AMS Weather Studies

AMS Weather Studies
Introduction to Atmospheric Science, 5th Edition
Chapter 6
Humidity, Saturation &
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
Water content of air
Monitoring water vapor
How air becomes saturated
Atmospheric Stability
Lifting Processes
Atmospheric Rivers
 Atmospheric Rivers (AR)
 Narrow band of concentrated
water vapor transport in the lower
 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
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)
Global Water Cycle
 Amount of water in Earth-atmosphere system neither increases
or decreases
 Internal processes continually generate and break down water
 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.
Where is the Water Stored?
Note the small
percentage of the
total water stored in
the atmosphere.
It is vital to weather
The Global Water Cycle
The Global Water Cycle
Water vapor image showing long range transport.
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
The Global Water Cycle
The percentage of annual precipitation over land that originally vaporized
from the ocean, averaged over 15 years.
The Global Water Cycle
Via precipitation and evaporation, the ocean has a net
loss of water and the land has a net gain.
The Global Water Cycle
Pathways taken by
precipitation after
falling on the surface.
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
 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
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.
How Humid is it?
Variation in saturation mixing ratio with
changing air temperature (at 1000 mb).
Variation in saturation vapor pressure with
changing air temperature.
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)
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.
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
 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
How Humid is it?
Average surface dewpoint for July.
Average surface dewpoint for January.
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
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
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.
Monitoring Water Vapor
 Sling psychrometer
 Wick is wetted in distilled water
 Instrument is ventilated by whirling
 Wet-bulb and dry-bulb temperatures
 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
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.
The dewpoint can be obtained from measurements of the
dry-bulb temperature and the wet-bulb depression.
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
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
 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
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
 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
offsets cooling as parcel rises
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.
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
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
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
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
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
Air stability is determined by
comparing the temperature
(density) of an ascending air
parcel with the temperature
(density) of the surrounding
air (sounding).
Atmospheric Stability
A Stüve diagram, with temperature on the horizontal axis, increasing
from left to right, and pressure on the vertical axis, decreasing upward.
Atmospheric Stability
An unsaturated air
parcel at point A is
subject to a dry
expansion to
point B (850 mb)
and then to point
C (700 mb).
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).
Lifting Processes
 Convection Current
 Updraft and a downdraft
 Cumulus clouds form where
air ascends
 Surrounding sky is cloud-free
where air descends
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
Lifting Processes
 Oragraphic lifting
 Air is forced upward by topography
 Ascend above hills, descend into
 Expansional cooling (windward
slope) and compressional warming
(leeward slope) of air affects clouds
and precipitation development
Lifting Processes
Mean annual
(1971-2000) in
the Pacific
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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