Chapter 4: Chapter 4: Humidity, Humidity,

Condensation and Condensation and CloudsClouds

By the end of this chapter you should:By the end of this chapter you should:

Know the hydrologic cycleKnow the hydrologic cycle Understand the idea of saturationUnderstand the idea of saturation Grasp the idea of humidityGrasp the idea of humidity

Understand the different cloudUnderstand the different cloudtypes, including fogtypes, including fog

Fig. 4-1, p. 80

Circulation of Water in Circulation of Water in the Atmospherethe Atmosphere

EvaporationEvaporation from oceans that take up 70% of the Earth’s from oceans that take up 70% of the Earth’s surface (solar radiation does the evaporation)surface (solar radiation does the evaporation)

CondensationCondensation this evaporated water (water vapor) turns this evaporated water (water vapor) turns back into liquid water in the form of cloudsback into liquid water in the form of clouds

PrecipitationPrecipitation clouds produce rain on land (runs back to clouds produce rain on land (runs back to the ocean) and over the oceanthe ocean) and over the ocean

Evaporation, Evaporation, Condensation and Condensation and

SaturationSaturation What is equilibrium?What is equilibrium?

It is a state in a system where competing forces are It is a state in a system where competing forces are balancedbalanced

For example, look at the two beakers belowFor example, look at the two beakers below

Evaporation, Evaporation, Condensation and Condensation and

SaturationSaturation In the first beaker is water with molecules bumping, In the first beaker is water with molecules bumping,

vibrating at different speeds (temperature)vibrating at different speeds (temperature)

Occasionally, one of the fast-moving molecules near Occasionally, one of the fast-moving molecules near the top of the water will break away and evaporatethe top of the water will break away and evaporate

Evaporation, Evaporation, Condensation and Condensation and

SaturationSaturation At the same time, some of the water vapor molecules At the same time, some of the water vapor molecules

outside the water outside the water change phasechange phase from vapor to liquid from vapor to liquid Now let’s look at the second beakerNow let’s look at the second beaker

Evaporation, Evaporation, Condensation and Condensation and

SaturationSaturation The second beaker has a lid on itThe second beaker has a lid on it Water molecules still evaporate into the air, and vapor still Water molecules still evaporate into the air, and vapor still condenses into liquidcondenses into liquid But, what will happen if we let this process go on for a But, what will happen if we let this process go on for a long time (think of a soda can in the sun)long time (think of a soda can in the sun)

Evaporation, Evaporation, Condensation and Condensation and

SaturationSaturation Eventually, the number of water molecules changing Eventually, the number of water molecules changing

phase into vapor will equal the number of vapor phase into vapor will equal the number of vapor molecules changing phase into watermolecules changing phase into water

This is called This is called saturationsaturation. Basically, the air is in equilibrium. Basically, the air is in equilibrium

Evaporation, Evaporation, Condensation and Condensation and

SaturationSaturation If we remove the lid and blow, water vapor molecules If we remove the lid and blow, water vapor molecules

would go away. The air isn’t saturated anymore. This would go away. The air isn’t saturated anymore. This allows for more evaporationallows for more evaporation

Thus, wind enhances evaporationThus, wind enhances evaporation

Evaporation, Evaporation, Condensation and Condensation and

SaturationSaturation In the air above the beakers is Nitrogen, Oxygen, etc. In the air above the beakers is Nitrogen, Oxygen, etc.

But also dirt, smoke, salt, etc.But also dirt, smoke, salt, etc.

Condensation nucleiCondensation nuclei – particles that allow vapor to – particles that allow vapor to condense into liquidcondense into liquid

Condensation is more likely when the air is cooledCondensation is more likely when the air is cooled

Vapor PressureVapor Pressure In a parcel of air, In a parcel of air,

the pressure is the pressure is 1000 millibars1000 millibars

Pressure is the sum Pressure is the sum of the forces of of the forces of every molecule every molecule pushing against the pushing against the side of the parcelside of the parcel

Vapor PressureVapor Pressure Thus, since air is Thus, since air is

78% Nitrogen, the 78% Nitrogen, the Nitrogen air Nitrogen air pressure is 780 mb pressure is 780 mb (1000 * 78%)(1000 * 78%)

Oxygen’s (21%) Oxygen’s (21%) pressure is 210 mbpressure is 210 mb

Water vapor’s (1%) Water vapor’s (1%) pressure is 10 mb. pressure is 10 mb. This is called This is called actual actual vapor pressurevapor pressure

Vapor PressureVapor Pressure So So actual vapor actual vapor

pressurepressure is a small is a small portion of the total portion of the total pressurepressure

Actual vapor Actual vapor pressure is an pressure is an indication of how indication of how much water vapor is much water vapor is in the airin the air

Vapor PressureVapor Pressure If the air inside the If the air inside the

balloon was balloon was saturated, the water saturated, the water molecules would molecules would exert a pressure. exert a pressure. This is called This is called saturation vapor saturation vapor pressurepressure

Vapor PressureVapor Pressure So, if we increase the temperature in the second beaker, So, if we increase the temperature in the second beaker,

more water molecules will escape to vapormore water molecules will escape to vapor To maintain equilibrium, the air must be able to hold To maintain equilibrium, the air must be able to hold

more vapormore vapor

At higher temperatures, it takes more vapor to saturated At higher temperatures, it takes more vapor to saturated the airthe air

Fig. 4-5, p. 83

Relative HumidityRelative Humidity Relative humidityRelative humidity – basically tells us how close the air is – basically tells us how close the air is

to being saturatedto being saturated Definition: RH = water vapor content / water vapor Definition: RH = water vapor content / water vapor

capacity OR RH = act vapor pressure / sat vapor capacity OR RH = act vapor pressure / sat vapor pressurepressure

So an RH of 50% tells us what? Or 100% How about So an RH of 50% tells us what? Or 100% How about 104%104%

• When the general public uses the term “humidity”, When the general public uses the term “humidity”, they mean “relative humidity.”they mean “relative humidity.”

Relative HumidityRelative Humidity SupersaturationSupersaturation – air with a relative humidity greater – air with a relative humidity greater

than 100%than 100% Name two ways we can change the humidityName two ways we can change the humidity

Change the water vapor content (put more vapor in or Change the water vapor content (put more vapor in or take some out)take some out)

Changing the air temperatureChanging the air temperature

Relative HumidityRelative Humidity

Higher temperatures – faster moving molecules and less Higher temperatures – faster moving molecules and less likely they are to collide and condenselikely they are to collide and condense

Condensation occurs at saturationCondensation occurs at saturation So what happens to RH during the day typically?So what happens to RH during the day typically?

Fig. 4-7, p. 85

Relative Humidity and Relative Humidity and Dew PointDew Point

Suppose the morning temperature is 10°C and the RH is Suppose the morning temperature is 10°C and the RH is 100%. From graph, saturation vapor pressure is 12 mb 100%. From graph, saturation vapor pressure is 12 mb at 10°C. What is the actual vapor pressure?at 10°C. What is the actual vapor pressure?

Suppose the afternoon temperature is 30°C with no Suppose the afternoon temperature is 30°C with no change is vapor content. What is the RH?change is vapor content. What is the RH?

Answer 1: 12 mbAnswer 1: 12 mb Answer 2: 29% (saturation vapor pressure is 412 mb at Answer 2: 29% (saturation vapor pressure is 412 mb at

30°C)30°C) For the second scenario, what temperature do we have For the second scenario, what temperature do we have

to cool the temperature to to get saturation?to cool the temperature to to get saturation?

Answer: 10°CAnswer: 10°C For this scenario, 10°C is the For this scenario, 10°C is the dew pointdew point

Relative Humidity and Relative Humidity and Dew PointDew Point

Dew pointDew point – the temperature to which the air would have – the temperature to which the air would have to be cooled to obtain saturation (with no moisture to be cooled to obtain saturation (with no moisture content change)content change)

So, temp of 15°C and RH of 100%. What is the dew So, temp of 15°C and RH of 100%. What is the dew point?point?

High dew points represent a lot of moisture. Low dew High dew points represent a lot of moisture. Low dew points, low moisturepoints, low moisture

Relative Humidity and Relative Humidity and Dew PointDew Point

Relative Humidity and Relative Humidity and Dew PointDew Point

Relative Humidity and Relative Humidity and Human DiscomfortHuman Discomfort

So why do humid days “feeling” hotter than drier days?So why do humid days “feeling” hotter than drier days?

• ““It’s not the heat, it’s the humidity”It’s not the heat, it’s the humidity”

First, why do we perspire?First, why do we perspire? The amount of evaporation from the skin is directly The amount of evaporation from the skin is directly

related to how saturated the air isrelated to how saturated the air is Drier air will evaporate sweat more quickly than air Drier air will evaporate sweat more quickly than air

that is closer to saturationthat is closer to saturation More evaporation, more coolingMore evaporation, more cooling

Relative Humidity and Relative Humidity and Human DiscomfortHuman Discomfort

Heat index – combines temperature and relative Heat index – combines temperature and relative humidity to determine what the air “feels like”humidity to determine what the air “feels like”

Dew and FrostDew and Frost On calm, clear nights, surface cools rapidly by what On calm, clear nights, surface cools rapidly by what

process?process?

Air near the ground cools to dew point quickly, Air near the ground cools to dew point quickly, reaches saturationreaches saturation

DewDew - vapor that condenses on blades of grass - vapor that condenses on blades of grass

Dew and FrostDew and Frost If the air temperature cools to the freezing point, you get If the air temperature cools to the freezing point, you get

frozen dewfrozen dew

FrostFrost - If the dew point - If the dew point is below the freezing is below the freezing point, then vapor goes point, then vapor goes directly to ice (what is directly to ice (what is this called)?this called)?

FogFog FogFog – a cloud resting near the ground – a cloud resting near the ground Types of fog:Types of fog:

Radiation fogRadiation fog Advection fogAdvection fog Upslope fogUpslope fog Evaporation fogEvaporation fog

FogFog Radiation fogRadiation fog – is produced as a result of radiational – is produced as a result of radiational

coolingcooling Forms best at night with a shallow moist layer below a Forms best at night with a shallow moist layer below a drier layerdrier layer Moist layer cools rapidly and reaches dew pointMoist layer cools rapidly and reaches dew point

Usually “burns off” by afternoon from bottom upUsually “burns off” by afternoon from bottom up

Radiation FogRadiation Fog

FogFog Advection fogAdvection fog – produced with warm, moist air flows over – produced with warm, moist air flows over

cool surfacecool surface Very common along West CoastVery common along West Coast

Cool water very near coast, warm water and moist air Cool water very near coast, warm water and moist air farther out. Afternoon breeze pushes air to coast, and farther out. Afternoon breeze pushes air to coast, and cools it to dew pointcools it to dew point

Moisture in fog important to redwood coastMoisture in fog important to redwood coast

Advection FogAdvection Fog

Advection FogAdvection Fog

Advection FogAdvection Fog

FogFog Upslope fogUpslope fog – produced as moist air flows up an – produced as moist air flows up an

elevated surfaceelevated surface

As the air rises, it cools and reaches its dew pointAs the air rises, it cools and reaches its dew point

Common on the eastern slopes of the Rocky Common on the eastern slopes of the Rocky MountainsMountains

Can last for daysCan last for days

Upslope FogUpslope Fog

FogFog Evaporation fogEvaporation fog – produced by mixing two unsaturated – produced by mixing two unsaturated

air massesair masses

Breathing on a cold day produces a cloudBreathing on a cold day produces a cloud

Occurs above swimming poolsOccurs above swimming poolsIf cold air moves over a swimming pool, warm If cold air moves over a swimming pool, warm water will evaporate into the air, raising the water will evaporate into the air, raising the dew pointdew point

Foggy WeatherFoggy Weather What areas will be the foggiest? Coastal or Interior?What areas will be the foggiest? Coastal or Interior?

Where the water is!!!! The coast!Where the water is!!!! The coast!

Table 4-2, p. 98

High CloudsHigh Clouds CirrusCirrus High clouds made of ice High clouds made of ice crystalscrystals

Can look like a pulled apart cotton ballCan look like a pulled apart cotton ball

High CloudsHigh Clouds Cirrocumulus (mackerel sky)Cirrocumulus (mackerel sky) Made of ice crystalsMade of ice crystals Looks a little like the scales of a fishLooks a little like the scales of a fish “ “Cumulus” means has a puffy lookCumulus” means has a puffy look

High CloudsHigh Clouds CirrostratusCirrostratus Made of ice crystalsMade of ice crystals ““Status” means layered across entire skyStatus” means layered across entire sky

Middle CloudsMiddle Clouds AltocumulusAltocumulus Mid-level clouds mostly made of liquid waterMid-level clouds mostly made of liquid water ““Cumulus” means puffyCumulus” means puffy Can cause very pretty sunsets or sunrisesCan cause very pretty sunsets or sunrises

Middle CloudsMiddle Clouds AltostratusAltostratus Mid-level clouds mostly made of liquid waterMid-level clouds mostly made of liquid water ““Stratus” means layered across skyStratus” means layered across sky

Low CloudsLow Clouds NimbostratusNimbostratus Low cloudLow cloud ““Nimbo” or “Nimbus” means rainNimbo” or “Nimbus” means rain ““Stratus” meansStratus” means

layered acrosslayered acrossthe entire skythe entire sky

Low CloudsLow Clouds StratocumulusStratocumulus Most common cloud type in the worldMost common cloud type in the world ““Cumulus” means puffy, “Strato” means layeredCumulus” means puffy, “Strato” means layered

Clouds With Vertical Clouds With Vertical DevelopmentDevelopment

CumulusCumulus ““Cumulus” means puffyCumulus” means puffy Not all cumulus clouds grow into thunderstormsNot all cumulus clouds grow into thunderstorms

Clouds With Vertical Clouds With Vertical DevelopmentDevelopment

Cumulus congestusCumulus congestus ““Cumulus” means puffyCumulus” means puffy

““Congestus” means bunched togetherCongestus” means bunched together

Clouds With Vertical Clouds With Vertical DevelopmentDevelopment

CumulonimbusCumulonimbus These are the big thunderstormsThese are the big thunderstorms

““Nimbus” means rainNimbus” means rain

Some Unusual CloudsSome Unusual Clouds

Lenticular cloudsLenticular clouds PileusPileus Mammatus cloudsMammatus clouds ContrailsContrails NacreousNacreous NoctilucentNoctilucent• Several alleged ‘flying

saucer’ reports have turned out to be lenticular clouds.

Mammatus CloudsMammatus Clouds

Lenticular CloudsLenticular Clouds

ContrailsContrails

Noctilucent CloudsNoctilucent Clouds

Nacreous CloudsNacreous Clouds