Stability & Cloud Development This chapter discusses: 1.Definitions and causes of stable and...
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Stability & Cloud Development This chapter discusses: 1.Definitions and causes of stable and unstable atmospheric air 2.Processes that cause instability
Stability & Cloud Development This chapter discusses:
1.Definitions and causes of stable and unstable atmospheric air
2.Processes that cause instability and cloud development
Slide 2
Stability & Movement A rock, like a parcel of air, that is
in stable equilibrium will return to its original position when
pushed.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.If the rock instead departs in the direction of the
push, it was in unstable equilibrium.
Slide 3
Key Terms When Discussing Stability Adiabatic Process - is when
an air parcel cools by expansion or warms by compression with no
exchange of heat from the the outside environment. Dry Adiabatic
Rate - the rate at which an unsaturated parcel is cooled or warmed
adiabatically (adiabatic process). The dry adiabatic rate is 10C
per 1000 m or 5.5F per 1000 ft and it remains constant. Moist
Adiabatic Rate - the rate at which a saturated parcel is cooled and
warms with ascending or descending motion. This rate varies but it
is less than the dry adiabatic rate due to latent heating from
condensation offsetting the cooling. A commonly used value for the
moist adiabatic rate is 6C per 1000 m or 3.3F per 1000 ft. This
rate is not an adiabatic process due to latent heating.
Environmental Lapse Rate - the rate at which ambient air
temperature decreases with height. This rate can vary as well and
must be measured by a radiosonde.
Slide 4
Close to the DALR, why?
Slide 5
Behavior of Rising and Sinking Air Rising air expands, using
energy to push out, which slows and adiabatically cools the
air.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 the environmental air,
which is sampled using radiosonde balloons.A parcel of air may be
forced to rise or sink, and change temperature relative to the
environmental air, which is sampled using radiosonde balloons. The
radiosonde balloon expands in size from approximately 6 feet to a
diameter between 24 and 32 ft before it bursts. The balloon carries
the instrument package to an altitude of approximately 25 mi (27-37
km) where the balloon bursts (at a pressure of approximately 10
mb).The radiosonde balloon expands in size from approximately 6
feet to a diameter between 24 and 32 ft before it bursts. The
balloon carries the instrument package to an altitude of
approximately 25 mi (27-37 km) where the balloon bursts (at a
pressure of approximately 10 mb). = Exerted Pressure
Slide 6
Fun Facts on Radiosondes Radiosondes contain sensors that
measure profiles of pressure, temperature, and relative
humidity.Radiosondes contain sensors that measure profiles of
pressure, temperature, and relative humidity. The balloon rises at
about 300 meters/minute (about 1,000 feet/minute).The balloon rises
at about 300 meters/minute (about 1,000 feet/minute). the balloon
expands in size from approximately 6 feet to a diameter 20+ ft
before it bursts.the balloon expands in size from approximately 6
feet to a diameter 20+ ft before it bursts. A small parachute slows
the descent of the radiosonde, minimizing the danger to lives and
property.A small parachute slows the descent of the radiosonde,
minimizing the danger to lives and property. The radiosonde flight
can last in excess of two hours, and during this time the
radiosonde can ascend to over 35 km (about 115,000 feet) and drift
more than 200 km (about 125 miles) from the release point.The
radiosonde flight can last in excess of two hours, and during this
time the radiosonde can ascend to over 35 km (about 115,000 feet)
and drift more than 200 km (about 125 miles) from the release
point. Less than 20 percent of the approximately 75,000 radiosondes
released by the NWS each year are found and returned to the NWS for
reconditioning.Less than 20 percent of the approximately 75,000
radiosondes released by the NWS each year are found and returned to
the NWS for reconditioning. All observations are usually taken at
the same time each day at 0000 and 1200 UTC, 365 days a year.All
observations are usually taken at the same time each day at 0000
and 1200 UTC, 365 days a year. Observations are made by the NWS at
92 stations - 69 in the contiguous United States, 13 in Alaska, 9
in the Pacific, and 1 in Puerto Rico.Observations are made by the
NWS at 92 stations - 69 in the contiguous United States, 13 in
Alaska, 9 in the Pacific, and 1 in Puerto Rico.
Slide 7
Dry & Moist Adiabatic Rates A parcel of rising dry air
cools at faster rate, while latent heat released from the saturated
parcel by condensing vapor keeps the environmental temperature
closer to the moist adiabatic rate. Due to the environmental air
cooling rate being slower, the parcel of air held by the helicopter
is always cooler and sinks back to its original position. This
example depicts an absolutely stable atmosphere. An absolutely
stable atmosphere occurs when the environmental lapse rate is less
than the moist adiabatic lapse rate.
Slide 8
Stable Atmospheric Conditions When cold air lies underneath
warm air, this setup is considered to be a stable atmosphere When
cold air lies underneath warm air, this setup is considered to be a
stable atmosphere because cold air has a higher density, and
because cold air has a higher density, and when displaced will
gravitate back below the warm, lighter, air. when displaced will
gravitate back below the warm, lighter, air. A morning atmosphere
has this stable vertical profile, keeping fog unmixed. A morning
atmosphere has this stable vertical profile, keeping fog unmixed.
Cooling of the surface air can be caused by: Cooling of the surface
air can be caused by: Nighttime radiational cooling of the surface
Nighttime radiational cooling of the surface An influx of cold
surface air brought in by wind (Cold Air Advection; CAA) An influx
of cold surface air brought in by wind (Cold Air Advection; CAA)
Air moving over a cold surface Air moving over a cold surface
Sinking of an entire layer, not just a parcel (subsidence
inversion) Sinking of an entire layer, not just a parcel
(subsidence inversion)
Slide 9
Subsidence Inversion As a layer sinks and shrinks by
compression of the atmosphere above it, its top lowers a greater
distance and warms more than the air at the bottom layer.As a layer
sinks and shrinks by compression of the atmosphere above it, its
top lowers a greater distance and warms more than the air at the
bottom layer. Subsidence inversion forms as air slowly sinks over a
large area.Subsidence inversion forms as air slowly sinks over a
large area. This phenomenon occurs most offer aloft.This phenomenon
occurs most offer aloft. It is associated with a large high
pressure area.It is associated with a large high pressure
area.
Slide 10
Neutral Stability Occurs when the environmental lapse rate is
equal to the dry adiabatic lapse rate in an unsaturated
parcel.Occurs when the environmental lapse rate is equal to the dry
adiabatic lapse rate in an unsaturated parcel. Occurs when the
environmental lapse rate is equal to the moist adiabatic lapse rate
in a saturated parcel.Occurs when the environmental lapse rate is
equal to the moist adiabatic lapse rate in a saturated parcel.
Therefore, in neutrally stable environment, air parcels will
neither rise nor sink.Therefore, in neutrally stable environment,
air parcels will neither rise nor sink.
Slide 11
Absolutely Unstable Atmosphere An absolutely unstable condition
indicates that a lifted parcel of air, either dry or moist, is
warmer (or less dense) than the surrounding environmental air, and
therefore continues to rise. An absolutely unstable condition
indicates that a lifted parcel of air, either dry or moist, is
warmer (or less dense) than the surrounding environmental air, and
therefore continues to rise. Since the dry and moist adiabatic
rates remain fairly constant in the troposphere, thus the greater
the environmental lapse rate (> 10C per 1000 m), the more
unstable the atmosphere will be. Since the dry and moist adiabatic
rates remain fairly constant in the troposphere, thus the greater
the environmental lapse rate (> 10C per 1000 m), the more
unstable the atmosphere will be. This example depicts an absolutely
unstable atmosphere. This example depicts an absolutely unstable
atmosphere.
Slide 12
Conditionally Unstable Conditional instability occurs when the
environmental lapse rate (ELR) is between the dry and moist
adiabatic ELR.Conditional instability occurs when the environmental
lapse rate (ELR) is between the dry and moist adiabatic ELR. 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.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.
Slide 13
Conditionally Unstable Atmosphere: Suppose an unsaturated air
parcel is somehow forced to rise from the surface, what would
happen? The level at which air cools to its dew point and RH = 100%
The level of free convection Rising air is colder than its
surroundings and must be forced up 30 20 10 00 -10 From the surface
to 4000 m, the atmosphere is said to be conditionally unstable
Slide 14
Conditional Instability (conditionally unstable atmosphere)
Unsaturated stable air is lifted to a level at which it becomes
saturated, instability may be released. (depends on if the rising
air is saturated or not) Exists whenever the environmental lapse
rate is between the dry and moist adiabatic rates. The atmosphere
is normally in a state of conditional instability since the average
lapse rate in the troposphere is approximately 6.5 C per 1000 m.
Stability does not remain constant but rather changes during the
day by: Daytime warming Nighttime (radiational) cooling
Slide 15
Stability Summary Environmental temperatures determine
stability for rising parcels of air.Environmental temperatures
determine stability for rising parcels of air. The atmosphere
is:The atmosphere is: absolutely unstable when the environmental
lapse rate (ELR) is steeper than the dry adiabatic lapse rate
absolutely unstable when the environmental lapse rate (ELR) is
steeper than the dry adiabatic lapse rate absolutely stable when
the ELR is less steep than the moist adiabatic lapse rate, and
absolutely stable when the ELR is less steep than the moist
adiabatic lapse rate, and conditionally unstable
otherwise.conditionally unstable otherwise.
Slide 16
Developing Instability, How? - must increase the ELR Cooling of
the air aloft may be due to: Wind bring in colder air aloft (CAA)
Clouds (or the air) emitting IR to space (radiational cooling)
Warming of the surface air may be due to: Daytime solar heating of
the surface An influx of warming air brought in by the wind (WAA)
Air moving over a warm surface When the environmental lapse rate
exceeds the dry adiabatic rate, this lapse rate is called
superadiabatic lapse rate.
Slide 17
Mixing as Instability Trigger Winds that mix upper and lower
atmospheric air by convection or mechanical mixing will increase
the environmental lapse rate.Winds that mix upper and lower
atmospheric air by convection or mechanical mixing will increase
the environmental lapse rate. As a result of mixing, the top layer
cools and bottom layer warms creating an increase in the ELR.As a
result of mixing, the top layer cools and bottom layer warms
creating an increase in the ELR.
Slide 18
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 levels cool at the dry adiabatic rate causing an
instability of the whole layer. 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 levels cool at the dry adiabatic rate
causing an instability of the whole layer. This layer instability
is called convective instability. This layer instability is called
convective instability.
Slide 19
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 unstable
environmental lapse rate.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 unstable environmental lapse
rate. Severe thunderstorms are associated with such
processes.Severe thunderstorms are associated with such processes.
Convective vs. Conditional Instability:Convective vs. Conditional
Instability: Conditional instability deals with lifting of a parcel
and the release of instabilityConditional instability deals with
lifting of a parcel and the release of instability Convective
instability refers to a layer being lifted and instability
releasing as the result.Convective instability refers to a layer
being lifted and instability releasing as the result.
Slide 20
Cloud Development Triggers (How air can be forced upward?)
1.Convection 2.Orographic uplift (topography) 3.Convergence of air
4.Lifting along weather fronts
Slide 21
Slide 22
Convection & Clouds Pockets of warm air rise as thermals
with water vapors. Pockets of warm air rise as thermals with water
vapors. Once the thermals reach its dew point temperature,
condensation occurs and creates a cloud base. Once the thermals
reach its dew point temperature, condensation occurs and creates a
cloud base. This cloud base is often referred to as the
condensation level or the lifting condensation level (LCL).This
cloud base is often referred to as the condensation level or the
lifting condensation level (LCL). Rising air from below is replaced
by sinking air from above, creating areas of blue sky. Rising air
from below is replaced by sinking air from above, creating areas of
blue sky.
Slide 23
Development of Cumulus Cloud The panels reveal vertical trends
for temperature and moisture during cloud development.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.The level of free convection
is where rising air becomes warmer than environmental air.
Slide 24
Sat Mix Ratio Sat Mix Ratio Ambient Temp Ambient Temp Dry
Adiabat Dry Adiabat Moist Adiabat Moist Adiabat Pressure Pressure
Skew T-log P diagram is used to depict thermodynamic
characteristics of the atmosphere.Skew T-log P diagram is used to
depict thermodynamic characteristics of the atmosphere. Lifting
condensations level (LCL) is the level at which a parcel of moist
air lifted dry adiabatically would become saturated.Lifting
condensations level (LCL) is the level at which a parcel of moist
air lifted dry adiabatically would become saturated.
Slide 25
Sat Mix Ratio Sat Mix Ratio Ambient Temp Ambient Temp Dry
Adiabat Dry Adiabat Moist Adiabat Moist Adiabat Pressure Pressure
Find the lifting condensation level (LCL) at Denver: If the temp is
59 F (15 C) and dew pt is 39 F (4 C). If the temp is 59 F (15 C)
and dew pt is 39 F (4 C). Recall Denver is 5280 ft above MSL
LCL
Slide 26
Sat Mix Ratio Sat Mix Ratio Ambient Temp Ambient Temp Dry
Adiabat Dry Adiabat Moist Adiabat Moist Adiabat Pressure Pressure
LCL Find the lifting condensation level (LCL) at Lubbock, TX: If
the temp is 86 F (30 C) and dew pt is 41 F (5 C) Lubbock is about
3000 ft above MSL
Slide 27
Stability & Cloud Height Environmental lapse rates (ELR)
determine the stability, and as the time of day changes the ELR and
cloud profile can change.
Slide 28
Topography & Clouds Semi-moist air parcels traveling toward
a mountain experience orographic lift to an elevation where the dew
point is eventually reached by the parcels and clouds are
formed.Semi-moist air parcels traveling toward a mountain
experience orographic lift to an elevation where the dew point is
eventually reached by the parcels and clouds are formed. As the
condensed moisture falls as rainfall on the windward side of the
mountain, the leeward side is kept in a rain shadow due to the
warming of the air parcels as they descends down the mountain.As
the condensed moisture falls as rainfall on the windward side of
the mountain, the leeward side is kept in a rain shadow due to the
warming of the air parcels as they descends down the mountain.
Slide 29
Mountain Wave Clouds As moist air condenses into clouds on the
windward side of the mountain, sinking and swirling winds on the
leeward side create and shape lenticular mountain wave clouds.As
moist air condenses into clouds on the windward side of the
mountain, sinking and swirling winds on the leeward side create and
shape lenticular mountain wave clouds. They appears motionless from
the ground.They appears motionless from the ground. Extremely
Turbulent
Slide 30
Changing Cloud Forms Uniform winds may align a large region of
cirrocumulus or stratocumulus clouds into rows of cloud
streets.Uniform winds may align a large region of cirrocumulus or
stratocumulus clouds into rows of cloud streets. Sometimes refer to
as street clouds.Sometimes refer to as street clouds. Can be the
result of CAA aloft.Can be the result of CAA aloft.
Slide 31
Billow & Castellanus Clouds Rapid changes in wind speed
with elevation in a stable environment can create wavelike billows
clouds.Rapid changes in wind speed with elevation in a stable
environment can create wavelike billows clouds. Conditionally
unstable air above a cloud may cause lifting and condensation in
the shape of castles.Conditionally unstable air above a cloud may
cause lifting and condensation in the shape of castles. Altocumulus
castellanus observed in the morning is often a good precursor to
thunderstorm activities.Altocumulus castellanus observed in the
morning is often a good precursor to thunderstorm activities.
Slide 32
Mixing & Stratocumulus When air is stable and close to
saturation, then forced wind mixing by the atmosphere:When air is
stable and close to saturation, then forced wind mixing by the
atmosphere: a) increases the environmental lapse rate b) moves
moist low-level air aloft The cooler and moister air aloft then
condenses into a deck of clouds. The cooler and moister air aloft
then condenses into a deck of clouds.