme that we still use today

Clouds and Cloud FormationClouds and Cloud Formation

Clouds are classified on the basis of the Clouds are classified on the basis of the presence or absence of vertical developmentpresence or absence of vertical development

CumuloformCumuloform clouds exhibit significant clouds exhibit significant vertical developmentvertical development

StratiformStratiform clouds do not exhibit significant clouds do not exhibit significant vertical developmentvertical development

Clouds and cloud formationsClouds and cloud formations

CirrusCirrus –– thin, wispy clouds of icethin, wispy clouds of ice

NimbusNimbus –– rain producing cloudsrain producing clouds

Cumuloform CloudsCumuloform Clouds

CumulusCumulus CloudsClouds11--2 km thick, 6002 km thick, 600--1200 meters high1200 meters high

Formation of fair weather cumulusFormation of fair weather cumulus

Cumuloform CloudsCumuloform Clouds

CumulusCumulus CloudsClouds11--2 km thick, base is 6002 km thick, base is 600--1200 meters high1200 meters high

Cumulus CongestusCumulus Congestus33--5 km thick5 km thick

Cumuloform CloudsCumuloform Clouds

CumulusCumulus CloudsClouds11--2 km thick, base is 6002 km thick, base is 600--1200 meters high1200 meters high

Cumulus CongestusCumulus Congestus33--5 km thick5 km thick

CumulonimbusCumulonimbus66--12 km thick 12 km thick –– thunderstorm cloudsthunderstorm clouds

Progression occurs with developmentProgression occurs with development

Stratiform CloudsStratiform Clouds

Classified on the basis of heightClassified on the basis of height

High CloudsHigh Clouds –– more than 6 km highmore than 6 km highMiddle CloudsMiddle Clouds –– between 2 and 6 km highbetween 2 and 6 km highLow CloudsLow Clouds –– less than 2 km highless than 2 km high

Stratiform CloudsStratiform Clouds

High CloudsHigh Clouds –– more than 6 km highmore than 6 km highComposed entirely of ice crystalsComposed entirely of ice crystalsWhitish appearanceWhitish appearanceTemperature is around Temperature is around --35 C35 CLittle water vapor (0.025 g/mLittle water vapor (0.025 g/m33))

High clouds High clouds ––CirrusCirrus: fine and wispy: fine and wispy

Cirrus clouds with fall streaksCirrus clouds with fall streaks

Stratiform CloudsStratiform Clouds

High CloudsHigh Clouds –– more than 6 km highmore than 6 km highComposed entirely of ice crystalsComposed entirely of ice crystalsWhitish appearanceWhitish appearanceCirrusCirrus –– fine and wispyfine and wispyCirrostratusCirrostratus –– forming a complete layerforming a complete layer

CirrostratusCirrostratus

When viewed through a When viewed through a layer of cirrostratus, the layer of cirrostratus, the Moon or Sun has a Moon or Sun has a whitish, milky appearance whitish, milky appearance but a clear outline. A but a clear outline. A characteristic feature of characteristic feature of cirrostratus clouds is the cirrostratus clouds is the halohalo, a circular arc around , a circular arc around the Sun or Moon formed by the Sun or Moon formed by the refraction (bending) of the refraction (bending) of light as it passes through light as it passes through the ice crystals.the ice crystals.

Stratiform CloudsStratiform Clouds

High CloudsHigh Clouds –– more than 6 km highmore than 6 km highComposed entirely of ice crystalsComposed entirely of ice crystalsWhitish appearanceWhitish appearanceCirrusCirrus –– fine and wispyfine and wispyCirrostratusCirrostratus –– forming a complete layerforming a complete layerCirrocumulusCirrocumulus –– light puffy cloudslight puffy clouds

“Mackerel Sky”“Mackerel Sky”

Stratiform CloudsStratiform Clouds

Middle CloudsMiddle Clouds –– between 2 and 6 km highbetween 2 and 6 km highComposed of a combination of ice crystals Composed of a combination of ice crystals and water dropletsand water dropletsThicker due to higher temperaturesThicker due to higher temperaturesAltostratusAltostratus –– forming a near complete forming a near complete layerlayer

Stratiform CloudsStratiform Clouds

Middle CloudsMiddle Clouds –– between 2 and 6 km highbetween 2 and 6 km highComposed of a combination of ice crystals Composed of a combination of ice crystals and water dropletsand water dropletsThicker due to higher temperaturesThicker due to higher temperaturesAltostratusAltostratus –– forming a near complete forming a near complete layerlayerAltocumulusAltocumulus –– rows of puffy cloudsrows of puffy clouds

Stratiform CloudsStratiform Clouds

Low CloudsLow Clouds –– less than 2 km highless than 2 km highComposed primarily of water dropletsComposed primarily of water dropletsThicker and somewhat darkerThicker and somewhat darkerStratusStratus –– a nearly unbroken layer of graya nearly unbroken layer of gray

Stratiform CloudsStratiform Clouds

Low CloudsLow Clouds –– less than 2 km highless than 2 km highComposed primarily of water dropletsComposed primarily of water dropletsThicker and somewhat darkerThicker and somewhat darkerStratusStratus –– a nearly unbroken layer of graya nearly unbroken layer of grayStratocumulusStratocumulus –– rows of puffy cloudsrows of puffy clouds

Cumulus or Stratocumulus?Cumulus or Stratocumulus?

CumulusCumulusflat bottomsflat bottomsoccur when significant lifting is presentoccur when significant lifting is present

StratocumulusStratocumulusrounded bottomsrounded bottomsoccur when vertical motions are minimaloccur when vertical motions are minimal

Stratiform CloudsStratiform Clouds

Low CloudsLow Clouds –– less than 2 km highless than 2 km highComposed primarily of water dropletsComposed primarily of water dropletsThicker and somewhat darkerThicker and somewhat darkerStratusStratus –– a nearly unbroken layer of graya nearly unbroken layer of grayStratocumulusStratocumulus –– rows of puffy cloudsrows of puffy cloudsNimbostratusNimbostratus –– stratus cloud from which stratus cloud from which rain is fallingrain is falling

Lenticular CloudsLenticular Clouds

Form downwind on mountain barriersForm downwind on mountain barriers

Have a curved (lenseHave a curved (lense--like) shapelike) shape

altocumulus standingaltocumulus standinglenticularuslenticularus

Lenticular CloudsLenticular Clouds

Stratiform CloudsStratiform Clouds

FogFog –– stratus cloud at the groundstratus cloud at the ground

Advection FogAdvection Fog –– moving air over a colder moving air over a colder surfacesurface

(e.g., the Grand Banks of Newfoundland)(e.g., the Grand Banks of Newfoundland)

Radiation FogRadiation Fog –– motionless air that cools motionless air that cools below the dew point due to longwave below the dew point due to longwave radiation loss at nightradiation loss at night

Radiation Fog in the Central Valley of CaliforniaRadiation Fog in the Central Valley of California

Stratiform CloudsStratiform Clouds

FogFog –– stratus cloud at the groundstratus cloud at the ground

Sea FogSea Fog –– forms when a cool moist air parcel forms when a cool moist air parcel comes in contact with cold oceancomes in contact with cold ocean

Occurs along the coasts of continents where Occurs along the coasts of continents where cold currents move toward the equator (e.g. cold currents move toward the equator (e.g. California, Peru)California, Peru)

What is the difference between the What is the difference between the environmental lapse rate and the dry environmental lapse rate and the dry

adiabatic lapse rate?adiabatic lapse rate?

Environmental Lapse Rate vs. Environmental Lapse Rate vs. DALR/WALRDALR/WALR

Is the average temperature lapse rate, an Is the average temperature lapse rate, an expression of how temperature of still air varies expression of how temperature of still air varies with altitudewith altitude

This rate will vary from time to time and from one This rate will vary from time to time and from one place to another, depending on the state of the place to another, depending on the state of the atmosphereatmosphere

No motion if air is implied for the environmental No motion if air is implied for the environmental lapse rate (both DALR and WALR rely on motion)lapse rate (both DALR and WALR rely on motion)