Chapter 6
Humidity, Saturation, and Stability
Driving Question
How is water cycled between Earth’s
surface and atmosphere?
Global Water Cycle
The supply of water is essentially fixed
Global Water Cycle

Endless flow of water between land,
atmosphere, ocean, and organisms
The driving force of this cycle is the sun
Oceans hold more than 97% of total
water
Transfer Process
Evaporation

Ocean is principle source of atmospheric
water vapor
Transpiration

Water taken up by roots that evaporates
through the leaves
Evapotranspiration

Direct evaporation plus transpiration
Transfer Process
Condensation: gas to liquid
Sublimation: solid to gas
Deposition: gas to solid
Precipitation

Water, in any form, that falls to the surface from
clouds
 Rain, snow, drizzle, freezing rain, hail, sleet, ice pellets
Global Water Budget
Net water gain over continents

Precipitation > Evapotranspiration
Net water loss over oceans

Evaporation > Precipitation
Balanced is achieved as land surplus
flows to the ocean

Runoff, rivers, ground water
Humidity
General term describing the amount or
concentration of water vapor in the air
Highly variable
Measures of Humidity





Vapor pressure
Mixing ratio
Specific, Absolute, and Relative Humidity
Dewpoint
Precipitable Water
Vapor Pressure
Water vapor mixes with with other
gases adding to total air pressure
Amount of pressure added by water
vapor is a measure of humidity
Vapor Pressure

Pressure exerted by water vapor alone
Considerably less than 40mb
Mixing Ratio, Specific
Humidity, Absolute Humidity
Mixing Ratio

Ratio of mass of water vapor per mass of
remaining dry air (g/kg)
Specific Humidity

Ratio of mass of water vapor to mass of total air,
dry and moist (g/kg)
Absolute Humidity


Mass of water vapor per unit volume of humid air
Density of water vapor in air (g/m3)
Saturation (not a measure of
humidity)
Air is saturated with respect to water
vapor at its maximum humidity
Occurs at equilibrium

When rate of evaporation equals the rate
of condensation
At equilibrium the air is saturated with
water vapor
Saturation VP and MR v.Temperature
Relative Humidity
Most common
Compares the actual amount of water
vapor in the air with the amount that
would be in the air if the air were
saturated (%)
RH is inversely proportional to temp.
RH = (vapor pressure/saturation vapor pressure) * 100%
RH = (mixing ratio/saturation mixing ratio) * 100%
Dewpoint
Temperature to which the air must be
cooled to reach saturation
A higher dewpoint indicates a greater
concentration of water vapor
If RH = 100%


Air is saturated
Temperature = Dewpoint
Dewpoint
Dew: tiny droplets of water formed
when water vapor condenses

Water vapor deposits as frost if the
temperature of saturation is below freezing
Average dewpoint across US is between
30-45oF

Can be higher than 80oF
Precipitable Water
Depth of water that would be produced if all
the water vapor in a vertical column of air
were condensed into liquid water

Column extends from surface to tropopause
Condensing all the water vapor would
produce a 1” layer of water covering the
entire earth’s surface
Values average from 4.0cm in tropics to
0.5cm in polar regions
Monitoring Water Vapor
Hygrometer: instrument that measures
water vapor concentration of air



Dewpoint hygrometer
Hair hygrometer
Electronic hygrometer
Hygrograph: continuous plot of relative
humidity with time
Monitoring Water Vapor
Sling Psychrometer



Two thermometers
mounted next to one
another
One is covered in
cloth and soaked with
water
Thermometers are
then “whirled”
causing the water to
evaporate
Monitoring Water Vapor
Dry Bulb thermometer measures actual air
temperature
Wet Bulb thermometer measures the wet bulb
temperature

Temperature to which air cools to due the
evaporation of the water in the air
Wet Bulb Depression

Difference between dry and wet bulb temperatures
Can use these numbers to find RH and
dewpoint
Monitoring Water Vapor
Water Vapor
emits radiation
at 6.7
micrometers
Satellite
imagery
displays water
vapor and
clouds above
3000m
How Air Becomes Saturated
Clouds

Visible collections of water droplets and/or
ice crystals suspended in the atmosphere
Clouds are most likely to form as RH
approaches 100%
So, what causes the RH to increase?
Warming and Cooling
Expansional Cooling

As a gas expands (rises), its temperature falls
Compressional Warming

As a gas contracts (falls), its temperature rises
As parcels of air move up and down in the
atmosphere the temperature of that parcel
changes
Lapse Rates
Adiabatic Process


No heat is exchanged between a parcel and
the environment
Temperature change is due to expansion and
compression only
Unsaturated Air – dry adiabatic lapse rate

9.8 oC / 1000m (5.5 oF/ 1000ft)
Saturated Air – moist adiabatic lapse rate


6.5 oC / 1000m (3.3 oF/ 1000ft)
Less because expansional cooling is offset by
release of latent heat
Problem:
Recall, DALR = 10 deg/1000m, WALR =
6 deg/1000m
Assume a parcel of 15 degrees C at the
surface
If parcel rises 2km dry adiabatically
what is the new temperature?
If the parcel then saturates and rises
another 1000m what is the
temperature?
-5 deg C
-11 deg C
Stable Air Layer
A rising air parcel becomes cooler
(denser) than the environment and thus
sinks back to its original position
A sinking air parcel becomes warmer
(less dense) than the environment and
thus lifts back to its original position
Vertical motion is inhibited
Unstable Air Layer
A rising air parcel becomes warmer
(less dense) than the environment and
thus continues to rise
A sinking air parcel becomes cooler
(denser) than the environment and thus
continues to sink
Vertical motion is enhanced
Types of Stability
When figuring stability it is helpful if the
following are known


Is the parcel saturated or unsaturated?
What is the vertical temperature profile
(sounding) of the atmosphere?
Types of Stability
Absolute Instability


Saturated and unsaturated parcels are unstable
Lapse rate is greater than 10 oC / 1000 m
Conditional Instability



Unsaturated parcels are stable
Saturated parcels are unstable
Lapse rate is between 10 oC / 1000 m and 6.5 oC /
1000 m
Types of Stability
Absolute Stability



Saturated and unsaturated parcels are stable
Lapse rate is less than 6.5 oC / 1000 m
Three types
 Lapse
 Isothermal (temperature is constant with height)
 Inversion (temperature increases with height)
Neutral Air


When environmental lapse rate equals dry or
moist adiabatic lapse rate
Neither impedes or provokes vertical motion
Stüve Thermodynamic Chart
Lifting Processes
Convection
Along Fronts
Topography (Orographic Lifting)
Converging Winds
Lifting Condensation Level (LCL)


The level in which rising air becomes saturated
and clouds form
Marked by the base of clouds
Convection
Frontal Lifting
Orographic Lifting
Converging Winds