Humidity, Condensation, Humidity, Condensation, Clouds, and FogClouds, and Fog

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Water in the AtmosphereWater in the Atmosphere

The Hydrologic CycleThe Hydrologic Cycle

Where the Water Exists onWhere the Water Exists on EarthEarth

Evaporation From the Evaporation From the Oceans and LandOceans and Land

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The Source of Water Vapor The Source of Water Vapor for the Atmospherefor the Atmosphere

!! Water molecules move in both Water molecules move in both directions between liquid and directions between liquid and gas.gas.

!! More molecules move from More molecules move from liquid to gas than from gas to liquid to gas than from gas to liquid, and thus there is a net liquid, and thus there is a net loss of liquid loss of liquid –– evaporation.evaporation.

!! Eventually liquid water will dry Eventually liquid water will dry up, unless replaced by a up, unless replaced by a source such as precipitation. source such as precipitation.

!! Some water molecules that Some water molecules that leave the liquid move away leave the liquid move away from the surface because the from the surface because the container is open.container is open.

OPEN TOP

Only water molecules are Only water molecules are shown in the air.shown in the air.

What affects evaporation rate?What affects evaporation rate?

!! Wind speedWind speed-- The stronger the wind, the more rapid the The stronger the wind, the more rapid the

evaporation.evaporation.-- Why? Why? -- Stronger wind over surface blows Stronger wind over surface blows

away vapor molecules from near away vapor molecules from near surface, so they will not resurface, so they will not re--enter liquid enter liquid surface. surface.

-- Example Example -- Clothes dry out faster in the Clothes dry out faster in the wind, we feel colder in the wind with wind, we feel colder in the wind with wet clotheswet clothes

!! Temperature of the waterTemperature of the water–– The higher the water temperature, the The higher the water temperature, the

more rapid the evaporation. more rapid the evaporation. -- Why? Why? -- The higher temperature means that The higher temperature means that

water molecules in the liquid are moving water molecules in the liquid are moving faster, and will escape to form vapor. faster, and will escape to form vapor.

-- Example Example -- wet clothes dry faster in the sunwet clothes dry faster in the sun

!! The humidity of the airThe humidity of the air (humidity = amount (humidity = amount of water vapor in the air) of water vapor in the air) –– The higher the humidity, the slower the The higher the humidity, the slower the

evaporation. evaporation. -- Why? Why? -- The more water molecules in the The more water molecules in the

air, the more that go back to the liquid. air, the more that go back to the liquid. -- Example Example -- clothes dry faster and your clothes dry faster and your

perspiration evaporates quickly in dry perspiration evaporates quickly in dry climatesclimates

Now Saturate the AirNow Saturate the Air!! Put a lid on container so no Put a lid on container so no

waterwater--vapor molecules blow vapor molecules blow awayaway

!! Water molecules evaporate from Water molecules evaporate from the liquid until enough the liquid until enough accumulate in the air so that accumulate in the air so that they move back to the liquid at they move back to the liquid at the same rate.the same rate.

!! At this point we say that the air At this point we say that the air is is saturatedsaturated with water vapor with water vapor –– it it has reached its capacity.has reached its capacity.

The Amount of Water Vapor the The Amount of Water Vapor the Air Can Hold Depends on Air Can Hold Depends on

TemperatureTemperature

!! The amount of water vapor that may be The amount of water vapor that may be evaporated into air is directly proportional to evaporated into air is directly proportional to the air temperature. That is, the higher the the air temperature. That is, the higher the temperature, the more water vapor the air temperature, the more water vapor the air can hold. can hold.

For ExampleFor Example!! Allow liquid water in a closed container to Allow liquid water in a closed container to

evaporate until the air is saturated.evaporate until the air is saturated.!! Warm the air by 20 F, and evaporation will begin Warm the air by 20 F, and evaporation will begin

again because the air is no longer saturated. At again because the air is no longer saturated. At the new saturation, when evaporation stops, twice the new saturation, when evaporation stops, twice the original amount of water will be in the air.the original amount of water will be in the air.

!! Warm the air another 20 F, and the water in the air Warm the air another 20 F, and the water in the air will double again when saturation is reached and will double again when saturation is reached and evaporation stops.evaporation stops.

SoSo!! For about every 20 F change in For about every 20 F change in

temperature, the capacity of the air temperature, the capacity of the air for water vapor changes by a factor for water vapor changes by a factor of twoof two

Condensation to Form Fog, Condensation to Form Fog, Clouds, Dew Clouds, Dew –– How it Happens in How it Happens in

the Atmospherethe Atmosphere!! Condensation occurs mainly when air is cooled Condensation occurs mainly when air is cooled

and its capacity for water vapor is decreased.and its capacity for water vapor is decreased.!! In the atmosphere, water vapor condenses on In the atmosphere, water vapor condenses on

particles of solid material rather than in the open particles of solid material rather than in the open air.air.

!! These particles may be natural or manThese particles may be natural or man--made, and made, and are called condensation nuclei.are called condensation nuclei.

!! Thus, the condensation that begins each cloud Thus, the condensation that begins each cloud droplet or fog droplet takes place on a solid droplet or fog droplet takes place on a solid particle.particle.

Humidity Humidity ––

Any way of specifying the Any way of specifying the amount of water vapor in airamount of water vapor in air

1. Vapor Pressure1. Vapor Pressure

ConceptConcept -- Each gas in the atmosphere contributes Each gas in the atmosphere contributes its own partial pressure (part of the total). Say the its own partial pressure (part of the total). Say the total pressure is 1000 total pressure is 1000 millibarsmillibars, and 1% of the air , and 1% of the air is water vapor.is water vapor.

nitrogen (78%) nitrogen (78%) –– partial pressure = 780 partial pressure = 780 mbmboxygen (21%) oxygen (21%) –– partial pressure = 210 partial pressure = 210 mbmbwater vapor (1%) water vapor (1%) –– partial pressure = 10 partial pressure = 10 mbmb

((Saturation vapor pressureSaturation vapor pressure = vapor pressure if the = vapor pressure if the air is saturated)air is saturated)

How saturation How saturation vapor pressure vapor pressure depends on depends on temperaturetemperature –– it it takes more water takes more water vapor to saturate vapor to saturate the air at higher the air at higher temperatures!!!!temperatures!!!!

2. Relative Humidity2. Relative HumidityRelative humidity tells us how close the air is to Relative humidity tells us how close the air is to

being saturatedbeing saturated

RH = RH = water vapor contentwater vapor content x 100x 100water vapor capacitywater vapor capacity

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RH = RH = actual vapor pressureactual vapor pressure x 100x 100saturation vapor pressuresaturation vapor pressure

Relative Humidity (cont.)Relative Humidity (cont.)

!! If air contains ½ the amount required for If air contains ½ the amount required for saturation the RH = 50%saturation the RH = 50%

!! If air is saturated, RH= 100%If air is saturated, RH= 100%!! How does RH change? How does RH change? –– two waystwo ways

–– Change the temperature (capacity for water Change the temperature (capacity for water vapor)vapor)

–– Change the amount of water vapor in the airChange the amount of water vapor in the air

Change of RH that results from daily Change of RH that results from daily temperature changetemperature change

Relative humidity and human comfortRelative humidity and human comfort

!! The body cools itself throughThe body cools itself through-- evaporation from the skinevaporation from the skin-- evaporation from the respiratory trackevaporation from the respiratory track-- radiationradiation-- conduction and convectionconduction and convection

!! When the body is under heat stress (large sources of heat When the body is under heat stress (large sources of heat from metabolism or external sources), evaporation of from metabolism or external sources), evaporation of perspiration from the skin is most important cooling perspiration from the skin is most important cooling mechanism.mechanism.

!! But, evaporation slows down when air is close to saturation But, evaporation slows down when air is close to saturation (high RH).(high RH).

!! Thus, we are uncomfortably hot when the RH is high. Thus, we are uncomfortably hot when the RH is high.

3. 3. DewpointDewpointThe temperature to which you have to cool the The temperature to which you have to cool the

air to produce saturationair to produce saturation

FogFog!! Air becomes saturated and droplets form on Air becomes saturated and droplets form on

condensation nucleicondensation nuclei!! There is no physical difference between a There is no physical difference between a

fog and a cloud, except that we tend to think fog and a cloud, except that we tend to think of a fog as being near the groundof a fog as being near the ground

!! There are different names for a fog, There are different names for a fog, depending on how it forms (that is, what depending on how it forms (that is, what causes the saturation) causes the saturation)

Fog typesFog types

Radiation fogRadiation fog

!! The surface of Earth cools at night because The surface of Earth cools at night because infrared radiation is emitted.infrared radiation is emitted.

!! The cool surface cools the air near it.The cool surface cools the air near it.!! If the air temperature cools to the If the air temperature cools to the dewpointdewpoint

temperature, a radiation fog forms.temperature, a radiation fog forms.

Radiation fog (continued)Radiation fog (continued)!! These fogs tend to form These fogs tend to form

-- in low areas (e.g., valleys) because the cool air in low areas (e.g., valleys) because the cool air near the ground drains downhill.near the ground drains downhill.

-- over moist surfaces (e.g., river valleys) because over moist surfaces (e.g., river valleys) because the dew point is higher.the dew point is higher.

-- when the sky is clear and air is dry, because more when the sky is clear and air is dry, because more infrared energy escapes to space rather than infrared energy escapes to space rather than being absorbed and radiated back to Earthbeing absorbed and radiated back to Earth

-- when the overall when the overall airmassairmass is moist, and thus the is moist, and thus the dewpointdewpoint is highis high

-- when nights are long in the winter, allowing more when nights are long in the winter, allowing more coolingcooling

-- when the winds near the surface are relatively lightwhen the winds near the surface are relatively light

Advection fogAdvection fog

!! When air moves horizontally (When air moves horizontally (advectsadvects) over ) over a cold surface, the air near the surface coolsa cold surface, the air near the surface cools

!! If the air cools to the If the air cools to the dewpointdewpoint, , condensation takes place and an advection condensation takes place and an advection fog forms.fog forms.

Advection fog (continued)Advection fog (continued)

!! These fogs tend to formThese fogs tend to form-- when air moves over a cold water surfacewhen air moves over a cold water surface-- when the air has a high relative humidity when the air has a high relative humidity

to begin withto begin with

Mixing fogMixing fog

!! Say two volumes of air are unsaturated but Say two volumes of air are unsaturated but have a high relative humidity.have a high relative humidity.

!! When they mix, the resulting mixture may be When they mix, the resulting mixture may be saturated.saturated.

Mixing Fog (examples)Mixing Fog (examples)

!! Your breath on a cold dayYour breath on a cold day!! Condensation trails from jetsCondensation trails from jets!! Exhaust from automobilesExhaust from automobiles!! “Steam fog” over a warm water surface“Steam fog” over a warm water surface!! Fog forming over snow that is melting on the Fog forming over snow that is melting on the

highway highway

Fog climatology Fog climatology –– average number of days a year average number of days a year with heavy fogwith heavy fog

Cloud TypesCloud Types

CirrusCirrus

CirrocumulusCirrocumulus

CirrostratusCirrostratus

AltocumulusAltocumulus

AltostratusAltostratus

NimbostratusNimbostratus

StratocumulusStratocumulus

StratusStratus

“Fair“Fair--weather” cumulusweather” cumulus

Cumulus Cumulus congestuscongestus

CumulonimbusCumulonimbus

LenticularLenticular or wave cloudsor wave clouds