Chapter 4: Clouds and Precipitation

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Midterm Results

• Class Mean 80.8
• Class Median 84.6
• Grade Breakdown
• A11
• B8
• C4
• D4
• F3
• Your total average up to this point is circled in
  the upper right corner.

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Chapter 4 Clouds and Precipitation
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Thermal Behavior of Air

• Air Parcel a chunk of air which we consider to
  be separated from the air of the environment
  (think of the chunk of air as being surrounded by
  an invisible balloon)
• No air from outside the balloon can mix with the
  air that is in the balloon and vice versa.

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Rising Motion

• Lets force the air parcel to rise
• As it rises, pressure on the parcel decreases.
  Therefore, the parcel expands. The number of
  molecular collisions is reduced, and the parcel
  will decrease in temperature.
• If (parcel temp environmental temp) the parcel
  will rise, since the parcel is less dense than
  environmental air.
• If (parcel temp will sink, since the parcel is more dense than
  the environmental air.

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LCL

• As the temperature of the parcel decreases, so
  does the saturation mixing ratio (remember from
  lab 3?)
• If the temperature decreases to the point where
  the actual mixing ratio is equal to the
  saturation mixing ratio, then condensation
  occurs.
• The level at which this occurs is the LCL
  (lifting condensation level)

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Calculating LCL

• Mixing Ratio is a function of pressure.
• Dew point is related to mixing ratio
• So, dew point will also change with pressure
• Td decreases at a rate of 1.8C/km.
• Under unsaturated conditions, the air temperature
  decreases at a rate of 10C/km
• The height of the LCL

___________T Td____________ Lapse rate of T
Lapse rate of Td
_______T Td_______ __T Td__ 10C/km
1.8C/km 8.2C/km
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How can a parcel be forced to lift?

• Unless a lifting mechanism is present,
  condensation will likely not occur and clouds
  will not form. There are four ways to lift air
  parcels
• 1. topographic lifting
• 2. convective lifting
• 3. convergence lifting
• 4. frontal lifting
• 3 and 4 are not discussed in chapter 4.

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Topographic lifting

• As air approaches a mountain, some of the air is
  forced over the mountain. As the air parcels
  rise, they cool. If the temperature falls to the
  dew point temperature, then clouds will form (see
  figure 4.2)

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Convective Lifting

• Due to differences in heating and differences in
  specific heats of the earths surface, some
  locations heat up faster than others.
• Wherever the temperatures are higher on the
  surface, the hot air, being less dense than the
  surrounding air, will rise.
• Ex. White puffy clouds (cumulus) on warm summer
  afternoons. Could be the result of a parking lot,
  open field with no vegetation, or an area of
  earth surrounded by water.
• See figure 4.1.
• Air also sinks over the cooler surfaces. Sinking
  air is called subsidence, and is often found
  around cumulus clouds.
• Areas of sinking air will be areas of clear
  skies, since sinking air compresses and warms.

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Level of free convection

• LFC The level at which the temperature of an air
  parcel is greater than the surroundings.
• A lifting mechanism is no longer required here,
  since the parcel will continue to rise due to its
  buoyancy.
• Ex. Forced topographic lifting, until the parcel
  reaches its LFC. Even above the level of the
  mountain, the air will continue to rise due to
  buoyancy.

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Cloud Types

• Classified according to the height of their bases
  and their appearance.
• Prefix or suffix nimbo implies rain.
• Nimbostratus clouds steady precipitation that
  lasts several hours.
• Form by non-buoyant processes.
• Relatively warm clouds with less vertical extent
  than cumulonimbus clouds.
• Cumulonimbus clouds - formed from convection
  heavy but brief precipitation.

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Collision coalescence process

• Main way precipitation develops and falls in
  warm, usually shallow, clouds (e.g. stratus
  clouds).
• Some droplets in the cloud are randomly larger
  than others.
• Due to larger condensation nuclei and other
  causes.
• Larger drops fall faster than small drops.
• As the larger drops fall, they collide with
  smaller drops and grow larger in size (coalesce).
• The droplets eventually grow large enough to fall
  to the ground.

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Aggregation and accretion

• In clouds where temperatures are below freezing,
  ice crystals grow by deposition (gas to solid)
• as larger ice crystals begin to fall, they
  encounter and stick to smaller ice crystals
  (aggregation). An accumulation of ice crystals is
  a snowflake.
• Accretion ice crystals can also fall through a
  region where liquid cloud droplets exist. The
  liquid water freezes to the crystal.
• Ice crystals that have grown by colliding with
  liquid droplets are called graupel.
• If the ice crystals grow large enough, they can
  fall to the ground, often melting along the way,
  producing rain.

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Bergeron Process (Occurs in the cold part of the
cloud)

• Liquid loses molecules more readily than a solid.
  
• Therefore, the vapor pressure around a liquid
  droplet is higher than around an ice crystal.
• Molecules will move from higher pressure to lower
  pressure.
• So water vapor molecules will tend to break away
  from the liquid droplets and move toward the ice
  crystals in the cloud.
• Ice also has a lower saturation vapor pressure
  than liquid water
• Fewer molecules are needed around a frozen
  surface for saturation to occur than are needed
  around a liquid surface.
• Layer of air around ice crystals can have RH
  100, so deposition readily occurs.

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Bergeron Process contd

• Therefore, when ice crystals and liquid water
  droplets (usually supercooled) co-exist, water
  molecules will move from the liquid droplets
  toward the ice crystals.
• Since the layer of air around the ice crystals is
  more easily saturated, and the ice crystals will
  grow larger through deposition of the water vapor
  molecules while the droplets grow smaller.
• The ice crystals eventually grow large enough to
  fall as either snow, sleet, or rain (most
  common), depending on the temperature profile of
  the air.

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Supercooled water

• Liquid water can exist and temperatures less than
  0 Celsius
• Liquid water whose temperature is below 0 Celsius
  is called supercooled
• Water can remain supercooled to about -40 C, at
  this temperature water will freeze spontaneously
• At temperatures between 0 and -40 C, ice nuclei
  are needed for freezing to occur. Ice nuclei are
  small particles whose molecular structure has a
  similar lattice to that of frozen water, which
  helps the water molecules to properly align.

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Why no supercooled water on land?

• Ice does not form until the molecules are aligned
  in a certain fashion.
• In a large body of water, it is likely that some
  of the molecules will properly align, especially
  if in contact with another surface that has a
  structure similar to frozen water.
• The rest of the molecules will quickly freeze by
  contact with the frozen molecules
• In the atmosphere, however, there are very few
  molecules and it is less likely that any of them
  will align into the required structure.

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Types of precipitation

• Rain droplet diameter 0.5 mm.
• Drizzle
• Most precipitation starts as ice, however when
  the droplets are completely melted before they
  hit the ground, the precipitation is considered
  rain (or drizzle).

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Freezing rain and sleet (figure 4.7-4.8)

• Freezing rain starts as snow in a cloud, but
  melts as it passes through a warmer layer of air.
  Closer to the ground there is a layer of sub
  freezing air. The layer is not thick enough to
  refreeze the raindrop however. When the droplets
  land on a frozen surface (street, house etc) the
  droplets freeze. Freezing rain leaves a layer of
  ice on the ground.
• Sleet same process as freezing rain, however
  the cold layer near the surface is deep enough
  that the liquid water freezes completely before
  hitting the ground. They refreeze into tiny, hard
  pellets of ice.

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Hail

• Within the environments of thunderstorms, there
  is an updraft (a column of rapidly rising air)
• Updrafts are strong enough to support large
  stones of ice, the ice crystals grow through
  accretion.
• The stronger the updraft, the heavier the stone
  must be before it can fall out of the cloud
• Size of hail depends on 2 factors
• Updraft strength stronger more hail
• Depth of the warm layer hail melts once reaching
  temperatures above freezing, so shallow layers of
  warm air lead to larger hail stones reaching the
  ground.

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Hail
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Problem Tips

• 1) Just circle the answer, theres nothing to
  write.
• 2) Use the Ideal Gas Law (P RDT) and solve
  for T. Then look at how density (D) would change
  for a warmer air parcel. Assume P and R dont
  change.
• 3a) Use the lapse rate for dry air, which is
  -10C/km. You know how high the cloud is, so you
  can find how much the temp. decreases and
  subtract that from your starting temp. This is
  just like 7 from Lab 1.
• 3b) Use the LCL equation on page 40 and solve for
  Td. Youre given the T and LCL (which is at the
  base of the cloud).
• 4) Remember that you need the air to be saturated
  to have a chance of rain. Also, from the last
  lab we know that the closer T and Td are, the
  closer the air is to saturation.

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Tips contd

• 5) This has to do with the mechanisms that cause
  cumulus and stratus clouds to form. Just keep in
  mind that they form from non-buoyant processes,
  unlike cumulus clouds. Non-buoyant processes
  often evenly affect a pretty large area.
• 6) Related to topographic lifting and figure 4.2
  on page 41. The cloud that forms from the air
  rising up the mountain is a result of expansional
  cooling. As the air rises, it expands and cools
  to the point where the TTd. After saturation,
  rain will eventually occur. Where will most of
  that rain fall? Also, if air expands and cools,
  which increases the Relative Humidity, as it
  rises, what will happen to it as it descends on
  the lee side of the mountain? Think about these
  points when answering this one.

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Tips contd

• 8) Remember, collision-coalescence processes
  occur when there are only liquid droplets present
  (warm clouds) and when there are a large amount
  of liquid droplets.
• 9) Ice crystals grow by deposition (gas converted
  directly to solid without being a liquid). Which
  one of the 2 choices (ice cube or frost) would
  most likely also for like this?
• 13) Use figures 4.7, 4.8, and 4.9 to guide you
  with this one.
• 14) This has to do with one of the two things
  that determine the size of hail (page 47 or slide
  18). Which one would vary between the upper
  Plains and Florida?