Transcript Lecture Packet#5

Chapter 5
Cloud Development and
Precipitation
Equlibrium
Atmospheric Stability
• Air is in stable equilibrium when after
being lifted or lowered, it tends to return to
its original position – resists upward and
downward air motions.
• Air Parcel- balloon like blob of air
• As air rises its pressure decreases and it
expands and cools
• As air sinks pressure increases and it is
compressed and warms
Adiabatic Process
• If an air parcel expands and cools, or
compresses and warms, with no interchange of
heat with its outside surroundings the situation is
called an adiabatic process.
• Dry Adiabatic lapse rate – 10˚C per 1km or
5.5˚F per 1,000 feet. (applies to unsaturated air)
• Moist Adiabatic lapse rate - ~6˚C per 1km or
3.3˚F per 1,000 ft (applies to saturated air). Not a constant.
Varies greatly. This number is used to keep things simple.
Determining Stability
• Determine stability by comparing the temperature of a
rising parcel to that of its surrounding environment.
• If it is colder than its environment it will be more dense
(heavier) and tend to sink back to its original level. This
is called stable air because the parcel resists moving
away from its original position.
• If the parcel is warmer (less dense) than its environment,
it will continue to rise until it reaches the same
temperature of its environment. This is called unstable
air because the parcel continues to move away from its
original position.
Stable Air
• Environmental Lapse Rate – rate at which the air
temperature of the environment would be changing if we
were to climb upward into the atmosphere.
• Absolutely stable – the lifted parcel of air is colder and
heavier than air surrounding it (its environment).
• Stable air strongly resists upward vertical motion, it will, if
forced to rise, tend to spread out horizontally.
• Atmosphere is stable when the environmental lapse rate
is small – when there is relatively small difference in
temperature between the surface air and the air aloft.
• The atmosphere stabilizes as the air aloft warms or as
the air near the surface cools.
Stable Air
Dry air example
Stable Air
Saturated air example
Cold surface air, on this morning, produces a stable atmosphere
that inhibits vertical air motions and allows fog and haze to linger
close to the ground.
Unstable Air
• Atmosphere is unstable when the air
temperature decreases rapidly as we move up
into the atmosphere.
• Absolutely unstable atmosphere – when
considering both moist and dry air – the rising air
is warmer than the environmental air around
them.
• Atmosphere becomes unstable when:
– Daytime solar heating of the surface
– An influx of warm air brought in by the wind near the
surface
– Air moving over a warm surface
Unstable Air
-
Dry air example
Unstable air. The forest fire
heats the air, causing instability
near the surface. Warm, lessdense air (and smoke) bubbles
upward, expanding and cooling
as it rises.
Conditionally unstable air.
The atmosphere is stable
if the rising air is
unsaturated...
Conditionally Unstable Air
• Suppose an unsaturated, but humid air parcel is forced to rise from
the surface.
• As it rises, it expands and cools at the dry adiabatic rate until it cools
to its dew point.
• The elevation above the surface where the air is saturated and
clouds form is called the condensation level.
• Above the condensation level rising air cools at the moist adiabatic
rate.
• Conditionally unstable atmosphere – the condition for stability being
where (if anywhere) the rising air becomes saturated. If unsaturated
stable air is lifted to a level where it becomes saturated, instability
may result.
(See text figure 5.7 on page 116)
When the environmental
lapse rate is greater than the
dry adiabatic rate, the
atmosphere is absolutely
unstable. When the
environmental lapse rate is
less than the moist adiabatic
rate, the atmosphere is
absolutely stable. And when
the environmental lapse rate
lies between the dry adiabatic
rate and the moist adiabatic
rate (shaded green area), the
atmosphere is conditionally
unstable
Cumulus clouds developing into thunderstorms in a conditionally unstable
atmosphere over the Great Plains. (Note the anvil in the distance)
Level of free convection
• The level of the atmosphere where an air
parcel, after being lifted, becomes warmer
than the environment surrounding it. This
air can then rise on its own and the
atmosphere is unstable.
Convection and Clouds
• Some areas of the earth surface absorb more
sunlight than others, and thus heat up more
quickly. (Discuss examples)
• Thermal – a hot bubble of air that breaks away
from the surface and rises, expanding and
cooling as it ascends.
• As a thermal rises, it mixes with cooler, drier air
aloft and gradually looses its identity. But, if it
cools to its saturation point, the moisture inside
will condense and the thermal becomes a
cumulus cloud.
Thermals forming cumulus clouds
Four primary means of convection
(ways to form clouds)
•
•
•
•
Surface heating (thermals)
Topographic (forced) lifting
Convergence at the surface
Frontal (forced) lifting
Topography and Clouds
• Orographic lift – forced lifting along a topographic barrier
(mountains)
• Rain Shadow – the region on the leeward side of a
mountain, where precipitation is noticeably low and the
air if often drier
• Lenticular clouds – (mountain wave clouds) form on the
lee side of mountains. Resemble waves that form in a
river downstream from a large boulder.
• Rotor clouds – Form beneath lenticular clouds. In the
large swirling eddy associated with the mountain wave,
the rising part may cool and condense enough to form a
cloud.
Orographic lift, cloud development, and the formation of a rain shadow
The air's stability greatly influences the growth of cumulus clouds.
The air's stability greatly influences the growth of cumulus clouds.
The air's stability greatly influences the growth of cumulus clouds.
The formation of lenticular clouds
Lenticular clouds (mountain wave clouds) over Mount Shasta in Northern California
Collision and Coalescence Process
• In clouds with tops warmer than -15oC collisions
between droplets can play a significant role in
producing precipitation.
• Large drops form on large condensation nuclei
or through random collisions of droplets.
• As the droplets fall (larger drops fall faster than
smaller drops) the larger droplets overtake and
collide with smaller drops in their path.
• The merging of cloud droplets by collision is
called coalescence. (Note: collision does not always
guarantee coalescence)
Relative sizes of raindrops, cloud droplets and condensation nuclei
Collision and Coalescence
• In a warm cloud
composed only of small
cloud droplets of uniform
size, the droplets are less
likely to collide as they all
fall very slowly at about
the same speed. Those
droplets that do collide,
frequently do not
coalesce because of the
strong surface tension
that holds together each
tiny droplet.
Collision and Coalescence
• In a cloud composed of
different size droplets,
larger droplets fall faster
than smaller droplets.
Although some tiny
droplets are swept aside,
some collect on the larger
droplet's forward edge,
while others (captured in
the wake of the larger
droplet) coalesce on the
droplet's backside.
Warm Clouds
• A cloud droplet rising
then falling through a
warm cumulus cloud
can grow by collision
and coalescence, and
emerge from the
cloud as a large
raindrop.
Factors in cloud formation and
raindrop production
• The cloud’s liquid water content
• The range of droplets sizes
• The cloud thickness
(heaviest precipitation occurs in those
clouds with most vertical development)
• The updrafts of the cloud
• The electric charge of the droplets and the
electric field in the cloud
Ice Crystal (Bergeron) Process
• Process of rain formation proposes that
both ice crystals and liquid cloud droplets
must co-exist in clouds at temperatures
below freezing.
• This process is extremely important to rain
formation in the middle and high latitudes
where cloud tops extend above the
freezing level (cold clouds)
Supercooled water
Collison coalescence occurs here
The distribution of ice and water in a cumulonimbus cloud.
Ice Nuclei
• Ice-forming particles that exist in
subfreezing air
• Small amount of these available in
atmosphere
• Clay materials, bacteria in decaying plant
leaf material and other ice crystals
Saturation Vapor Pressure
Ice vs Water
• In a saturated
environment, the water
droplet and the ice crystal
are in equilibrium, as the
number of molecules
leaving the surface of
each droplet and ice
crystal equals the number
returning. The greater
number of vapor
molecules above the
liquid indicates, however,
that the saturation vapor
pressure over water is
greater than it is over ice.
Ice Crystal (Bergeron) Process
•
•
•
The ice-crystal process. The
greater number of water vapor
molecules around the liquid
droplets causes water molecules
to diffuse from the liquid drops
toward the ice crystals. The ice
crystals absorb the water vapor
and grow larger, while the water
droplets grow smaller.
It takes more vapor molecules to
saturate the air directly above the
water droplet than it does to
saturate the air directly above the
crystal.
Ice crystals grow at the expense
of the surrounding water droplets.
Accretion
• In some clouds ice
crystals might collide with
supercooled liquid
droplets. Upon contact,
the liquid droplets freeze
into ice and stick to the
ice crystal – accretion or
riming.
• The icy matter that forms
is called graupel or snow
pellets.
Secondary Ice particles
• In colder clouds the
ice crystals may
collide with other ice
crystals and fracture
into smaller ice
particles or tiny seeds
which freeze
hundreds of
supercooled droplets
on contact.
Aggregation
• As the crystals fall,
they may collide and
stick to one another
forming an aggregate
of crystals called a
snowflake.
Cloud Seeding
• To inject (or seed) a cloud with small particles
that will act as nuclei, so that the cloud particles
will grow large enough to fall to the surface as
precipitation.
• First experiments in late 1940s using dry ice.
• Silver Iodide is also used today because it’s
structure is similar to that of ice crystals.
• Natural seeding – cirriform clouds lie directly
above a lower cloud deck, ice crystals descend
into lower clouds.
Natural seeding by cirrus clouds may form bands of precipitation
downwind of a mountain chain.
Precipitation Types
• Rain
– Drizzle
– Virga
– Showers
• Snow
–
–
–
–
–
Snow grains and snow pellets
Fallstreaks
Flurries
Squalls
Blizzard
• Sleet and Freezing Rain
• Hail
Rain
• Falling drop of liquid water that has a diameter
equal to or greater than .5 mm (.02 in)
• Drizzle – drops too small to qualify as rain
• Virga – raindrops that fall from a cloud but
evaporate before reaching the ground
• Shower – intermittent precipitation from a
cumuliform cloud usually of short duration but
often heavy intensity
• Acid rain – rain that is mixed with gaseous
pollutants (sulfur, nitrogen) and becomes acidic
Virga
Snow
• A solid form of precipitation composed of ice crystals in
complex hexagonal form
• Much of the precipitation reaching the ground actually
begins as snow.
• Fallstreaks – Ice crystals and snowflakes falling from
high cirrus clouds. Behave similar to Virga – fall into
drier air and disappear before reaching the ground.
Change from ice to vapor (sublimation)
• Flurries – light snow showers that fall intermittently for
short durations. Light accumulation.
• Squall – more intense snow shower, brief but heavy
snowfall.
• Blizzard – severe weather condition. Low temperatures
and strong winds (greater than 30 kts) bearing a great
amount of falling or blowing snow.
Fallstreaks
Dendrite snowflakes – most common form of snow.
Sleet and Freezing Rain
• Sleet – type of precipitation consisting of
transparent pellets of ice 5 mm or less in
diameter (ice pellets)
• Freezing Rain/drizzle – rain/drizzle that falls in
liquid form and then freezes upon striking a cold
object or ground. (glaze)
• Rime – an accumulation of white or milky
granular ice. Formed when supercooled cloud
or fog droplets strike an object whose
temperature is below freezing.
Sleet forms when a partially melted snowflake or a cold raindrop freezes
into a pellet of ice before reaching the ground.
Rime -An accumulation of rime forms on tree branches as supercooled
fog droplets freeze on contact in the below-freezing air.
A heavy coating of freezing rain during this ice storm caused tree limbs to
break and power lines to sag.
Vertical temperature profiles
(solid red line) associated
with snow.
Vertical temperature
profiles (solid red line)
associated with sleet.
Vertical temperature profiles
(solid red line) associated with
freezing rain.
Vertical temperature
profiles (solid red line)
associated with rain.
Hail
• Hailstones are pieces of ice either transparent or
partially opaque, ranging in size from that of a
small pea to that of a golf ball or larger.
• Produced in cumulonimbus clouds when
graupel, large frozen raindrops or just about
anything (insects) act as embryos that grow by
accumulating supercooled liquid water droplets.
• Golf ball size hail has remained aloft for between
5 and 10 minutes.
Hailstones
• Hailstones begin as embryos
(usually ice particles) that
remain suspended in the cloud
by violent updrafts. When the
updrafts are tilted, the ice
particles are swept horizontally
through the cloud, producing
the optimal trajectory for
hailstone growth. Along their
path, the ice particles collide
with supercooled liquid
droplets, which freeze on
contact. The ice particles
eventually grow large enough
and heavy enough to fall
toward the ground as
hailstones.
The accumulation of small hail after a thunderstorm. The hail formed as
supercooled cloud droplets collected on ice particles called graupel inside a
cumulonimbus cloud.
The giant Coffeyville hailstone first cut then photographed under regular light...
September 1970 weighed 1.67 lbs.
Measuring Precipitation
• Rain gauge – instrument used to collect and
measure rainfall.
• Trace – an amount of precipitation less than .01
in
• Snow depth – determined by measuring in three
or more representative areas and taking an
average.
• Water equivalent – generally about 10 inches of
snow will melt down to about 1 inch of water.
Varies greatly and depends on texture and
packing of snow.
Standard Rain Gauge
• A nonrecording rain
gauge with an 8 inch
diameter collector
funnel and a tube that
amplifies rainfall by
ten.
Tipping Bucket Rain Gauge
• The tipping bucket
rain gauge. Each time
the bucket fills with
one-hundredth of an
inch of rain, it tips,
sending an electric
signal to the remote
recorder.
Doppler Radar
• Radar – radio detection and ranging
• Used to examine the inside of clouds
• Doppler Radar – a radar that determines the
velocity of falling precipitation either toward or
away from the radar unit by taking into account
the Doppler shift
• Doppler shift (effect) the change of frequency of
waves that occurs when the emitter or the
observer is moving toward or away from the
other
Doppler radar display showing precipitation intensity over Oklahoma for April
24, 1999. The numbers under the letters DBZ represent the logarithmic
scale for measuring the size and volume of precipitation particles
Doppler radar display showing 1-hour rainfall amounts over Oklahoma for
April 24, 1999.