Chapter 4: Chapter 4: Humidity, Humidity,

Condensation and Condensation and CloudsClouds Circulation of water in the atmosphereCirculation of water in the atmosphere

Evaporation, condensation and saturationEvaporation, condensation and saturation HumidityHumidity Dew and frostDew and frost FogFog Foggy weatherFoggy weather CloudsClouds

Circulation of Circulation of Water in the Water in the AtmosphereAtmosphere

Q: do we have more water vapor in the hot, `dry’ air the Sahara desert than in the cold, `damp’ polar air? (A: yes and no, depending on the type of humidity we mean)

Circulation of Water in Circulation of Water in the Atmospherethe Atmosphere

evaporationevaporation condensationcondensation PrecipitationPrecipitation RunoffRunoff hydrologic cyclehydrologic cycle

• The total amount of water vapor stored in the The total amount of water vapor stored in the atmosphere amounts to only one week’s supply of atmosphere amounts to only one week’s supply of precipitation for the planet.precipitation for the planet.

Fig. 4-1, p. 80

Evaporation, Evaporation, Condensation and Condensation and

SaturationSaturation•Saturation saturation is more likely to occur in cool air

•Evaporation is increased by stronger wind; Higher T

Evaporation, Condensation and Evaporation, Condensation and SaturationSaturation

condensation condensation nucleinuclei• In very clean air, about 10,000 condensation nucleiIn very clean air, about 10,000 condensation nuclei

are typically found in one cubic centimeter of air,are typically found in one cubic centimeter of air,a volume approximately the size of your fingertip.a volume approximately the size of your fingertip.

• Condensation occurs primarily when the air is cooled

HumidityHumidity•Water vapor density (absolute humidity (kg/m3)

•Specific humidity (kg/kg)

•Mixing ratio (kg/kg)

•Vapor pressure (mb)

•Relative humidity

Vapor PressureVapor Pressure

actual vapor pressureactual vapor pressure saturation vapor pressuresaturation vapor pressure

• ““Saturation” describes a condition of equilibrium: Saturation” describes a condition of equilibrium: liquid water is evaporating at exactly the same rate that liquid water is evaporating at exactly the same rate that water vapor is condensing.water vapor is condensing.

Fig. 4-5, p. 83

The actual values areAlso provided in TableB.1 on p. 440.

Relative HumidityRelative Humidity

definition of definition of relative humidity:relative humidity:

vapor pressure divided by vapor pressure divided by saturation vapor saturation vapor pressurepressure

saturation and saturation and supersaturationsupersaturation

relative humidity relative humidity and temperatureand temperature

Fig. 4-7, p. 85

Relative Humidity and Relative Humidity and Dew PointDew Point

dew point temperaturedew point temperature

dew point depression (T – Tdew point depression (T – Tdd) and ) and relative humidityrelative humidity

Higher (T – THigher (T – Tdd) indicates lower RH) indicates lower RH

• The It is the T to which air would have to be cooled (with The It is the T to which air would have to be cooled (with no change in air pressure and moisture content) for no change in air pressure and moisture content) for saturation to occursaturation to occur• Higher THigher Tdd indicates higher water vapor content indicates higher water vapor content

Fig. 4-9a, p. 87

Ta = -2C Ta = 35CTd = -2C Td = 10CTa – Td = 0C Ta – Td = 25CRH = 100% RH = 21%

Relative Humidity and Relative Humidity and Human DiscomfortHuman Discomfort

wet bulb temperaturewet bulb temperature TTww: attained by evaporating water into the air;: attained by evaporating water into the air;

a good measure of how cool the skin can a good measure of how cool the skin can becomebecome

TTdd: reached by cooling the air to saturation;: reached by cooling the air to saturation;

a good measure of actual vapor contenta good measure of actual vapor content

Heat indexHeat index• ““It’s not the heat, it’s the humidity” - both temperature andIt’s not the heat, it’s the humidity” - both temperature andrelative humidity contribute to warm-weather discomfort.relative humidity contribute to warm-weather discomfort.

Fig. 4-10, p. 89

Measuring HumidityMeasuring Humidity PsychrometersPsychrometers Wet-bulb T (Tw);Wet-bulb T (Tw);

Dry-bulb T (Ta);Dry-bulb T (Ta); wet-bulb depression (Ta-Tw);wet-bulb depression (Ta-Tw);

Find TFind Tdd and RH (based on and RH (based on Table D on p. 443-446) forTable D on p. 443-446) for Ta = 20C, Ta-Tw = 5C; orTa = 20C, Ta-Tw = 5C; or Ta = 90F, Ta-Tw = 10FTa = 90F, Ta-Tw = 10F

HygrometersHygrometers hair hygrometer and electrical hygrometer: RHhair hygrometer and electrical hygrometer: RH

infrared hygrometer and dew cell: moisture content; infrared hygrometer and dew cell: moisture content; dew-point hygrometer (for ASOS) dew-point hygrometer (for ASOS)

Dew and FrostDew and Frost

dewdew frostfrost frost point and depositionfrost point and deposition

• Frost is one of the few examples of deposition in nature.Frost is one of the few examples of deposition in nature.

FogFog radiation fog: radiation fog: cooling from groundcooling from ground advection fogadvection fog: : warm, moist air over cold surfacewarm, moist air over cold surface upslope fogupslope fog:: coolingcooling

evaporation (mixing) evaporation (mixing) fogfog

when moist air from your mouth when moist air from your mouth or nose meets the cold air and or nose meets the cold air and mixes with it mixes with it

haze:haze: hygroscopic (`water hygroscopic (`water seeking’) condensation nuclei seeking’) condensation nuclei

allows water vapor to allows water vapor to condense when RH is below condense when RH is below 100%100%

Foggy WeatherFoggy Weather

coastal vs. interior areascoastal vs. interior areas hazard to aircrafthazard to aircraft

• Some airports use fog-dispersal equipment.Some airports use fog-dispersal equipment.

CloudsClouds

The cloud chart is provided at the end of the textbook

Classification of CloudsClassification of Clouds

major cloud typesmajor cloud types low, middle, high, verticallow, middle, high, vertical

cloud appearancecloud appearance sheetlike, puffy, wispy, rain cloudsheetlike, puffy, wispy, rain cloud

cloud basecloud base 0-2 Km, 2-6 km, 6-10km0-2 Km, 2-6 km, 6-10km

• It’s easy to identify clouds, but it takes practice.It’s easy to identify clouds, but it takes practice.The ability to identify clouds allows you to forecastThe ability to identify clouds allows you to forecastmany aspects of the weather using nothing but yourmany aspects of the weather using nothing but youreyes.eyes.

Table 4-2, p. 98

High CloudsHigh Clouds

Cirrus: Cirrus: thin, wispythin, wispy

Cirrocumulus: Cirrocumulus: small, white puffssmall, white puffs

Cirrostratus: Cirrostratus: usually thin, often producing a usually thin, often producing a

halohalo • Cirrostratus clouds can sometimes be quite thick.Cirrostratus clouds can sometimes be quite thick.

Middle CloudsMiddle Clouds

Altocumulus: Altocumulus: gray, puffy (larger, darker than gray, puffy (larger, darker than Cc)Cc)

Altostratus: Altostratus: gray layer cloud with `watery gray layer cloud with `watery sun’ sun’

(difference from Cs: darker, dimly visible, no (difference from Cs: darker, dimly visible, no ground ground

shallows)shallows)

Low CloudsLow Clouds NimbostratusNimbostratus dark gray with light raindark gray with light rain

Stratocumulus:Stratocumulus: larger cloud elements larger cloud elements

with lower cloud base than with lower cloud base than AcAc

StratusStratus uniform grayish cloud;uniform grayish cloud;

has a more uniform base has a more uniform base than Ns;than Ns;

has a lower base and dark has a lower base and dark gray than As;gray than As;

Clouds with Vertical Clouds with Vertical DevelopmentDevelopment

Cumulus: Cumulus: puffy, floating `cotton’ with flat puffy, floating `cotton’ with flat basebase

cumulus congestus: cumulus congestus: line of towering Culine of towering Cu

Cumulonimbus: Cumulonimbus: thunderstorm with anvilsthunderstorm with anvils• Not all cumulus clouds grow to be thunderstorms, butNot all cumulus clouds grow to be thunderstorms, but

all thunderstorms start out as cumulus clouds.all thunderstorms start out as cumulus clouds.

Some Unusual CloudsSome Unusual Clouds

lenticular clouds: lenticular clouds: lens-like; UFOlens-like; UFO

Pileus: `cap’Pileus: `cap’ mammatus clouds: mammatus clouds:

baglikebaglike

Contrails: Contrails: condensation trail from condensation trail from engine exhaustengine exhaust