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AMS Weather Studies
Introduction to Atmospheric Science, 4th Edition
Chapter 6
Humidity, Saturation,
and Stability
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Case-in-Point
 Cloud forests are forests that are perpetually
shrouded in clouds or mist
– They are found from 2000-3000 m (6500-9800 ft) in
elevation in the tropics and subtropics
 Onshore and upslope winds that are warm and
humid supply the moisture
– Warm air blowing upslope cools through expansion
– Expansional cooling raises the relative humidity to
saturation and water vapor condenses into low clouds
and fog
– The tree canopy strips moisture from the clouds and this
water drips to the forest floor
 Deforestation reduces available moisture and
raises air temperature → clouds form less readily
and at higher elevations
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Case-in-Point
 If global warming translates into higher sea surface
temperatures (SSTs) in the tropics, cloud forests could be
affected
– Air flowing onshore would be warmer
– Greater ascents would be required to
produce clouds
– Clouds would be at higher elevations
 Perhaps even lift off the mountains
– Cloud forests are extremely sensitive
to climate variations
 They may prove to be early indicators
of effects of global-scale climate change
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Driving Question
 How does the cycling of water in the Earthatmosphere system help maintain a
habitable planet?
– This chapter will tell us:
 How the global water cycle functions
– Especially as it relates to transference between
the Earth’s surface and the atmosphere
 How to quantify the water content of air
 How air becomes saturated through uplift and
expansional cooling
 How atmospheric stability affects the ascent of
air
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Global Water Cycle
 Assumption – the amount of water in the Earthatmosphere system is neither increasing or
decreasing
– Internal processes continually generate and break down
water molecules
 Volcanoes and meteors (minute amount) add water
 Photodissociation of water vapor and chemical reactions break
down water molecules
– Fixed quantity of water in Earth-atmosphere system is
distributed in 3 phases among various reservoirs, mostly
the ocean (97.2%) and ice sheets and glaciers (2.15%)
– The sun powers the global water cycle and gravity
keeps water from escaping to space, causing water to
fall from the sky as precipitation and flow to oceans
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The Global Water Cycle
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Where is the Water Stored?
Note the small
percentage of the
total water that is
stored in the
atmosphere.
Even though small
in percentage, this is
vital to weather
processes
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Water vapor image showing long range transport
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The Global Water Cycle
 Transfer processes
1. Phase changes
 Evaporation – more molecules enter the atmosphere as vapor
then return as liquid to the water surface
 Condensation – more molecules return to the water surface as
liquid then enter the atmosphere as vapor
 Transpiration – Water that is taken up by plant roots escapes as
vapor from plant pores
– Evapotranspiration is the total of evaporation and
transpiration
 Sublimation – ice or snow become vapor without first becoming
liquid
 Deposition - water vapor becomes solid without first becoming
liquid
 All 3 phases of water exist in the atmosphere
2. Precipitation
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 Rain, drizzle, snow, ice pellets, and hail
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Percent of Precipitation Originating
from Land Sources
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Ocean evaporation is the origin of most precipitation.
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Pathways Taken by
Precipitation Falling
on Land
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The Global Water Budget
Via precipitation and evaporation, the ocean has a net
loss of water and the land has a net gain.
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How Humid is it?
 Humidity describes the amount of water vapor in
the air
– This varies with time of year, from day-to-day, within a
single day, and from place-to-place
– Humid summer air, and dry winter air cause discomfort
 Ways of measuring humidity:
–
–
–
–
–
–
–
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Vapor pressure
Mixing ratio
Specific humidity
Absolute humidity
Relative humidity
Dewpoint
Precipitable water
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How Humid is it?
 Vapor pressure
– Water vapor disperses among the air molecules and
contributes to the total atmospheric pressure
 This pressure component is called the vapor pressure
 Mixing ratio
– Mass of water vapor per mass of the remaining dry air
 Expressed as grams of water vapor per kilograms of dry air
 Specific humidity
– Mass of the water vapor (in grams) per mass of the air
containing the vapor (in kilograms)
 In this case, the mass of the air includes the mass of the water
vapor
 Mixing ratio and specific humidity are so close
they are usually considered equivalent
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How Humid is it?
 Absolute humidity
– The mass of the water vapor per unit volume of humid
air; normally expressed as grams of water vapor per
cubic meter of air
 Saturated air
– This is the term given to air at its maximum humidity
– A dynamic equilibrium develops where the liquid water
becomes vapor at the same rate as vapor becomes
liquid
– “Saturation” may be added to various humidity terms
 Saturation vapor pressure, saturation mixing ratio, saturation
specific humidity, saturation absolute humidity
– Changing the air temperature disturbs equilibrium
temporarily
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 Example: heating water increases kinetic energy of water
molecules and they more readily escape the water surface as
vapor. If the supply of water is sufficient, a new dynamic
equilibrium is established with more vapor at higher temp.
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Variations with Air Temperature of:
Vapor Pressure
Saturation Mixing Ratio
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How Humid is it?
 Relative humidity
– Probably the most familiar measure
– Compares the amount of water vapor present to the
amount that would be present if the air were saturated
– Relative humidity (RH) can be computed from vapor
pressure or mixing ratio
 RH = [(vapor pressure)/ (saturation vapor pressure)] x 100
 RH = [(mixing ratio)/(saturation mixing ratio)] x 100
– At constant temperature and pressure, RH varies directly
with the vapor pressure (or mixing ratio)
– If the amount of water vapor in the air remains constant,
relative humidity varies inversely with temperature
 See next slide
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The Relationship of Relative
Humidity to Temperature
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How Humid is it?
 Dewpoint
– The temperature to which the air must be cooled
at constant pressure to reach saturation
 At the dewpoint, air reaches 100% relative
humidity
 Higher with greater concentration of water vapor in
air
 With high relative humidity, the dewpoint is closer
to the current temperature than with low relative
humidity
– Dew is small drops of water that form on
surfaces by condensation of water vapor
– If the dewpoint is below freezing, frost may form
on the colder surfaces through deposition
 Dewpoints below freezing are sometimes referred
to as frostpoints
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How Humid is it?
 Precipitable water
– The depth of the water
that would be produced if
all the water vapor in a
vertical column of
condensed into liquid
water
 Condensing all the water
vapor in the atmosphere
would produce a layer of
water covering the entire
Earth’s surface to a depth of
2.5 cm (1.0 in.)
– Highest in the tropics
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Monitoring Water Vapor
 Humidity instruments
– Hygrometer
 Measures the water vapor concentration of air
– Dewpoint hygrometer
 Uses a temperature-controlled mirror and an infrared beam
– When the mirror temperature reaches a point that
condensation forms, the reflectivity of the mirror is changed,
altering the reflection of the beam. The temperature is
recorded as the dewpoint.
 These are common at NWS forecast stations
– Hair hygrometer
 Relates changes in length of a humid hair to humidity – hair
lengthens as relative humidity increases
– Hygrograph
 Provides a record of humidity variations over time
– Electronic hygrometer
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 Based on changes in resistance of certain chemicals as they
absorb or release water vapor to the air
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Monitoring Water Vapor
The temperature/dewpoint sensor
(hygrothermometer) used in the NWS Automated
Surface Observing System (ASOS)
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Monitoring Water Vapor
 Sling psychrometer
– Wick is wetted in distilled water
– Instrument is ventilated by whirling
– Wet-bulb and dry-bulb temperatures are
recorded
– Dry bulb – actual air temperature
– Water vapor vaporizes from the wick as it
is whirled and evaporated cooling lowers
the temp. to the wet-bulb temperature
– Important to remember – use the
depression of the wet bulb on the chart
 This is the difference between the wet
and dry bulb temperatures
 Aspirated psychrometers do the same
thing, but use a fan instead whirling
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The difference between the dry-bulb temperature and the wet-bulb
temperature, known as the wet bulb depression, is calibrated in
terms of percentage relative humidity on a psychrometric table.
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The dewpoint can be obtained from measurements of the dry-bulb
temperature and the wet-bulb depression.
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Monitoring Water Vapor
 Water vapor satellite
imagery
– IR imagery using
infrared wavelengths
that detect water vapor
Water vapor imagery indicates
presence of water vapor above
3000 m (10,000 ft) The whiter
the image, the greater the
moisture content of the air
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How Air Becomes Saturated
 As relative humidity nears 100%, condensation or
deposition becomes more likely
 Condensation or deposition will form clouds
– Clouds are liquid and/or ice particles
 Humidity increases when:
– Air is cooled; saturation vapor pressure decreases while
actual vapor pressure remains constant
– Water vapor is added at a constant temperature; vapor
pressure increases while saturation vapor pressure
remains constant
 As ascending saturated air (RH about 100%)
expands and cools, saturation mixing ratio and
actual mixing ratio decline and some water vapor
is converted to water droplets or ice crystals
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How Air Becomes Saturated
 Adiabatic process and lapse rates (review from
Chapter 5)
– During an adiabatic process, no heat is exchanged
between the air parcel and its environment
– Expansional cooling and compressional heating of
unsaturated air are referred to as adiabatic processes if
no heat is exchanged with surroundings
– Air cools adiabatically as it rises
 Lower pressure with altitude allows the air to expand
 Unsaturated ascending air cools at 9.8° C/1000 m (5.5° F/1000 ft)
and it warms at the same rate upon descent.
– This is called the dry adiabatic lapse rate
– Upon saturation, air continues to cool, but at the
moist adiabatic lapse rate of 6° C/1000 m (3.3° F/1000
ft) → rate is lower because latent heat released upon
condensation partially offsets cooling as parcel rises
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Atmospheric Stability
 Air parcels are subject to buoyant forces caused
by density differences between the surrounding air
and the parcel itself
 Atmospheric stability is the property of ambient air
that either enhances (unstable) or suppresses
(stable) vertical motion of air parcels
– In stable air, an ascending parcel becomes cooler and
more dense than the surrounding air
 This causes the parcel to sink back to its original altitude
– In unstable air, an ascending parcel becomes warmer
and less dense than the surrounding air
 This causes the parcel to continue rising
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Stable Air
 Note that movement of the
parcel upward means it is
colder than the
surrounding air, so it sinks
back down to its original
altitude
 Also, in movement of the
parcel downward, it
becomes warmer than the
surrounding air, and
returns to its original
altitude
 Stable air inhibits vertical
motion
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Unstable Air
 Note that movement of the
parcel upward means it is
warmer than the
surrounding air, so it
continues rising.
 Also, in movement of the
parcel downward, it
becomes colder than the
surrounding air, and
continues descending
 Unstable air enhances
vertical motion
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Atmospheric Stability
 Soundings
– These are the temperature profiles of the ambient air
through which air parcels are moving
– Soundings (and hence stability) can change due to:
 Local radiational heating and cooling
– At night, cold ground cools and stabilizes the overlying air
– During day, warm ground warms and destabilizes the overlying air
 Air mass advection
– Air mass is stabilized as it moves over a colder surface
– Air mass is destabilized as it moves over a warmer surface
 Large-scale ascent or descent of air
– Subsiding air generally becomes more stable
– Rising air generally becomes less stable
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Atmospheric Stability
 Absolute instability
– Occurs when the air temperature is dropping more
rapidly with altitude than the dry adiabatic lapse rate
(9.8° C/1000 m)
 Conditional instability
– Occurs when the air temperature is dropping with
altitude more rapidly than the moist adiabatic lapse rate
(6° C/1000 m), but less rapidly than the dry adiabatic
lapse rate
– Air layer is stable for unsaturated air parcels and
unstable for saturated air parcels
– Implies that unsaturated air must be forced upwards in
order to reach saturation
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Atmospheric Stability
 Absolute stability
– Air layer is stable for both unsaturated and saturated air
parcels and occurs when:
 Temperature of ambient air drops more slowly with altitude than
moist adiabatic lapse rate
 Temperature does not change with altitude (isothermal)
 Temperature increase with altitude (inversion)
 Neutral air layer
– Rising or descending parcel always has same
temperature as ambient air
– Neither impedes nor spurs upward or downward motion
of air parcels
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Stüve Diagrams
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Temperature – Horizontal axis, increasing from left to right
Pressure – vertical axis, decreasing upward
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Lifting Processes - Convective
Lifting
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Lifting Processes - Frontal Lifting
 Frontal uplift occurs where contrasting air masses
meet – leads to expansional cooling of rising air,
and possible cloud and precipitation development
 Warm front – as a cold and dry air mass retreats,
the warm air advances by riding up and over the
cold air
– The leading edge of advancing warm air at the Earth’s
surface is the warm front
 Cold front – cold and dry air displaces warm and
humid air by sliding under it and forcing the warm
air upwards
– The leading edge of advancing cold air at the Earth’s
surface is the cold front
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Lifting Processes – Orographic Lifting
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Lifting Processes – Convergent Lifting
 When surface winds converge, associated upward
motion leads to expansional cooling, increasing
relative humidity, and possible cloud and
precipitation formation
 For example, converging winds are largely
responsible for cloudiness and precipitation in a
low-pressure system
 In another example, converging sea breezes
contribute to high frequency of thunderstorms in
central Florida
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