Download Chapter 4: Moisture and Atmospheric Stability

Chapter 4
Moisture and Atmospheric Stability
This chapter covers:
 Water
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states of matter
heat capacity and latent heat
Humidity and dew point
Adiabatic temperature changes in the
atmosphere
Atmospheric stability
The Hydrologic Cycle
States of Matter: Solid, Liquid and Vapor
Water’s Changes of State
Gas (Vapor)
 widely
spaced molecules
 no bonding between molecules
 molecules move at high speeds
 very compressible
Liquid
 Closely
spaced molecules
 Moderate bonding between
molecules
 molecules move at medium
speeds
 Slightly compressible
Solid (i.e., ice)
 closely
spaced molecules
 Strong, rigid bonding between
molecules
 No molecule movement – only
vibrations
 Fairly incompressible
Solid Water: Ice
Liquid Water
Water Vapor
Heat Capacity and Latent Heat of Water
Saturation
Condition in which the air is holding
the maximum amount of water
vapor possible
 Amount of water vapor present at
saturation depends on

Temperature; more vapor at higher
temp. Very strong effect
 Pressure; more vapor at higher
pressure.
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Absolute Humidity
 Amount
of water vapor present in
air
 Given as grams water vapor per
cubic meter of air
 Value is affected by air pressure
Mixing Ratio
 Amount
of water vapor present
in air, but more useful than
absolute humidity
 Given as grams water vapor per
kilogram of air
 Typically ranges from 0 to 4%
 Value is not affected by air
pressure
Saturation
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Air is limited in how much water
vapor it can hold without water
droplets forming
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Saturation is the point at which air
can’t hold more water vapor

Mixing ratio at saturation depends on
temperature, and somewhat on
pressure
Contrail: engine exhaust contains water
vapor, exhaust cools, becomes saturated
with water vapor and condensation occurs
Contrail: engine exhaust contains water
vapor, exhaust cools, becomes saturated
with water vapor and condensation occurs
Saturation Mixing-Ratio:
How much water vapor can be present in air
at different temperatures
Relative Humidity
the humidity we feel
Amount of water vapor in air
relative to maximum possible
amount (saturation mixing ratio)
Example
Temperature: 20oC
Saturation mixing ratio=14g vapor per 1 kg air
Actual vapor content = 7 g per 1 kg air
Relative humidity = 7 g / 14 g x 100% = 50%
Dew Point
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Temperature to which air must
be cooled to become saturated
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Assumes no change in mixing ratio
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Relative humidity is 100% in air
that’s at its dew point
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Stating air’s dew point is
essentially the same as stating its
mixing ratio
Air’s saturation mixing ratio and relative
humidity change with temperature
Which has
larger mixing
ratio?
Death Valley
Antarctica
Which has
higher relative
humidity?
Hotter:
Higher mixing
ratio,
Lower relative
humidity
Colder:
Lower mixing
ratio
Higher relative
humidity
Relative Humidity, Mixing Ratio
and Air Temperature

Hotter air can hold much more
water vapor than cold air
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Hotter air can have more vapor in it
than cold air, yet have lower
relative humidity
Temperature
Relative Humidity
Relative Humidity Changes with
Temperature Daily
Air Temp, Dew Pt. & Relative
Humidity in Heber
Dew Point Temperatures
Adiabatic Temperature Changes
• Air cools when it expands, warms
when its compressed
• Rising air expands and cools
• Sinking air is compressed and warms
• Adiabatic refers to temperature
changes w/o heat transfer
Very important!
Adiabatic Temperature Changes
Dry & Wet Adiabatic Rates
• Saturated air cools less as it rises
because condensation of water
releases heat
• Dry adiabatic rate = 10oC /
1000m = 5.5oF / 1000 feet
• Wet adiabatic rate = 5 to
9oC / 1000m (2.75 to 5oF/1000ft)
Dry & Wet Adiabatic Rates
Lifting Condensation Level

As air rises, it expands and cools
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Level (altitude) at which it is cooled
to its dew point is the lifting
condensation level
– Clouds form
above this level
if air is rising
Causes of Lifting
Orographic – wind blows over
mountains
 Frontal wedging – warm air forced
over colder air
 Convergence – winds blowing
together
 Convection – solar heating creates
hot air that rises
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Orographic Lifting
Important along Wasatch Front, much of Western U.S.
Frontal Wedging
“Storm Fronts”
Convergence
Convection
Cause of Rain Shadow Desert
Atmospheric Stability
Stable Air = Air that tends to not
rise
 Unstable Air = Air that tends to
keep rising (regardless of orographics,
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fronts, etc.)
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Importance – rising air cools,
makes clouds, precipitation, even
tornados
What Controls Stability

Depends on adiabatic cooling
rate (dry and wet) vs.
Environmental Lapse Rate

Environmental Lapse Rate = the
actual, existing decrease in air
temperature with altitude
Atmospheric Stability, cont.
 Three
types of stability:
 Absolute stability
 Absolute
instability
 Conditional
instability
Absolute Stability
 Environmental
Lapse rate is
less than wet adiabatic rate

As air rises, it cools so much (even if
its saturated) that it becomes cooler
than surrounding air so it stops
rising
Absolute stability
Absolute instability
 Environmental
lapse rate is
greater than dry adiabatic
rate
 As
air rises, despite cooling at
dry adiabatic rate, it becomes
progressively warmer than
surrounding air and rises
faster
Absolute instability
Absolute Instability
Conditional Instability
 Environmental
Lapse rate is
greater than wet adiabatic rate,
less than dry adiabatic rate

As air rises, if it is unsaturated it
tends to not rise, but once its
saturated it keeps rising
Conditional Instability
Conditional Stability
NWS Storm Prediction Center
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Focuses on dangerous thunderstorms
Produces estimates of convective stability for
locations across the country twice daily
Main website: http://www.spc.noaa.gov
Soundings (weather balloon data which provide
information on environmental lapse rate and
more) with stability analysis (somewhat advanced
scientifically):
http://www.spc.noaa.gov/exper/soundings/
Chapter 4
END