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Chapter 6
Stability and Cloud Development
Stability & Cloud Development
This chapter discusses:
1. Definitions and causes of stable and unstable
atmospheric air
2. Processes that cause instability and cloud
development
Stability & Movement
A rock, like a parcel of air, that is in stable equilibrium will return
to its original position when pushed.
If the rock instead departs in the direction of the push, it was in
unstable equilibrium.
Movement & Temperature
Rising air
expands, using
energy to push out,
which slows and
adiabatically cools
the air.
A parcel of air
may be forced to
rise or sink, and
change
temperature
relative to
environmental air,
which is sampled
using radiosonde
balloons.
Dry & Moist Adiabatic Rates
A parcel of rising dry air cools at approximately 10° C/km, while
latent heat released into the parcel by condensing vapor keeps the
moist adiabatic lapse rate nearer to 6° C/km.
Because the environmental air cools more slowly, the parcel of air
held by the helicopter is always cooler and sinks back to its
original position.
Stable Atmospheric Conditions
Cold air below warm air is considered stable because cold air has a
higher density, and when displaced will gravitate back below the
warm, lighter air.
A morning atmosphere has this stable vertical profile, keeping fog
unmixed.
Subsidence Inversion
Cooler surface air is called a temperature inversion, caused by
radiational cooling, cold advection, or subsiding air that shrinks in
total thickness.
As it sinks and shrinks, its top lowers a greater distance and warms
more than the air resting at earth's surface.
Absolutely Unstable Atmosphere
Absolutely unstable
conditions indicate that a
lifted parcel of air,
whether dry or moist,
will be warmer than the
surrounding
environmental air, and
hence continue to rise.
Conditionally Unstable
Conditions are initially stable
when the environmental lapse
rate (LR) is between the dry
and moist adiabatic LR.
Air that is dry is always cooler
and sinks, but when the
parcel's dew point is reached
it cools more slowly, and may
become warmer than the
surrounding air, creating
instability.
Stability Summary
Environmental
temperatures determine
stability for rising parcels
of air.
The atmosphere is
absolutely unstable when
the environmental lapse
rate (ELR) is steeper than
the dry LR, absolutely
stable when the ELR is less
steep than the moist LR,
and conditionally unstable
otherwise.
Heat as an Instability Trigger
As the environmental lapse rate
(ELR) steepens (ie, drops more
rapidly with height), it becomes
more unstable.
Heating air below, through
contact with a hot surface or
fire, will cause the ELR to
steepen.
Cooling air aloft by clouds
radiating energy or cold
advection will also increase
instability.
Mixing as an Instability Trigger
Winds that mix
upper and lower
atmospheric air
will steepen the
environmental
lapse rate.
Wind (forced
convection) will
warm descending
and lower air
while cooling
rising and upper
air.
Layer of Air Instability
Lifting a stable layer of air causes it to stretch out vertically, with
the top section experiencing a greater elevation change and more
cooling than the lower section, even when both cool at the dry
adiabatic rate.
Convective Instability & Storms
Lifting a stable layer of
air that is saturated
below and unsaturated
aloft will cause the
moist air to cool more
slowly, and create a very
steep and unstable
environmental lapse
rate.
Severe thunderstorms
are associated with such
processes.
Convection & Clouds
Pockets of warm
air rise as
thermals with
invisible water
vapor, and at the
dew point
temperature
condensation
creates the cloud
base.
Rising air from
below is replaced
by sinking air
from above,
creating areas of
blue sky.
Cumulus Clouds & Clear Sky
Development of a Cumulus Cloud
The panels reveal vertical trends for temperature and moisture
during cloud development.
The level of free convection is where rising air becomes warmer
than environmental air.
Stability & Cloud Height
Environmental
lapse rates (ELR)
determine the
stability, and as the
time of day
changes the ELR,
the cloud profile
can change.
Cumulus to Cumulonimbus
Topography & Clouds
Winds blowing moist air toward a mountain will experience
orographic uplift to an elevation where dew point is reached and
clouds are formed.
When the condensed moisture falls as rainfall, the leeward side of
the mountain is kept in a rain shadow.
Mountain Wave Clouds
As moist air condenses into clouds on the windward side of the
mountain, sinking and swirling winds on the leeward side shape
a lenticular mountain wave cloud.
Changing Cloud Forms
Uniform winds may
align a large region
of cirrocumulus and
stratocumulus clouds
into rows of cloud
streets.
Billow “KH” & Castellanus Clouds
Rapid changes in wind speed with elevation can create
wavelike billows at the cloud top, while conditionally unstable
air above a cloud may cause lifting and condensation in the
shape of castles.
Mixing & Stratocumulus
When air is stable and close to saturation, then forced wind mixing
of the atmosphere:
a) steepens the environmental lapse rate
b) moves moist low-level air aloft
The cooler and moister air aloft then condenses into a deck of
clouds.