Clouds

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Clouds

• EAS 211
• Spring 2005
• 02/07/05

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Clouds and their Classifications

• In order to have cloud formation, we need to lift
  air.
• There are four ways to do so
• Orographic Uplift
• Frontal Lift
• Convergence
• Localized Convection

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Orographic Uplift

• Air is forced to rise over a mountainous barrier.
• This causes adiabatic cooling and leads to
  condensation on the WINDWARD side of the
  mountain.
• As air begins to descend on the LEEWARD side of
  the mountain, it compress and warms to create a
  rain shadow (an area of little cloud cover and
  low precipitation).
• Example Eastern slopes of the Sierra Nevada
  Mountains, Great Basin, The Gobi Desert of
  Mongolia, The Takla Makan of China and the
  Patagonia Desert of Argentina are all deserts b/c
  they are on the leeward sides of mountains.

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Orographic Uplift and Rain Shadow
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Frontal Lift or Wedging

• Front a transition zone (boundary) b/w two air
  masses of different density (usually warm and
  cool ones)
• Cold Front causes uplift as cold air advances
  toward warmer, less dense air, forcing the warm
  air to rise
• Warm Front causes uplift as warmer less dense
  air overruns a cold wedge of air ahead of it.

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Convergence

• When there is a low pressure center near the
  surface, low level winds blow into the low from
  all directions.
• The accumulating air must go somewhere it cannot
  go downward b/c the surface acts as a barrier, so
  it must rise.

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Localized Convection

• During the day the surface heats up due to
  incoming solar radiation.
• The heated air becomes more buoyant than the
  surrounding environment.
• The less buoyant air rises, and cools to
  saturation to form clouds and precipitation.

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Convection
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Stability

• The ability of air to continue to rise after
  being initially lifted by one of the previous
  methods is measured by the airs stability.
• Static Stability The state of the atmosphere
  that inhibits or favors vertical displacement
  (upward motion) of air parcels
• Suppose an air parcel is forced (displaced)
  upward some distance. When the forcing stops,
  what happens to the parcel? Does it continue to
  rise, stop or sink?
• Statically Unstablecontinues to rise
• Statically StableSinks back to original level
• Statically NeutralParcel stops rising, but does
  not sink
• Stability is related to buoyancy if a parcel is
  less dense than its surrounding environment, it
  has positive buoyancy and floats upward.
• This in turn is related to Temp.

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• Consider a parcel of air (that does not mix with
  the environment) near the surface that is lifted
  through the surrounding air. The lifted air
  cools at the DALR or the MALR while the
  surrounding air maintains its original Temp.
  profile (ELR ?)
• The relative density of the rising parcel depends
  on the differences in lapse rates (differences in
  Temp. at new levels)
• Absolutely Stable Air (? lt ?m lt ?d)
• Environment cools more slowly than the parcel.
• Suppose we are lifting a parcel (saturated or
  unsaturated) to 1 km AGL. In this case, the
  parcel cools more quickly than the environment,
  so that at 1 km the parcel will be colder and
  more dense than its surroundings. This will
  cause the parcel to sink back down to its
  original level if the forcing (lifting mechanism
  stops).

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Absolute Stability
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How does the air become stable?

• Warm the air aloft (Warm Air Advection) (WAA)
• Cool the air aloft (Cold Air Advection) (CAA)
• Sinking AirA column sinks, so the top of
  column warms more than the bottom (Called a
  subsidence inversion)
• Cases of EXTREME stability
• InversionA region of extremely stable air in
  which Temp increases with height.
• Vertical motion will be suppressed
• Caps off a cloud or a thunderstorm.

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Radiation (Nocturnal) Inversion

• Forms near the ground at night.
• Caused by radiational cooling
• Dew pt can inc. or dec. with height through the
  inversion.
• Air in contact with the ground cools rapidly,
  while air slightly above does not (air is a poor
  conductor)
• If Temp Dew pt. fog will form

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Subsidence Inversion

• Formed by sinking air (NOTE air does not sink
  all the way to the surface).
• There is a rapid decrease in Td throughout the
  inversion.
• Recall that a layer of air compresses and warms
  during descent. As it compressed, the thickness
  of the layer decreasesso the top of the layer
  descends a greater distance than the bottom.
  Therefore, the top part warms more to form an
  inversion.

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Frontal Inversion

• When a cold or warm front is present, it
  separates the cold air from the warm air.
• This causes a wedge of cold air to form under the
  warm air
• The closer the front, the lower the height of the
  inversion

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Which inversion would occur where?

• Radiation
• Subsidence
• Frontal

• Radiation C
• Subsidence B
• Frontal A

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More Stability

• Neutrally Stable
• If the parcel is saturated, the environment is
  neutral if ? ?m
• If the parcel is unsaturated, the environment is
  neutral if ? ?d
• Here the environment cools at the DALR (MALR) An
  unsaturated (saturated) parcel will have the same
  T as the surrounding environment at all levels.
  Therefore, the density of the air parcel and the
  surroundings is the same. The parcel will not
  rise or sink, but just stop whenever the lifting
  mechanism stops.

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Absolutely Unstable

• In this case the environment cools faster (with
  height) than the parcel.
• For instance if we lift an unsaturated (or
  saturated) parcel of air to a height of 1 km AGL.
• As the parcel rises it cools at the DALR (MALR)
  We said that the environment cools faster,
  therefore, at 1 km AGL the parcel will be warmer
  then its surroundings. If the parcel is warmer
  than the environment, the parcel will have
  positive buoyancy (it is less dense), and will
  continue to rise. As the parcel continues to
  risethe T differences (buoyancy differences) b/w
  the parcel and the environment will increase,
  causing the parcel to accelerate upward.
• NOTE IN THIS CASE IT DOESNT MATTER IF THE
  PARCEL IS SATURATED OR UNSATURATED ? IS GREATER
  THAN ?D WHICH IS GREATER THAN ?M

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Absolutely Unstable
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How do we make the environment unstable?

• Cool the air aloft
• Warm the surface (WAA or daytime heating.
• Dry air aloft and moist air below (called
  Convective Instability).

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Conditionally Unstable

• In this case, the environmental lapse rate (?) is
  between the DALR )and MALR. This means that the
  air is stable or unstable depending on whether
  the air parcel is saturated or unsaturated.
• If the parcel is saturated, the parcel cools at
  the MALR, while the environment cools more
  quickly ( ? gt ?m ). Therefore, at 1 km AGL the
  parcel is warmer and more buoyant than the
  surrounding air and continues to rise. UNSTABLE.
• If the parcel is unsaturated, the parcel cools at
  the DALR while the environment cools more slowly
  ( ?d gt ? gt ?m ). Therefore, at 1 km AGL the
  parcel would be cooler and more dense than the
  surrounding environment. Since the parcel is
  negatively buoyant, it will sink back to its
  original position. STABLE.

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Convective (Potential) Instability

• This type of instability relates Temp and
  moisture to stability
• Suppose we have an inversion b/w points A and B
  in which Td decreases. In other words we have a
  moist layer of air below A and a dry layer of air
  above B. If we lift a parcel from point A it
  will reach it LCL fairly quickly as it is moist
  already. A parcel lifted from point B will rise
  at the DALR for a much longer time to reach its
  LCL because it is drier.
• Therefore, if we lift an inversion layer, the top
  part cools more rapidly (dry adiabatically) than
  the bottom part (moist adiabatically). An
  environment in this condition is called
  Convectively Unstable.

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Summary

• The air temp in a rising parcel of unsaturated
  air cools at the DALR while a rising parcel of
  saturated air cools at the MALR.
• The DALR is greater than the MALR due to the fact
  that the release of latent heat during the ascent
  of a saturated parcel works against cooling.
• In a stable atmosphere, a lifted parcel of air
  will be colder than (thus more dense, and
  negatively buoyant) the surrounding air. B/c of
  this the lifted parcel will sink back to its
  original position.
• In an unstable atmosphere, a lifted parcel of air
  will be warmer than its surrounding environment
  (thus less dense, and positively buoyant). B/c of
  this, the lifted parcel will continue to rise
  away from its original position.
• The atmosphere becomes more stable as the surface
  air cools, the air aloft warms, or subsidence
  occurs over a large area.
• The atmosphere becomes more unstable as the
  surface air warms, the air aloft cools, or a
  layer of air is lifted.

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Cloud Types
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Cloud Types

• Clouds are grouped according to their height and
  form. There are 10 main types which fit into the
  following categories
• High cloudsCirrus (Ci), Cirrostratus (Cs),
  Cirrocumulus (Cc)
• Middle cloudsAltostratus (As), Alto Cumulus (Ac)
• Low cloudsStratus (St), Stratocumulus (Sc),
  Nimbostratus (Nb)
• Clouds with Vertical developmentCumulus (Cu),
  Cumulonimbus (Cb)

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High Clouds(Cirro)

• Base height in mid-latitudes b/w 5000 and 13000
  meters (generally above 6000 m)
• Almost always composed of ice cystals.
• Cirrus (Ci)
• Thin, white-colored wispy clouds forming in
  patches or streaks.
• Very low water content (0.025 g m-3)
• Sometimes the ice crystals become large enough
  that their weight overcomes updrafts and can be
  seen as fall streaks beneath the cirrus.
• Cirrostratus (Cs)
• Very thin
• More extensive horizontally
• Lower concentrations of ice crystals than Ci
• Can see the sun or moon through them
• Cirrocumulus (Cc)
• Small rounded puffs or ripples arranged in long
  rows
• Form during episodes of wind shear, which often
  occurs ahead of advancing storm systems
• Can be helpful in predicting precipitation 12-24
  hours ahead

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Middle Clouds(Alto)

• Form b/w 2000 and 6000 m
• Usually composed of liquid droplets
• Altostratus (As)
• Similar to Cs but more extensive in size and
  composed of liquid droplets
• Scatters more radiation (sun does not produce
  shadows)
• Sun or moon may be visible but there is no halo
  present
• Grey or blue-grey in color
• Alto Cumulus (Ac)
• Layered clouds forming long bands or puffy clouds
  arranged in rows.
• Grey in color
• Can pop-up out of nowhere
• During spring and summer they may signal the
  chance of afternoon t-storms

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Low Clouds(Strato)

• Cloud bases are below 2000 ft (1 km)
• Shallow (only .5 1 km thick) but large
  horizontally (500 km)
• Form under generally stable conditions in which
  vertical motion is suppressed (explains their
  shallowness)
• Often forms when winds are strong (mixing moist
  air with cool air).
• Stratus (St)
• Uniform grey color
• Cover whole sky (cant see sun or moon)
• Stratocumulus (Sc)
• Low, layered clouds with slight vertical
  development
• Thin layered Sc clouds appear light grey
• Thick layered Sc clouds appear dark grey
• Sky is usually visible in parts
• No precipitation
• Nimbostratus (Ns)
• Low, layered clouds yielding precipitation
  (usually light)
• Dark grey
• Continuously falling precipitation
• Often produces fogmaking visibility poor

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Clouds w/ Vertical Development

• Base height varies from a few hundred feet to
  3000 m
• Occurs when air is absolutely or conditionally
  unstable
• Requires moderately strong upward vertical motion
  (a few m/s)
• Has a high liquid water content (1 g cm-3)
• Cumulus (Cu)
• Fair weather Cumulus (Cumulus humilis)
• Form due to localized heating at the surface
• No precipitation
• Short lifespan, often evaporate quickly.
• Towering Cumulus (Cumulus Congestus) (Tcu)
• Multiple towers of varying heights
• Form in unstable conditions
• Can reach such large heights that the tops become
  glaciated while lower portion remains liquid
• Cumulonimbus (Cb)Cumulus clouds with rain and
  lightning.
• Produce intense thunderstorms
• Have tremendous vertical extentBase near the
  surface and tops 13 km or 39,000 ft AGL (in the
  stratosphere)
• Distinguished by the presence of an anvil a wedge
  of ice crystals at the top of the cloud that
  gradually this out further from the cloud (blown
  downstream by high winds aloft.)

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Unusual Clouds

• Lenticular Clouds
• Lens shaped clouds that form downwind of mountain
  barriers.
• Banner Clouds
• Similar to lenticulars, but is a single cloud
  located immediately above isolated mountain peaks
• Mammatus
• Downward extending protrusions extending from
  parts of some Cbs
• Formed by downdrafts at the edge of a Cb
• Looks like a boiling surface
• Nacreous clouds
• Consists of supercooled droplets or ice crystals
  in the stratosphere (30km)
• Soft whitish appearance
• Can be seen during twilight hours of winter at
  high latitudes
• Noctulucent Clouds
• Located in the mesosphere (gt 75km AGL)
• Visible after sunset or before sunrise when the
  surface and lower atmosphere are in earths
  shadow.
• Often visible during winter in high latitudes
• Pileus
• Forms a cap or a hood above, or attached to, the
  upper part of a cumuliform cloud, particularly
  during its developing stage
• Occurs when moist winds are deflected up and over
  a cumuliform

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