CHAPTER 4
MOISTURE AND
ATMOSPHERIC
STABILITY
“Too Much, Too Little,
Too Bad”
All life on Earth is directly tied to acquiring
water in forms of sufficient quality and
quantity
Additionally, moisture is fundamental to:
Hydrologic Cycle
CO2 Sink
Distribution of Solar Energy
Generation of Pressure Systems
Habitat
Pollution Dispersion
A closed system
Hydrologic Cycle
-
The continuous moisture circulation
system of the Earth
--- Consists of three major “processes”
(1) evaporation / transpiration
(2) condensation
(3) precipitation
--- and two minor “processes”
(1) sublimation
(2) deposition
Across time, evaporation =
precipitation worldwide
In reality:
(1) continents – precipitation
exceeds evaporation
(2) Oceans – evaporation
exceeds precipitation
Even here there are variations according to region and
conditions
- Energy drives the cycle and its
processes (learn diagram p. 98)
--- addition and subtraction of
energy changes the “state”
(solid, liquid, gas) of moisture
- This energy is measured in
calories
(energy to raise 1 g of H2O 1o C)
Process Energy
(1) Evaporation – 600 cal. to change
1 g of H2O from liquid to gas
--- this energy is absorbed
--- this is latent heat;
specifically, latent heat of
vaporization
… absorbing energy cools
the atmosphere
Process Energy, cont
(2) Condensation – 600 cal. to change 1 g of
H2O from gas to liquid state
--- energy is released (latent heat
of condensation)
(3) Melting – 80 cal. to change 1 g of
H2O from solid to liquid state
--- energy is absorbed
(4) Freezing - 80 cal. to change 1 g of
H2O from liquid to solid state
(latent heat of fusion)
--- energy is released
Process Energy, cont
minor processes
(1) Sublimation – passage from
solid-to-gaseous state
without passage through a
liquid state
--- 680 cal. absorbed
(2) Deposition – passage from
gaseous-to-solid state without
passage through a liquid state
--- 680 cal. released
Humidity: Water Vapor
in the Air
Humidity – general term for the
measure of the amount of water
vapor in the air
… “… scientists agree that water
vapor is the most important gas in
the atmosphere when it comes to
understanding atmospheric
processes.”
Humidity, cont
Atmospheric water vapor (humidity) is
recorded in a number of ways:
(1) absolute humidity – amount of
water vapor present in a unit
volume of air
(2) specific humidity
(3) mixing ratio - mass of water
vapor in a unit of air, compared to
the remaining mass of air
specialized / limited general usage
Vapor Pressure and
Saturation
(4) Vapor pressure – that part of
total atmospheric pressure
that can be attributed to
water vapor present
--- it is directly proportional
to the density
(molecules/unit volume)
of H2O in the air
Vapor Pressure and
Saturation, cont
Water vapor “flows” from high
vapor pressure to lower vapor
pressure (vapor pressure
gradient)
--- see p. 100 evaporation /
condensation discussion
--- your perspiration illustrates
this
Saturation
In meteorology, means much the same
as general use… [full of]
When the amount of H2O vapor present
is such that the atmosphere can hold
no more
… an equilibrium of evaporation and
condensation at a given
temperature
Dew Point
Defined as the temperature at
which saturation will occur for a
given mass of water vapor
… dew point is changed by
adding or subtracting H2O
vapor
Relative Humidity
(5) Relative humidity
- most familiar and understood
way of describing H2O vapor
content in the air
– ratio of observed H2O vapor to
possible H2O vapor at a given
temperature
(H2O vapor obs / H2O vapor pos) x 100
Relative Humidity, cont
- Also defined as:
(actual vapor pressure / saturation vapor
pressure) x 100
for a given temperature
- RH can be changed by
(1) changing temperature
(2) adding / subtracting H2O
Measuring Relative
Humidity
Chemically
--- absorption hygrometer
utilizes chemical compounds
that absorb moisture at a rate
that can be quantified
… or measured by color
change
Measuring Humidity, cont
Mechanically
--- hair hygrometer takes
advantage of the fact that
human hair expands about
2.5% over a 0-100% humidity
Measuring Humidity, cont
Mechanically, cont
--- sling psychrometer
… most common device
… takes advantage of
evaporative cooling
into the atmosphere
Dry bulb temperature
(ambient air temperature)
minus
Wet bulb temperature
equals
Wet bulb depression
(from tables can read RH and dew
point)
Conditions for Condensation
The air must be saturated
--- cooling below dew point
--- water vapor added to air
(2) Surface on which the vapor can
condensate
--- condensation surface
(for dew; frost)
--- condensation nuclei
(dust; salt particles; industrial
emissions)
(1)
*with health, motivation for 10-to-2.5 micron CAA*
- Condensation can occur without
such particles
… at 400% RH
- Normally, humidity will not
exceed 101%
(I don’t think I’ve heard of 101% RH)
Clouds
- Result of trapping of vapor by
billions of condensation nuclei
- Cloud formation (type; amount;
elevation) is directly tied to
adiabatic temperature change
Adiabatic
- Term for temperature change without
the addition or subtraction of energy
--- result of pressure change
… expansion – cooling
… compression – heating
--- increasing altitude lessens
pressure promoting cooling
--- decreasing altitude increases
pressure, promoting heating
Adiabatic Rates
- In unsaturated air this is called
the dry adiabatic rate equal to
1o C/100 m (1o F/183 ft) of
elevation change
- If the air is allowed to rise and
cool far enough it will saturate
Adiabatic Rates, cont
- At the point of saturation the
rate at which the air cools will
slow from the release of latent
heat
… this is wet adiabatic rate
… average 0.6o C/100 m
(0.6o f/100 ft)
Illustrating the Difference
Utilizing graph paper graph the
adiabatic rate for:
30o C sea level
0o C cloud base
Sea level – to – 7 km
Continue dry adiabatic rate line
Lifting Condensation Level (LCL)
Altitude at which a parcel of air
reaches saturation and cloud
formation begins
… usually reported in millibars
… a factor in lifting of large
volumes of air
(small scale air parcel lifting is termed
convection condensation lifting)
Stability (of the atmosphere)
-
-
Determined by examining
atmospheric temperature at
various altitudes
Stability resists vertical
movement
Stability, cont
Two forms of stability
(1) Stable air – situation where
air resists rising
(a) temperature inversion
(b) katabatic winds
Stability, cont
(2) Unstable air – convective
movement of air to increasing
altitudes… can be:
(a) absolutely unstable –
temperature change with height is
greater than dry adiabatic rate.
Rare, but may occur in desert or
intense ground heating regions
(b) conditionally unstable –
temperature change with height is
between the wet and dry adiabatic
rates
Clouds that form under conditions of
stability are light, have little vertical
formation, and little or no
precipitation
Clouds forming under conditions of
instability have vertical displacement
and generally bring heavy
precipitation
Changes in Instability
(1) Movement of air - vertical
(a) subsidence
(b) lifting
(2) Movement of air – horizontal
(a) advection
Changes in Instability, cont
(3) Forceful lifting
- convergence
- orographic
- frontal wedging
How Stability Changes
Instability is enhanced by:
(1) intense insolation in the
lower atmosphere
(2) heating of an airmass
passing over a warm surface
(3) forces lifting
(4) radiation cooling from cloud
tops
How Stability Changes, cont
Stability is enhanced by:
(1) radiation cooling by the
Earth’s surface after
nightfall
(2) cooling of airmass passing
over a cool surface
(3) subsidence in an air column