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