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AMS Weather Studies
Introduction to Atmospheric Science, 5th Edition
Chapter 6
Humidity, Saturation &
Stability
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Driving Question
 How does the cycling of water in the Earth-atmosphere system
help maintain a habitable planet?
 This chapter covers:
 The global water cycle
 Transfer processes between Earth’s surface and the atmosphere
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Water content of air
Monitoring water vapor
How air becomes saturated
Atmospheric Stability
Lifting Processes
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Case-in-Point
Atmospheric Rivers
 Atmospheric Rivers (AR)
 Narrow band of concentrated
water vapor transport in the lower
atmosphere
 Responsible for most of the
horizontal flow of water outside
of the tropics
 Especially in the Pacific coast states
 Play important role in fresh water supply
 30-50% of the average annual precipitation for the West Coast states
 Responsible for more than 90% of the global north/south
transport of water vapor
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Case-in-Point
Atmospheric Rivers
 Atmospheric Rivers
 ARs that affect Pacific Coast states form, move and develop with
winter storms in the North Pacific Ocean
 Warm humid air flows poleward ahead of the cold front; water vapor is
concentrated into narrow ribbons in the warm sector.
 ARs occasionally dip southward and entrain moisture and heat directly from
the Pacific subtropics and tropics.
 Pineapple Express – originates near Hawaii, flows toward the northeast, and
makes landfall along California coast
 Greatest flood potential exists where AR encounters coastal/inland
mountain ranges (orographic lifting)
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Global Water Cycle
 Amount of water in Earth-atmosphere system neither increases
or decreases
 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
 Various reservoirs store water
 Mostly the ocean (97.2%), ice sheets and glaciers (2.15%)
 Sun powers the global water cycle, gravity stops water from
escaping to space
 Water falls from the sky as precipitation and flow to oceans.
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Where is the Water Stored?
Note the small
percentage of the
total water stored in
the atmosphere.
It is vital to weather
processes
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The Global Water Cycle
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The Global Water Cycle
Water vapor image showing long range transport.
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The Global Water Cycle
 Transfer Processes
 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 taken up by plant roots escapes as vapor from
plant pores
 Evapotranspiration – 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.
 Precipitation
 Rain, drizzle, snow, ice pellets, hail9
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The Global Water Cycle
The percentage of annual precipitation over land that originally vaporized
from the ocean, averaged over 15 years.
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The Global Water Cycle
Via precipitation and evaporation, the ocean has a net
loss of water and the land has a net gain.
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The Global Water Cycle
Pathways taken by
precipitation after
falling on the surface.
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How Humid is it?
 Humidity describes amount of water vapor in the air
 Varies within a year, from day-to-day, within a single day, and from
place-to-place
 Humid summer air and dry winter air cause discomfort
 Measuring humidity
 Vapor pressure, mixing ratio, specific humidity, absolute humidity,
relative humidity, dewpoint, precipitable water
 Vapor pressure
 Water vapor disperses among the air molecules and contributes to
the total atmospheric pressure
 Pressure component is vapor pressure
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How Humid is it?
 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
 Absolute humidity
 The mass of the water vapor per unit volume of humid air
 Normally expressed as grams of14water vapor per cubic meter of air© AMS
How Humid is it?
 Saturated air
 Air at its maximum humidity
 Dynamic equilibrium develops when liquid water becomes vapor at
the same rate as vapor becomes liquid
 “Saturation” added to various humidity terms
 Saturation vapor pressure, saturation mixing ratio, saturation specific
humidity, saturation absolute humidity
 Changing the air temperature disturbs equilibrium
 Example: Heating water increases kinetic energy of water molecules,
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 temperature.
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How Humid is it?
Variation in saturation mixing ratio with
changing air temperature (at 1000 mb).
Variation in saturation vapor pressure with
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changing air temperature.
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How Humid is it?
 Relative humidity
 Most familiar measure
 Compares amount of water vapor present to amount that would be
present if 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 vapor
pressure (or mixing ratio)
 If the amount of water vapor in the air remains constant, relative
humidity varies inversely with temperature (next slide)
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How Humid is it?
The vapor pressure varies only slightly through the day so the relative
humidity varies inversely with temperature; the relative humidity increases
as the temperature drops and decreases as the temperature rises.
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How Humid is it?
 Dewpoint
 Temperature to which air must cool, at
constant pressure, to reach saturation
 At dewpoint, air reaches 100% relative humidity
 Greater concentration of water vapor, then
higher dewpoint
 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
– frost points20
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How Humid is it?
Average surface dewpoint for July.
Average surface dewpoint for January.
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How Humid is it?
 Precipitable water
 Depth if all water vapor in a vertical column was condensed into liquid
 Condensing all the atmosphere’s water vapor would produce a layer of
water covering the entire Earth’s surface 2.5 cm (1.0 in.) deep
 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 infrared beam
 The mirror temperature reaches a point that condensation forms,
reflectivity of the mirror changes, altering the reflection of the
beam: dewpoint
 Common at NWS forecast stations
 Hair hygrometer
 Relates changes in length of a humid hair to humidity
 Hair lengthens as relative humidity increases
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Monitoring Water Vapor
 Humidity Instruments
 Hygrograph
 Provides a record of humidity variations over time
 Electronic hygrometer
 Based on changes in resistance of certain chemicals as they absorb or
release water vapor to the air
The temperature/dewpoint
sensor (hygrothermometer)
used in the NWS’s 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
recorded
 Dry bulb – actual air temperature
 Water vapor vaporizes from the wick as it is whirled and evaporated cooling
lowers the temperature of the wet-bulb temperature
 Important to remember – use the depression of the wet bulb on the chart
 The difference between the wet and dry bulb temperatures
 Aspirated psychrometers do the same, but use a fan instead of whirling
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The difference between the dry-bulb temperature and the wet-bulb
temperature, known as the web bulb depression, is calibrated as a
percentage of 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
Long-distance transport of water vapor in a
plume off the Pacific and through Mexico.
Hurricane Irene shown off the East Coast.
 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); whiter the image, 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 forms 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 added at a constant temperature
 Vapor pressure increases while saturation vapor pressure remains constant
 As ascending saturated air (RH ~100%) expands and cools,
 Saturation mixing ratio and actual mixing ratio decline, some water vapor
converted to water droplets or ice crystals
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How Air Becomes Saturated
 Adiabatic process (Chap 5)
 No heat is exchanged between the air parcel and environment
 Expansional cooling and compressional heating of unsaturated air
referred to as adiabatic processes if no heat is exchanged with
surroundings
 Air cools adiabatically as it rises
 Lower pressure with altitude allows air to expand
 Unsaturated ascending air cools at 9.8° C/1000 m (5.5° F/1000 ft), it
warms at the same rate upon descent. (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 lower because latent heat released upon condensation partially
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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
 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
 Causes parcel to sink back to original altitude
 In unstable air, an ascending parcel becomes warmer and less
dense than the surrounding air
 Causes the parcel to continue rising.
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Atmospheric Stability
 Stable air
 Movement of parcel up means
it is colder than surrounding air
 Sinks down to original altitude
 Movement down means it is
warmer than surrounding air
 Rises to its original altitude
 Stable air inhibits vertical
motion
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Atmospheric Stability
 Unstable air
 Movement of parcel upward
means it is warmer than the
surrounding air
 Continues rising
 Movement of the parcel
downward, becomes colder
than the surrounding air
 Continues descending
 Unstable air enhances
vertical motion
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Atmospheric Stability
 Soundings
 Temperature profiles of ambient air through which air parcels move
 Soundings (and 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
 Soundings
 Absolute instability
 Air temperature dropping more rapidly with altitude than
dry adiabatic lapse rate (9.8° C/1000 m)
 Conditional instability
 Air temperature 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 stable for unsaturated air parcels and unstable for
saturated air parcels
 Implies unsaturated air must be forced upward to reach
saturation
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Atmospheric Stability
 Absolute stability
 Air layer stable for both unsaturated and saturated air parcels
 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 up or down motion of air parcels
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Air stability is determined by
comparing the temperature
(density) of an ascending air
parcel with the temperature
(density) of the surrounding
air (sounding).
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Atmospheric Stability
A Stüve diagram, with temperature on the horizontal axis, increasing
from left to right, and pressure on the vertical axis, decreasing upward.
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Atmospheric Stability
An unsaturated air
parcel at point A is
subject to a dry
adiabatic
expansion to
point B (850 mb)
and then to point
C (700 mb).
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Atmospheric Stability
A saturated air
parcel at point D
(700 mb) is subject
to a moist adiabatic
expansion to point E
(500 mb) and then
to a dry adiabatic
compression to
point F (700 mb).
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Lifting Processes
 Convection Current
 Updraft and a downdraft
 Cumulus clouds form where
air ascends
 Surrounding sky is cloud-free
where air descends
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Lifting Processes
 Frontal uplift
 When contrasting air masses meet, leads to expansional cooling of
rising air, possible cloud and precipitation development
 Warm front – as a cold and dry air mass retreats, 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, 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
 Oragraphic lifting
 Air is forced upward by topography
 Ascend above hills, descend into
valleys
 Expansional cooling (windward
slope) and compressional warming
(leeward slope) of air affects clouds
and precipitation development
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Lifting Processes
Mean annual
precipitation
(1971-2000) in
the Pacific
Northwest.
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Lifting Processes
 Convergent Lifting
 When surface winds converge, associated upward motion
leads to
 Expansional cooling, increasing relative humidity, possible
cloud and precipitation formation
 Example: converging winds are largely responsible for
cloudiness and precipitation in a low-pressure system
 Example: converging sea breezes contribute to high
frequency of thunderstorms in central Florida
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