Chapter 6: Atmospheric Moisture – p. 1 of 13
Chapter 6: Atmospheric Moisture
I.
II.
III.
The Impact of Atmospheric Moisture on the Landscape
A. 3 physical states of water in the atmosphere:
1. solid: snow, hail, sleet, etc
2. liquid: rain, water droplets
3. gas: water vapor (most abundant state of atmospheric moisture)
B. gas state
1. most important in atmospheric dynamics
2. water vapor stores energy that can incite atmosphere into action
The Hydrologic Cycle
A. hydrologic cycle: unending circulation of our planet’s water supply
B. liquid water (primarily from oceans) evaporates into the air  condenses
to liquid or solid state  returns to Earth as precipitation
C. important determinant of climate
The Nature of Water: Commonplace but Unique
A. water
1. most widespread substance on Earth’s surface; covers more than
70% of Earth’s surface
2. properties:
a. no color, no taste, no smell
b. turns solid at 0oC/32oF; boils at 100oC/212oF
B. The Water Molecule
1. covalent bonds: holds 2 hydrogen atoms and 1 oxygen atom together
to form water molecule (H2O)
2. electrical polarity of water gives it many of its interesting properties,
including cohesion
C. Properties of Water
1. Liquidity: the fact that water is liquid at the temperatures found most
places on Earth enhances its versatility as active agent in the
atmosphere, lithosphere and biosphere
2. Ice Expansion: water expands as it cools from 39oF to freezing
point 32oF
a. important component of weathering
b. ice floats: ice is less dense than water
c. lakes freeze from the top down
3. Surface Tension: because of electrical polarity water molecules stick
together
4. Capillarity
a. action by which water can climb upward in restricted confinement
as a result of its surface tension and adhesion
b. enables water to circulate upward through rock, soil and the roots
and stems of plants
5. Solvent Ability
a. “universal solvent” – water can dissolve almost any substance
b. water in nature is always impure
Chapter 6: Atmospheric Moisture – p. 2 of 13
IV.
c. moving water carries dissolved minerals and nutrients and tiny
solid particles
6. Specific Heat
a. water has very high heat capacity: when water is warmed it can absorb
an enormous amount of energy with small increase in temperature
b. bodies of water are slow to warm up during the day or in summer
and slow to cool off at night or in winter → water has moderating
effect on surrounding temperature
Phase Changes of Water
A. water found naturally in 3 states on Earth: liquid, solid, gas
1. great majority of world’s moisture in form of liquid water
2. phase changes (Fig 6-4):
a. evaporation:
1) change of state from liquid to gas (water vapor)
2) 540-600 calories absorbed
b. freezing:
1) change of state from liquid to solid
2) 80 calories released
c. condensation:
1) change of state from water vapor (gas) to liquid water
2) 540-600 calories released
d. melting:
1) change of state from solid to liquid
2) 80 calories absorbed
e. sublimation: gas to solid (~680 calories released) or solid to gas
directly (~680 calories absorbed)
3. in each phase change, there is an exchange of latent heat energy
B. Latent Heat
1. latent heat: energy exchanged in a phase change – the
temperature of water does not change while it is undergoing a
phase change
2. latent heat of melting: energy required to melt ice (80 cal/g/oC)
3. latent heat of fusion: energy released as water freezes (80 cal/g/oC)
4. latent heat of vaporization: energy required to change water from
liquid to gas (540 cal/g/oC)
5. latent heat of condensation: energy released when water vapor
changes back to liquid (540 cal/g)
6. latent heat of evaporation: energy required for evaporation
(540-600 cal/g/oC, depending on the temperature of the water)
7. about 7 times more heat is needed to evaporate 1 gram of liquid
water than is needed to melt 1 gram of ice
C. Importance of Latent Heat in the Atmosphere
1. especially of significance between liquid and gas phase
2. evaporation
a. energy is removed from the liquid
b. temperature of remaining liquid is reduced
Chapter 6: Atmospheric Moisture – p. 3 of 13
V.
c. cooling process
3. condensation
a. latent heat is released
b. warming process
c. water vapor represents a “reservoir” of heat
Water Vapor and Evaporation
A. water vapor
1. water’s gaseous state
2. minor constituent in atmosphere
a. variable from place to place and time to time
b. varies from 0-4% of total atmospheric volume
c. restricted to lower troposphere
B. Evaporation and Rates of Evaporation
1. amount and rate of evaporation from a water surface depend on 3
factors:
a. temperature of both the air and water
b. amount of water vapor in the air
c. whether the air is still or moving
2. Temperature
a. high temperature increases energy of molecules  accelerates
the rate of evaporation
3. Water Vapor Content of Air
a. vapor pressure: pressure exerted by water vapor
b. at any given air temperature there is a maximum vapor
pressure that water vapor molecules can exert
c. the higher the temperature, the higher the maximum vapor
pressure
d. saturated air
1) results when there are enough water molecules in the air to
exert the maximum vapor pressure at any given temperature
2) when maximum water vapor pressure is exceeded 
condensation
3) evaporation takes place more rapidly when there is relatively
little water vapor in air
4. Windiness
a. dispersing of water vapor  air further from saturation  rate of
evaporation increases
b. higher temperature, greater windiness, and drier air 
greater net evaporation
C. Evapotranspiration
1. evapotranspiration: combined process of water vapor entering the air
from land sources (soil and other inanimate objects, and plants)
a. evaporation: process by which liquid water is converted to gaseous
water vapor
b. transpiration: process by which plants give up moisture through
their leaves
Chapter 6: Atmospheric Moisture – p. 4 of 13
2. potential evapotranspiration:
a. amount of evapotranspiration that would occur if the ground at the
location in question were sopping wet all the time
b. maximum evapotranspiration that could result under local
environmental conditions if the moisture were available
D. water surplus: locations where precipitation exceeds potential
evapotranspiration
E. water deficit: locations where potential evapotranspiration exceeds
precipitation
VI.
Focus: GOES Weather Satellites
A. GOES
1. Geostationary Operational Environmental Satellites
2. operated by the National Oceanic and Atmospheric Administration (NOAA)
3. geostationary: orbits in fixed position relative to the surface of the Earth
B. instrumentation
1. sounder – detects vertical temperature and moisture variations within
atmosphere and ozone distribution
2. imager: sensors detect radiant and reflected electromagnetic radiation
in different wavelengths
C. products
1. visible light images measure albedo
2. infrared images
a. longwave – thermal IR radiation basically shows differences in
temperature
b. warm objects (dark) emit more longwave radiation than cold objects
(whiter)
VII. Measures of Humidity
A. Absolute Humidity
1. humidity: amount of water vapor in the air
2. absolute humidity: amount of water vapor in a given volume of air
(g/m3)
3. the maximum possible absolute humidity (water vapor capacity)
for a parcel of air is limited by the temperature
4. warm air capable of much higher absolute humidity than cold air
(Fig 6-7)
5. if the volume of air changes, the value of absolute humidity changes
B. Specific Humidity
1. specific humidity: mass of water vapor in a given mass of air (g/kg)
2. changes only if quantity of water vapor changes
3. both absolute and specific humidity are indications of potential for
precipitation in a parcel of air
C. Vapor Pressure
1. vapor pressure: contribution of water vapor to the total pressure of
the atmosphere
2. saturation vapor pressure: maximum possible vapor pressure at a
given temperature
Chapter 6: Atmospheric Moisture – p. 5 of 13
3. warm air has potential to contain much more water vapor than
cold air
D. Relative Humidity
1. relative humidity: ratio (expressed as a percentage) that compares the
actual amount of water vapor in the air (absolute or specific humidity) to
the water vapor capacity of the air (where capacity is the maximum
amount of water vapor that can be in the air at a given temperature)
2. describes how close the air is to saturation with water vapor
3. relative humidity = (actual water vapor in air/capacity) x 100
4. relative humidity changes
a. if water vapor content changes
b. if temperature changes (water vapor capacity changes):
1) air can be brought to saturation (100% relative humidity) through
a decrease in temperature with no water vapor added
2) cooling is most common way air is brought to saturation point and
condensation
5. Temperature-Relative Humidity Relationship
a. the relationship between temperature and relative humidity is
one of the most important in all of meteorology:
d. as temperature increases, relative humidity decreases
e. as temperature decreases, relative humidity increases
b. on a typical day, relative humidity tends to be lowest in midafternoon and highest in the early morning (Fig 6-9)
E. Related Humidity Concepts
1. Dew Point Temperature
a. dew point (temperature): temperature at which saturation is reached
b. varies with the moisture content of the air
2. Sensible Temperature
a. sensible temperature: temperature as it feels to a person’s body
b. involves
1) actual temperature
2) relative humidity
a) on warm, humid days, the air is near saturation so little
evaporative cooling takes place; sensible temperature is high
b) on cold, humid days, body heat is conducted away more rapidly
because of damp air; sensible temperature is low
3) wind: influences evaporation and the convecting away of body heat
VIII. Focus: Water Vapor Satellite Images
A. satellite water vapor images show regions of dry air and moist air in the
atmosphere
1. longwave radiation of wavelengths 6.7 µm – 7. 3 µm (micrometers) is
readily absorbed and reradiated by water vapor – high emission of
radiation in this band indicates relatively large amounts of water vapor
2. darker gray shades indicate relatively dry air; lighter gray indicates
relatively moist air
Chapter 6: Atmospheric Moisture – p. 6 of 13
IX.
X.
XI.
Condensation
A. condensation: change in state from gas to liquid
1. opposite of evaporation
2. for condensation to take place, air must be saturated
3. usually result of air being cooled to below dew point temperature
B. hygroscopic particles/condensation nuclei: tiny atmospheric particles of
dust, smoke and salt that serve as collection centers for water molecules during
condensation
1. required for condensation
2. most concentrated over cities, seacoasts and volcanoes
C. supercooled water: water that persists in liquid form at temperatures
below freezing; relative humidity > 100%
1. clouds often composed of supercooled water
2. promote growth of ice particles in cold clouds
Adiabatic Processes
A. only way in which large masses of air can be cooled to the dew
point is by expansion as the air masses rise
B. only prominent mechanism for the development of clouds and the
production of rain is adiabatic cooling
C. as air rises its pressure decreases; it expands and cools adiabatically
D. dry adiabatic rate
1. constant
2. non-saturated air
E. lifting condensation level: altitude at which air cools to the dew point,
condensation begins, and clouds form
1. visible as the base of clouds
2. latent heat is released as soon as condensation begins
F. saturated (wet) adiabatic lapse rate: diminished rate of cooling
(because of release of latent heat during condensation) of rising air
above the lifting condensation level
1. varies but is less than the dry rate
2. averages 3.3oF/1,000 ft (6oC/km)
G. descending air warms at the dry adiabatic lapse rate (5.5oF/1,000 ft
-- 10oC/km)
1. causes saturated air to become unsaturated
2. reason descending air does not make clouds
H. adiabatic temperature change only applies to air in vertical motion,
air that either rising or descending (environmental or average lapse
rate pertains to still air)
I. air moving up and over a mountain range: (Fig 6-14)
1. cools adiabatically as it rises on windward side
2. warms adiabatically as it descends on leeward side
3. arrives on leeward side significantly warmer and drier than it started
4. one way in which deserts are formed
Clouds
A. intro
Chapter 6: Atmospheric Moisture – p. 7 of 13
1. clouds: collections of minute droplets of water or tiny crystals of ice
2. visible expression of condensation
3. at any given time 50% of Earth is covered by clouds
4. not all clouds precipitate, but all precipitation comes from clouds
5. important influence on radiant energy
B. Classifying Clouds
3. classified on 2 factors
a. form
b. altitude
4. Cloud Form
a. cirriform clouds: thin wispy clouds formed of ice crystals rather
than water droplets
b. stratiform clouds: grayish sheets that cover most or all of the sky
c. cumuliform clouds: massive, rounded clouds with a flat base and
limited horizontal extent but often billowing upward to great
heights
d. 3 cloud forms are subclassified into 10 types based on shape
1) cirrus, stratus, and cumulus clouds are purely 1 form
2) the other 7 types are combinations
e. nimbus: indicates precipitation clouds
5. Cloud Families
a. cloud families based on altitude
b. high clouds
1) above 20,000’
2) thin, white, composed of ice crystals
3) cirrus, cirrocumulus, and cirrostratus
4) harbingers of approaching weather system or storm
c. middle clouds
1) 6,500 – 20,000’
2) either stratiform or cumuliform
3) composed of liquid water
4) altocumulus – puffy; indicate settled weather
5) altostratus – associated with changing weather
d. low clouds
1) below 6,500’
2) often general overcast
3) stratus, stratocumulus, and nimbostratus
4) associated with drizzly rain
e. clouds of vertical development
1) grow from low bases to heights of 60,000’
2) restricted horizontal spread
3) indicate very active vertical air movement
4) cumulus – indicate fair weather
5) cumulonimbus – storm clouds
C. Fog
1. fog: a cloud on the ground
Chapter 6: Atmospheric Moisture – p. 8 of 13
2. formation:
a. when air at Earth’s surface cools to below its dew point
b. when enough water vapor is added to air to saturate it
3. types of fog
a. radiation fog: produced by condensation near the ground, where air
is cooled to the dew point by contact with the colder ground; usually
at night
b. advection fog: produced by condensation that results when warm,
moist air moves horizontally over a cold surface
1) most common source: air moving from sea to land
c. upslope/orographic fog: condensation that occurs when humid air is
caused to ascend a topographic slope and consequently cools
adiabatically
d. evaporation fog: condensation that results from the addition of water
vapor to cold air that is already near saturation
4. fog distribution in North America
a. areas of heavy fog are mostly coastal
b. radiation fog: western mountains and Appalachians
c. areas of minimal fog
1) Southwest, Mexico, and Great Plains
2) limited available atmospheric moisture and strong winds
D. Dew
1. originates from terrestrial radiation
2. nighttime radiation cools objects  adjacent air cooled by conduction
 air cooled enough to reach saturation
3. if temperature is below freezing, frost is formed
XII.
People and the Environment: Global Dimming
A. atmospheric aerosols
1. influence weather and climate
2. release of sulfate aerosols during 1991 Mount Pinatubo eruption
lowered global temperatures by ~ 0.9o F for a year
B. global dimming
1. anthropogenic (human-released) aerosols
2. global dimming: cooling of Earth’s surface and lower troposphere as
a result of anthropogenic aerosols directly blocking sunlight, absorbing
solar energy high in the atmosphere, and creating more reflective
clouds
3. contrails
a. condensation trails from jet airliners can reflect radiation
b. absence of contrails during three day grounding of all commercial
airline traffic over the US after Sept 11 increased daily temperature
range by > 2oF; days were hotter, nights were cooler
4. paradoxical concern for atmospheric scientists: if human-produced
global dimming has been masking some of the effects of global
warming, if we continue to reduce “global pollutants” we will face
higher temperatures from global warming
Chapter 6: Atmospheric Moisture – p. 9 of 13
XIII.
The Buoyancy of Air
A. Atmospheric Stability
1. buoyancy: tendency of an object to rise in a fluid
a. parcel of air moves vertically until it reaches a level at which the
surrounding air is of equal density
b. warm air is more buoyant than cool air
1) warm, less dense air tends to rise
2) cool, more dense air tends to sink
2. stable air: a parcel of air that resists vertical movement
a. stable air is non-buoyant and thus provides little opportunity for
adiabatic cooling unless there is forced uplift
b. high stability is promoted when there is a temperature inversion –
when cold air is beneath warm air
c. cold winter night is a typical, highly stable situation
d. highly stable air is normally not associated with cloud formation and
precipitation
3. unstable air: air that either rises without any external force other than
the buoyant force or continues to rise after such an external force has
ceased to function
a. unstable air is buoyant
b. a mass of air that heated enough so it is warmer than the
surrounding air is unstable
c. typical condition on a warm summer afternoon
d. unstable air rises until it reaches the equilibrium level, the level at
which the surrounding air has the same temperature and density
4. conditional instability: when the environmental lapse rate is between
the dry and saturated adiabatic rates
a. left alone the parcel acts like stable air
b. forced to rise the parcel might become unstable at the condensation
level
5. Determination of Stability via Temperature and Lapse Rate
a. stable: if the environmental lapse rate of the surrounding air is less
than the dry adiabatic lapse rate of the rising air (Fig 6-23)
1) the rising air is cooler than surrounding air at every elevation
2) air rises only when forced to do so
b. instability: if the environmental lapse rate of the surrounding air is
greater than the dry adiabatic lapse rate of the rising air (Fig 6-24)
1) the rising air is warmer than the surrounding air at every elevation
2) it rises until it reaches an elevation where the surrounding air is of
similar temperature and density
c. if rising air is cooled to its dew point (Fig 6-25)
1) condensation begins
2) latent heat is released so rising air cools at a slower rate – the
saturated adiabatic lapse rate
3) increases tendency toward instability
4) reinforces rising
Chapter 6: Atmospheric Moisture – p. 10 of 13
6. Visual Determination of Stability
a. cumulus clouds suggest instability
b. towering cumulonimbus clouds indicate pronounced instability
c. stratiform clouds characterize stable air forced to rise
d. cloudless skies indicate stable, immobile air
1) although no clouds form unless air is cooled to the dew point
regardless of stability
XIV. Precipitation
A. The Processes
1. intro
a. all precipitation originates in clouds, but most clouds do not yield
precipitation
b. many droplets must join together to form a drop large enough to
overcome turbulence and evaporation and fall to ground under
influence of gravity
2. Collision/Coalescence
a. in warmer clouds
b. rain is produced by the collision and coalescing (merging) of water
droplets
c. condensation allows water molecules to grow large enough to coalesce
then fall and coalesce more as they collide with other droplets
d. accounts for most precipitation in the tropics and some in
midlatitudes
3. Ice-Crystal Formation
a. in high, cold clouds
b. Bergeron process
1) ice crystals grow by attracting water vapor supplied by
evaporating water droplets
2) when large enough the ice crystals fall as snowflakes or melt and
precipitate as raindrops
c. accounts for majority of precipitation outside the tropical regions
B. Forms of Precipitation
1. Rain
a. drops of liquid water
b. most common and widespread form of precipitation
2. Snow
a. solid precipitation in form of ice crystals, small pellets or flakes
b. formed when water vapor is converted directly to ice through
sublimation with no intermediate liquid stage
3. Sleet
a. small raindrops that freeze during descent
4. Glaze
a. rain that turns to ice the instant it collides with a solid object
b. produced by raindrops that are supercooled when they pass through
a shallow layer of subfreezing air near the ground and convert to an
icy surface when they land
Chapter 6: Atmospheric Moisture – p. 11 of 13
5. Hail
a. small pellets or lumps of ice composed of concentric layers of ice
crystals
b. produced in highly unstable air with strong updrafts in cumulonimbus
clouds with above freezing temperature in the lower part and below
freezing temperature in the upper part
c. record hailstones:
1) largest: 7” diameter – Nebraska 2003
2) heaviest: 1.67 pounds – Kansas 1970
XV. Atmospheric Lifting and Precipitation
A. all significant precipitation originates through rising air and adiabatic
cooling - caused by 1 of 4 types of atmospheric lifting (Fig 6-32)
B. Convective Lifting
1. convective precipitation: unequal heating of different surface areas
causes a parcel of air to rise, cool to the dew point, condense and
precipitate
2. characteristics: showery, large drops, fast and furious, short duration
3. associated with warm regions and warm seasons
C. Orographic Lifting
1. orographic precipitation: air forced to rise up a topographic barrier
cools to the dew point, condenses, and precipitates
a. windward side is wet
b. rain shadow: dry leeward slope and dry area beyond
2. characteristics: upslope flow, with prolonged precipitation
3. occurs at any latitude, any season, any time of day – where there is a
topographic barrier and moist air to move over it
D. Frontal Lifting
1. frontal precipitation: warm air forced to rise over cooler air along a
front cools to the dew point, condenses and precipitates
2. characteristic of midlatitudes
a. where cold polar air and warm tropical air collide
b. less common in high latitudes and rare in the tropics
E. Convergent Lifting
1. convergent precipitation: converging air is forced upward because of
crowding, cools to the dew point, condenses and precipitates
2. characteristic of low latitudes
a. ITCZ
b. hurricanes and easterly waves
XVI. Global Distribution of Precipitation (Fig. 6-34)
A. Average Annual Precipitation
1. isohyet: isoline joining points of equal quantities of precipitation
2. Regions of High Annual Precipitation
a. tropical latitudes
1) ITCZ
Chapter 6: Atmospheric Moisture – p. 12 of 13
2) eastern coasts of tropical landmasses where trade winds forced
over topographic barriers – east coast of Central America,
northeastern South America, and Madagascar
3) monsoon areas
b. narrow zones along western coasts of North and South America
between 40o and 60o N/S
1) combines frequent onshore westerly flow, storminess and mountain
barriers
2) north-south trending mountain ranges near coast
a) restrict precipitation to narrow zone
b) produce rain shadows to the east of the mountains
3. Regions of Low Annual Precipitation
a. subtropical latitudes centered at 25o or 30o N and S along
western sides of continents
1) subtropical highs – descending air is warm and dry
2) largest deserts in the world – North Africa and Australia
b. midlatitudes
1) large continental masses – central and southwest Asia which is a
long distance from any moisture source
2) rain shadows in areas of westerly flow – North and South America
c. very high latitudes
1) water surfaces are scare and cold
2) cold deserts
4. because they are much closer to moisture sources, continental
margins (coastal regions) tend to have more precipitation than
interior locations
B. Seasonal Precipitation Patterns (Fig 6-35)
1. in interior areas most of the year’s precipitation occurs during the
summer months
a. summer heating results in greater convective activity
b. anticyclonic conditions in winter
2. coastal areas have a more balanced seasonal precipitation regime
3. global precipitation generalizations
a. seasonal shifting of the high sun causes shifting of major pressure
and wind systems that is reflected in displacement of wet and dry areas
b. summer is time of maximum precipitation over most of the world
1) July in northern hemisphere; January in southern hemisphere
2) exceptions: narrow zone along western coasts between 35 o and
60o latitude which experience summer dryness associated with
the seasonal shift of the subtropical high
c. monsoon regions tend to have very wet summers and generally dry
winters
C. Precipitation Variability
1. expected departure from average precipitation in any given year
2. regions of normally heavy precipitation experience the least variability
Chapter 6: Atmospheric Moisture – p. 13 of 13
3. regions of normally low precipitation experience the most variability; dry
regions experience great fluctuations in precipitation from one year to the next
XVII. Acid Rain
A. acid rain: deposition of either wet or dry acidic materials from the
atmosphere on Earth’s surface
1. sulfuric and nitric acids are principal culprits
2. major human-induced source:
a. sulfur dioxide (SO2) emissions from smokestacks
b. nitrogen oxides (NOx) from motor vehicle exhaust
B. acidity
1. measured on a pH scale
a. logarithmic scale based on relative concentration of hydrogen ions
b. range: 0 – 14 (acidic to alkaline)
2. acid rain: precipitation with pH < 5.6 (normal rain is slightly acidic)
C. surface reaction
1. naturally alkaline soils or bedrock such as limestone neutralizes acid
precipitation
2. granitic soils have no neutralizing components
D. Damage from Acid Precipitation
1. sterilizing aquatic ecosystems
2. forest diebacks
3. destroying buildings and monuments
4. geographic complication: much of the pollution is deposited at great
distances downwind from its source – half of acid rain falling on
Canada in the 1980s came from US sources
5. Title IV of 1990 Clean Air Act Amendments created the Acid Rain
Program → 50% reduction of SO2 from 1980 levels by 2007