air parcels a parcel is a “blob” of air a parcel is a “blob” of air small enough to have...

Air parcelsAir parcels A parcel is a “blob” of airA parcel is a “blob” of air Small enough to have only one value of T, Small enough to have only one value of T,

p, ρ, etc.p, ρ, etc. Large enough to contain a significant Large enough to contain a significant

number of molecules. (Are there enough number of molecules. (Are there enough particles to talk about temperature as particles to talk about temperature as average kinetic energy, for example?)average kinetic energy, for example?)

Lapse RatesLapse Rates Parcel lapse rate – the rate at which Parcel lapse rate – the rate at which

temperature changes as the parcel is temperature changes as the parcel is lifted to a higher altitudelifted to a higher altitude

Environmental lapse rate – the rate at Environmental lapse rate – the rate at which the air surrounding the parcel which the air surrounding the parcel changes as altitude increaseschanges as altitude increases

The Adiabatic The Adiabatic Lapse RateLapse Rate

An An adiabatic process adiabatic process is one during is one during which no heat is exchanged between the which no heat is exchanged between the substance in question and its substance in question and its surroundingssurroundings

Many atmospheric motions occur rapidly Many atmospheric motions occur rapidly enough that parcels do not exchange a enough that parcels do not exchange a significant amount of heat with the significant amount of heat with the environmentenvironment

Examples: Examples: • rising air in a thunderstormrising air in a thunderstorm• Air rising over a topographic barrier (like a Air rising over a topographic barrier (like a

mountain)mountain)


The adiabatic lapse rate for The adiabatic lapse rate for DRYDRY air on air on Earth isEarth is

ΓΓdd = g/c = g/cpp

ΓΓdd = 9.81 m s = 9.81 m s-2-2 / 1004 J kg / 1004 J kg -1 -1 C C-1 -1

ΓΓdd = 0.00977 C m = 0.00977 C m-1-1

ΓΓdd = 9.77 C km = 9.77 C km-1-1


This means that a rising(This means that a rising(sinkingsinking) air parcel ) air parcel will cool(will cool(warmwarm) at a rate of about 10 ) at a rate of about 10 ooC C per km of ascent(per km of ascent(descentdescent) unless:) unless:• It exhanges significant mass or heat with the It exhanges significant mass or heat with the

environmentenvironment• It becomes saturated with respect to water It becomes saturated with respect to water

vaporvapor• It rises(sinks) so slowly that radiation heat It rises(sinks) so slowly that radiation heat

transfer is possibletransfer is possible

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 CloudsClouds

Circulation of Water in Circulation of Water in the Atmospherethe Atmosphere

evaporationevaporation condensationcondensation precipitationprecipitation 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

Stepped Art

Evaporation, Evaporation, Condensation and Condensation and

SaturationSaturation saturationsaturation condensation nucleicondensation nuclei

• In very clean air, about 10,000 condensation nucleiIn very clean air, about 10,000 condensation nucleiare 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.

HumidityHumidity

Mixing Ratio (Mixing Ratio (ww))The ratio of the mass of water vapor in air to the The ratio of the mass of water vapor in air to the mass of dry air:mass of dry air:

w = mw = mvv / m / mdd

Usually expressed in g kgUsually expressed in g kg-1-1

Some typical values:Some typical values:• Tropical marine boundary layer air: w ≈ 18 g kgTropical marine boundary layer air: w ≈ 18 g kg-1-1

• Polar air: w ≈ 1 g kgPolar air: w ≈ 1 g kg-1-1

• Stratospheric air: w ≈ 0.1 g kgStratospheric air: w ≈ 0.1 g kg-1-1

Specific HumiditySpecific Humidity

The ratio of the mass of water vapor in air to the The ratio of the mass of water vapor in air to the total mass of the air (dry air plus water vapor):total mass of the air (dry air plus water vapor):

SH = mSH = mvv / (m / (mdd + m + mvv))

w = SH / (1 – SH)w = SH / (1 – SH)SH = w / (1 + w)SH = w / (1 + w)

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.


Saturation vapor pressure depends only on Saturation vapor pressure depends only on temperature…temperature…

Formula:Formula:

TTR

Lee

vss

11exp

00

es :

es0 :

L :

Rv :

T0 :

T :

Saturation vapor pressureSaturation vapor pressure

Saturation vapor pressure Saturation vapor pressure at 273 K = 6.11 mbat 273 K = 6.11 mb

Latent heat of Latent heat of vaporization = 2.5x10vaporization = 2.5x1066 J kgJ kg-1-1

Gas constant for water Gas constant for water vapor = 461 J kgvapor = 461 J kg-1-1 K K-1-1

273 K273 K

TemperatureTemperature


Saturation vapor pressure depends only on Saturation vapor pressure depends only on temperature…temperature…

Formula:Formula:

es 6.11 exp 54231

273

1

T


Saturation vapor pressure depends only on temperature…Saturation vapor pressure depends only on temperature…

GraphGraph::

Relative HumidityRelative Humidity

definition of relative humiditydefinition of relative humidity saturation and supersaturationsaturation and supersaturation condensationcondensation relative humidity and temperaturerelative humidity and temperature

• 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

The ratio of the actual vapor pressure to the The ratio of the actual vapor pressure to the saturation vapor pressure.saturation vapor pressure.

f = e / ef = e / ess

Since eSince ess depends on temperature, the depends on temperature, the relative humidity measures closeness to relative humidity measures closeness to

saturation, not actual water vapor content.saturation, not actual water vapor content.

Fig. 4-5, p. 83

Fig. 4-7, p. 85

Relative Humidity and Relative Humidity and Dew PointDew Point

dew point temperature: the dew point temperature: the temperature to which air must be temperature to which air must be lowered to reach 100% relative lowered to reach 100% relative humidityhumidity

dew point depression and relative dew point depression and relative humidityhumidity• The dew point temperature is useful for forecasting heatThe dew point temperature is useful for forecasting heat

index, precipitation probabilities, and the chance of frost.index, precipitation probabilities, and the chance of frost.

Measuring HumidityMeasuring Humidity

psychrometerspsychrometers hygrometershygrometers

Topography and CloudsTopography and Clouds

orographic upliftorographic uplift rain shadowrain shadow

• The rain shadow works for snow too. Due to frequentThe rain shadow works for snow too. Due to frequentwesterly winds, the western slope of the Rocky Mountainswesterly winds, the western slope of the Rocky Mountainsreceives much more precipitation than the eastern slope.receives much more precipitation than the eastern slope.

Collision and Collision and Coalescence ProcessCoalescence Process

terminal velocityterminal velocity coalescencecoalescence warm cloudswarm clouds

• A typical cloud droplet A typical cloud droplet falls at a rate of 1 falls at a rate of 1 centimeter per second.centimeter per second.At this rate it would take At this rate it would take 46 hours to fall one mile.46 hours to fall one mile.

InstrumentsInstruments

standard rain gaugestandard rain gauge tipping bucket rain gaugetipping bucket rain gauge

• It is difficult to capture rain in a bucket when theIt is difficult to capture rain in a bucket when thewind is blowing strongly.wind is blowing strongly.

Doppler Radar and Doppler Radar and PrecipitationPrecipitation

radarradar Doppler radarDoppler radar

Fig. 5-39, p. 135

Stepped Art

air parcels a parcel is a “blob” of air a parcel is a “blob” of air small enough to have...

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