Cloud Development and Precipitation

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Chapter 5

• Cloud Development and Precipitation

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Equlibrium
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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

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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 10C per 1km or 5.5F
  per 1,000 feet. (applies to unsaturated air)
• Moist Adiabatic lapse rate - 6C per 1km or
  3.3F per 1,000 ft (applies to saturated air).
  Not a constant. Varies greatly. This number is
  used to keep things simple.

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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.

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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.

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Stable Air
Dry air example
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Stable Air
Saturated air example
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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.
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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

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Unstable Air
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Dry air example
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Unstable air. The forest fire heats the air,
causing instability near the surface. Warm,
less-dense air (and smoke) bubbles upward,
expanding and cooling as it rises.
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Conditionally unstable air.The atmosphere is
stable if the rising air is unsaturated...
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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)

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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
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Cumulus clouds developing into thunderstorms in a
conditionally unstable atmosphere over the Great
Plains. (Note the anvil in the distance)
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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.

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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.

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Thermals forming cumulus clouds
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Four primary means of convection(ways to form
clouds)

• Surface heating (thermals)
• Topographic (forced) lifting
• Convergence at the surface
• Frontal (forced) lifting

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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.

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Orographic lift, cloud development, and the
formation of a rain shadow
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The air's stability greatly influences the growth
of cumulus clouds.
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The air's stability greatly influences the growth
of cumulus clouds.
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The air's stability greatly influences the growth
of cumulus clouds.
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The formation of lenticular clouds
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Lenticular clouds (mountain wave clouds) over
Mount Shasta in Northern California
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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)

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Relative sizes of raindrops, cloud droplets and
condensation nuclei
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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.

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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.

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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.

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Factors in cloud formation and raindrop production

• The clouds 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

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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)

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Supercooled water
Collison coalescence occurs here
The distribution of ice and water in a
cumulonimbus cloud.
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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

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Saturation Vapor PressureIce 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.

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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.

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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.

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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.

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Aggregation

• As the crystals fall, they may collide and stick
  to one another forming an aggregate of crystals
  called a snowflake.

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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 its
  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.

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Natural seeding by cirrus clouds may form bands
of precipitation downwind of a mountain chain.
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Precipitation Types

• Rain
• Drizzle
• Virga
• Showers
• Snow
• Snow grains and snow pellets
• Fallstreaks
• Flurries
• Squalls
• Blizzard
• Sleet and Freezing Rain
• Hail

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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

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Virga
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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.

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Fallstreaks
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Dendrite snowflakes most common form of snow.
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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.

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Sleet forms when a partially melted snowflake or
a cold raindrop freezes into a pellet of ice
before reaching the ground.
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Rime -An accumulation of rime forms on tree
branches as supercooled fog droplets freeze on
contact in the below-freezing air.
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A heavy coating of freezing rain during this ice
storm caused tree limbs to break and power lines
to sag.
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Vertical temperature profiles (solid red line)
associated with snow.
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Vertical temperature profiles (solid red line)
associated with sleet.
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Vertical temperature profiles (solid red line)
associated with freezing rain.
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Vertical temperature profiles (solid red line)
associated with rain.
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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.

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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.
   

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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.
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The giant Coffeyville hailstone first cut then
photographed under regular light... September
1970 weighed 1.67 lbs.
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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.

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Standard Rain Gauge

• A nonrecording rain gauge with an 8 inch diameter
  collector funnel and a tube that amplifies
  rainfall by ten.

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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.

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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

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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
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Doppler radar display showing 1-hour rainfall
amounts over Oklahoma for April 24, 1999.
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