ES 1111

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ES 1111

• Precipitation

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Formation of Cloud Droplets

• When the air reaches saturation, the condensation
  of water vapor into tiny cloud droplets may begin
• In order for water vapor to condense, particles
  in the atmosphere called cloud condensation
  nuclei (CCN) are required
• Without CCN, a relative humidity of several
  hundred percent would be needed to keep a tiny
  cloud droplet from evaporating away

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Cloud Condensation Nuclei

• Common types of cloud condensation nuclei are
• Dust
• Clay
• Organic particles from land surfaces
• Salt crystals from sea spray
• Gas-to-particle conversion particulates

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Marine vs. Continental CCN

• Division between marine and continental depends
  primarily on the origin of the air and the
  particles it contains (not where the cloud itself
  is located)
• Marine clouds tend to have smaller numbers of
  particles but a larger range of droplet sizes
  than continental clouds

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Droplet Distributions

• Figure 5.1, Page 76

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The Solute Effect

• When CCN is dissolved in the water droplet, a
  solution is formed
• This solution droplet is more stable than a pure
  water droplet (does not evaporate as easily)
• Therefore, it is possible for the impure droplet
  to exist and grow with a relative humidity below
  100

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The Curvature Effect

• Droplets are curved, but the saturation curves
  you saw applied to a flat surface
• For a tiny droplet, the surface area of the
  droplet is extreme compared to the mass of the
  droplet
• This means it is easy for the tiny droplet to
  evaporate away
• In order to survive, a tiny droplet requires
  supersaturated air (relative humidity more than
  100)
• Supersaturation values in excess of 101 are rare

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Growth of Cloud Droplets

• The solute effect and the curvature effect both
  work together to dictate how a droplet will grow
• At small sizes, the solute effect dominates
• At large sizes, the solution becomes more diluted
  and the curvature effect dominates

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Growth of a Cloud Droplet

• Figure 5.2, Page 76

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Failure of a Cloud Droplet

• Continental clouds have a high number of tiny
  CCN.
• All these CCN compete for water vapor and affect
  the way the collection of droplets can grow
• If the competition is high for water vapor, the
  supersaturation values may not reach the
  necessary amount for the droplets to continue
  growing
• Haze results when many tiny CCN particles fail to
  grow any further
• Marine clouds, with some larger droplets in the
  distribution, will have a better chance of
  getting over the hump and continue growing into
  raindrops

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Supercooled Water and Ice Nuclei

• Recall that in order to have ice, we need to have
  the water arrange itself into a crystalline
  structure
• Particulates know as ice nuclei have a structure
  similar to ice and allow ice to form readily
• Without ice nuclei in the droplet, it is possible
  for the water droplet to remain a liquid when its
  temperature is below freezing! This is called a
  supercooled water droplet
• It is possible to have liquid water present at
  temperatures down to -40 C!

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Growth of a Raindrop

• The growth of a cloud droplet is extremely slow,
  and simple growth by condensation is not what
  forms raindrops
• There are two processes that explain the
  formation of precipitation-sized particles
• Collision and Coalescence Process
• Bergeron-Findeisen Mechanism

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Falling Droplets

• All particles suspended in the air have a
  terminal velocity, which is the speed at which
  the force of gravity downward is equal to the
  force of friction acting upward
• The more massive an object, the faster the
  terminal velocity will be
• A droplet 10 µm in diameter will have a terminal
  velocity of 1/10 of a mm per second, while a
  droplet 1 mm in diameter will fall at 10 m/s
• The fall velocity of a particle is the terminal
  velocity of the particle minus any updraft
  velocity that will oppose the objects freefall
  down to the ground

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Collision-Coalescence Process

• This process is important in warm clouds where
  the precipitation particles are all liquid
• Larger droplets, having a higher fall speed, will
  collide with smaller drops and coalesce together
  into a bigger drop
• Collision is more efficient as the larger droplet
  increases in size
• A smaller droplet is more likely to collide with
  a larger drop if the difference in size is
  significant (having a wide variety of drop sizes
  in the cloud is helpful, as in the marine
  environment)

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Bergeron-Findeisen Mechanism

• This process dominates in cold clouds, where we
  have all three phases in water present solid,
  liquid, and gas
• In order to have ice, we must have a cloud whose
  temperature is below 32 F
• Recall from the previous chapter that there was a
  difference in the saturation vapor pressure over
  ice and over water

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Saturation Curve Revisited

• Figure 4.3, Page 62

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Bergeron-Findeisen Mechanism

• The saturation vapor pressure over ice is less
  than the saturation vapor over water
• Lets start with a water droplet that is in
  saturated conditions
• When we are saturated, the condensation of water
  vapor into the droplet is balanced by the
  evaporation of water off of the droplet
• The addition of an ice crystal nearby will be in
  a supersaturated environment

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Goodbye Water Droplet!

• If the ice crystal is in an environment that is
  supersaturated, water vapor will deposit on the
  ice crystal faster than it sublimates off of the
  ice crystal
• The water vapor content in the cloud will drop,
  and we no longer will have saturated conditions
  with respect to the liquid water drop
• Finding itself in subsaturated air, the water
  droplet has evaporation exceed condensation,
  which adds water vapor to the cloud but causes
  the droplet to shrink in size
• After evaporation of water vapor sufficient to
  make the conditions saturated with respect to
  liquid water once again, the droplet stops
  shrinking
• But, we still have that ice crystal and it is
  still in supersaturated conditions
• Through this process, the ice crystal will grow
  at the expense of the liquid water drop

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Other Cold Cloud Processes

• Riming is when supercooled water droplets collide
  with an ice crystal and instantly freeze on the
  ice crystal. Hail formation is due to riming
• Ice crystals at or above -5 C tend to be sticky
  due to a film of water on the surface. Ice
  crystals can then stick together in a process
  called aggregation

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Warm vs. Cold Clouds

• Not all clouds are exclusively warm or cold
• The bottom part of the cloud may be warm (and
  have the collision and coalescence process
  dominate)
• The top part of the cloud will most like find
  itself in sufficiently cold air to have the
  Bergeron-Findeisen Process dominate
• Clouds may assist in the growth of precipitation
  by throwing particles into other clouds

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Cloud Seeding

• Man has a number of substances that can be used
  to attempt to assist growth
• Silver Iodide (AgI) has a crystalline structure
  similar to ice, so it serves as a great ice
  nucleus
• Dry ice chills the air in contact with it to
  temperatures below -40 C and allow ice to
  spontaneously form without ice nuclei
• Cloud seeding is done to attempt to reduce hail
  size
• Conclusions whether this technique is successful
  are still being debated

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Precipitation

• The fancy term for the solid or liquid water that
  falls to the surface from the sky is hydrometeor
• The type of hydrometeor that one gets at the
  surface depends on
• What the hydrometeor started off being
• What temperature conditions existed during the
  fall

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Precipitation Types

• Figure 5.3, Page 78

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Precipitation Types

• Disregard the books use of the term sleet the
  British have a different meaning than our use of
  the term
• Sleet and ice pellets means the same thing in
  the US solid chunks of ice that are NOT hail
• Hail requires a thunderstorm and may exhibit
  concentric rings when sliced open
• The controlling factor in getting sleet vs.
  freezing rain is the time it takes to get down to
  the surface
• Sleet (ice pellets) The falling liquid water
  has sufficient time to freeze solid before it
  hits the ground
• Freezing rain The falling liquid water does not
  have sufficient time to freeze solid (though may
  be supercooled) before hitting the ground, but it
  freezes once contact is made with the surface
  that is below freezing

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Precipitation Type and Duration

• The intensity and duration of the precipitation
  depends on the cloud
• Cumulus clouds have more vigorous vertical
  motions that result in larger drops and more
  intense precipitation over a shorter period of
  time and over a smaller geographical area
• Stratus clouds are more persistent, have less
  vertical motion, and have prolonged, steadier,
  and less intense precipitation
• Intensity decreases as the duration of
  precipitation increases

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Precipitation Measurement

• Each form of precipitation is not measured
  independently of each other
• Most locations report the 24-hour total liquid
  amount that fell. Anything that fell solid is
  melted for this computation
• Precipitation is measured at most once an hour,
  so intensity information is not given
• Precipitation is highly variable in space and
  time. There can be significant differences
  between nearby stations over short time periods

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Rain Days

• A rain day is the number of 24-hour periods that
  had 0.2 mm or more of rainfall
• The number of rain days can vary from less than
  one in arid regions to over 180 per year in
  humid, rainy areas
• The rainiest location could be on top of Mount
  Wai-ale-ale in Hawaii that gets over 11 meters of
  rain and has an average of 335 rain days per
  year!
• Most locations will have seasonal influences that
  result in variations in rain days and amounts

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Global Mean Annual Precipitation

• Figure 5.5, Page 84

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Tropical Precipitation

• Most due to convective activity
• Intense, short-lived storms
• Highly localized events, so one location may not
  get rain every day even though rain falls in the
  area every day
• Topographic effects may combine to produce high
  rainfall totals

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The Monsoon

• The term monsoon refers to a seasonal shift in
  wind direction
• The most pronounced monsoon occurs in Asia
  (India, Bangladesh, etc.)
• In the wet season (summer), warm and moist air
  flows off the Indian Ocean and rises over the
  Himalayas, producing copious rainfall
• In the dry season (winter), the winds shift to
  blowing drier air from the Asian continent

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Mid-Latitude Precipitation

• Associated with low pressure systems and fronts
• Widespread rainfall over extended periods
• Amounts depend on the moisture content of the air
  at that particular time
• Convective activity limited to summer months for
  the most part

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Regions of Low Precipitation

• Subtropics, as you will see in the next chapter,
  lack means to lift the air to form clouds and
  precipitation
• The subtropics have moisture in the atmosphere,
  but high pressure prevents the formation of
  clouds
• The polar regions lack water vapor to begin with,
  so they have low amounts of precipitation

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The Thunderstorm

• Common to almost all regions of the globe
• Small-scale feature that, due to frequency, may
  contribute significant precipitation
• Warm, moist air rises due to an unstable
  environment
• A single thunderstorm undergoes a life cycle of
  three stages

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Annual Number of Thunderstorms

• Figure 5.6, Page 86

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Thunderstorm Life Cycle

• Cumulus Stage All updrafts, the storm is
  building vertically upward. Cloud droplets form
  in the mid-levels and grow
• Mature Stage Part updraft and part downdraft,
  the updraft strength is not able to keep the
  hydrometeors aloft, so precipitation falls at the
  surface
• Dissipating Stage All downdraft, the lack of
  updraft strength allows all precipitation to fall
  down and choke the storm of updraft air

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Thunderstorm Life Cycle

• Figure 5.7(a), Page 87

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Thunderstorm Life Cycles

• A typical thunderstorm will go through all 3
  stages in the period of 30 minutes to an hour
• Thunderstorms may group together to form a
  complex, such as a squall line, that can remain
  active for hours

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The Surface Water Budget

• If we focus on the amount of soil moisture (for
  agriculture), the surface water budget will look
  like the following
• ?S (PI) (ERDT)
• ?S change in the water content of the soil
• P Precipitation, I Irrigation
• E Evapotranspiration, R Runoff, D Drainage
  and T storage (in lakes, ponds, etc.)
• This has an impact on vegetation types found

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The Surface Water Budget

• Figure 5.9, Page 89

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Vegetation vs. Precipitation

• Figure 5.10, Page 90

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Latitudinal Hydrological Imbalance

• On a global scale, there is a balance between
  evaporation and precipitation
• Horizontal motions can transport moisture to
  other areas
• The primary source region of water vapor is the
  subtropics, and this vapor is transported towards
  the Equator and the midlatitudes
• Because of latent heat, the movement of water
  vapor also involves a movement of energy
• This plays a role in the general circulation of
  our atmosphere, the topic for the next chapter

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Latitudinal Hydrological Imbalance

• Figure 5.11, Page 90