Chapter - Valdosta State University

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Chapter 4
Moisture and Atmospheric Stability
Steam Fog over a Lake
The Hydrologic Cycle
Moisture-related Terms
• Transpiration
– The release of water vapor into the atmosphere
by plants
• Latent Heat
– Energy absorbed or released to change the state
of water
– Measured in Calories
• A calorie is the amount of heat needed to raise the
temperature of 1g of water by 1°C
States of water
States of Water
• Ice
– Frozen
• Liquid Water
– Liquid at room temperature
• Water Vapor
– Gas
More Moisture Terms
• Evaporation
– The phase change from liquid to gas
– Endothermic / absorbs latent heat
• Condensation
– The phase change from gas to liquid
– Exothermic / latent heat is released
• Sublimation
– Phase change of solid directly to gas
• Deposition
– Phase change of gas directly to solid
• Frost is an example of deposition
Water’s Changes of State
Water’s Changes of State
Condensation of Water Vapor
Generates Fog
Humidity – Water Vapor in Air
• Humidity
– The general term for the amount of water vapor
in the air
• Absolute Humidity
– Mass of water vapor in a given volume of air
• Mass of water vapor (g) / volume of air (m3)
• Mixing Ratio
– Mass of water vapor in a unit of air compared
to the remaining mass of dry air
• Mass of water vapor (g) / mass of dry air (kg)
Is the
Content of
Water Vapor
in the Air
Vapor Pressure and Saturation
• Vapor Pressure
– The part of total atmospheric pressure
attributable to its water vapor content
• Saturation
– Balance between evaporation and condensation
• Saturation Vapor Pressure
– Pressure of water vapor in a saturated
Vapor Pressure
Varies with
Relative Humidity
• Relative Humidity
– Ratio of the air’s actual water vapor content
compared with the amount of water vapor
needed for saturation at that temperature and
Relative Humidity
Changes with Added Moisture
Saturation Mixing-Ratio
For every 10°C increase in temperatures, the saturation vapor pressure doubles
Changes with Temperature
Higher Temperature
Lower Relative Humidity
with MORE Moisture!
Lower Temperature
Higher Relative Humidity
with LESS moisture!
Daily Changes
Relative Humidity
Dewpoint temperature
• Dewpoint
– The temperature to
which a parcel of air
needs to be cooled to in
order to reach
Sling Psychrometer
to the
Condensation on Cold Drinking-glasses
Dew Point Temperatures
Dew Point Temperatures
Adiabatic Temperature Changes
• The Basis of Cloud Formation
• Adiabatic Temperature Changes mean no
heat added or subtracted
– When air expands, it cools
– When air is compressed it warms
– Example: Pumping up a tire
• Air compressed in tire causes it to warm
• Escaping air is cool
Adiabatic Temperature Changes
Adiabatic Cooling
• Parcel
– A volume of air
• Dry Adiabatic Rate
– Unsaturated air / 10°C per 1000 m (1km)
• Lifting Condensation Level
– Parcel reaches saturation / condensation begins
• Wet Adiabatic Rate
– Latent heat absorbed from the evaporation
processed is released at the LCL due to
condensation– this reduces the adiabatic rate /
BTW 5°C and 9°C per 1000 m (1km)
Dry & Wet Adiabatic Rates
Processes that lift air
• Orographic Lifting
– Air is forced to rise over mountains
• Frontal Wedging
– Warmer, less dense air is forced over cooler,
denser air
• Convergence
– A “pile-up” of horizontal air flow results in
upward movement
• Localized Convective Lifting
– Unequal surface heating causes small parcels to
rise due to buoyancy
Lifting Processes
Heavy Precipitation in Mountains
Snow Pack in the Rocky Mountains
Rain Shadow Desert
• When air climbs over a mountain, most of
the moisture is lost as precipitation on the
windward side
• The air descends on the leeward side
• As it descends, it compresses, warms, and
becomes very dry
• Very little precipitation occurs in the “Rain
Death Valley
Rain Shadow Desert
and Leeward
Rain Shadows
Rain Shadow Deserts
• On the windward side of the Sierra Nevada
in California, giant sequoias and Douglas
firs are found
• On the leeward side, you can find Death
• In addition, fast moving windward systems
can produce downslope warm dry winds
• These winds are often called “Chinook” and
can warm the adjacent areas by 10°C (18°F)
during winter
Frontal Wedging
• Masses of warm and cold air collide,
producing a front
• Warmer, less dense, air rises over colder,
denser, air
Frontal Wedging
• When air flows in from more than one
• Air ascends, cools, and forms clouds
• Florida provides an excellent example of
Localized Convective Lifting
• Unequal heating causes some places to be
warmed more than other places, i.e.,
parking lot versus wooded area
• Parcel of air that is heated will rise – these
parcels are often called thermals
• Birds and hang gliders ride on thermals
• This is called localized convective lifting
As long as
in a
than the
surrounding air,
it will rise
Atmospheric Stability
• If an air parcel is cooler than the
surrounding environment, it tends to sink,
and does not rise – called stable air
• If an air parcel is warmer than the
surrounding environment, it tends to rise –
called unstable air / like a hot air balloon
• How high does the parcel rise? Until its
temperature is the same as the surrounding
Environmental Lapse Rate
• The actual temperature of the atmosphere at
any height in the atmosphere, based on
• Adiabatic changes are based on a parcel of
air moving vertically in the atmosphere –
does not include horizontal movement or
• Air that rises, that is cooler than the
surrounding environment, will sink if
allowed to
Absolute Stability
• The environmental lapse rate is less than the
wet adiabatic lapse rate
• If temperature increases with altitude, an
inversion exists and conditions are
relatively stable
Atmospheric Stability
Absolute Stability
Absolute Instability
• The environmental lapse rate is greater than
the dry adiabatic lapse rate
• The ascending parcel of air is always
warmer than its environment
• Often occurs in summer and warmer months
• Generally confined to the first few km of the
Absolute Instability
Conditional Instability
• Moist air has an ELR between the wet and
dry adiabatic rates
• The atmosphere is stable in respect to an
unsaturated parcel of air, but unstable in
respect to a saturated parcel of air
Conditional Stability
Stability and Weather
• Days with low clouds and light precipitation
probably involve stable air forced aloft
• Days with puffy clouds, such as in summer,
likely result from unstable conditions
• Unstable conditions
– Intense solar heating, air masses heated from below,
orographic lifting, fronts, and convergence
• Stable conditions
– Radiative cooling of earth’s surface after dark,
An Unstable Atmosphere
Sinking air is called subsidence
Associated with High Pressure systems
Usually associated with blue cloudless skies
More subsidence-warming occurs aloft than
at the surface
Chapter 4
Humidity and Moisture
Next – Condensation and Precipitation