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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 CycleCO2 SinkDistribution of Solar EnergyGeneration of Pressure SystemsHabitatPollution 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 precipitationEven 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, contminor 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, contAtmospheric 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 airspecialized / 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
SaturationIn 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 agiven 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 coolinginto the atmosphere
Dry bulb temperature (ambient air temperature)
minusWet bulb temperature
equalsWet bulb depression(from tables can read RH and dew
point)
Conditions for Condensation
(1) 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)
*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 baseSea 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