Clouds And Precipitation

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BBCC Aviation Meteorology

  • Chapter 13
  • Clouds “Signposts In The Sky”
  • Cloud Forms (The Core Four)
  • The Height of Clouds
  • CLOUD CLASSIFICATIONS
  • High clouds – cirrus, cirrocumulus, cirrostratus
  • Middle clouds – altocumulus, altocumulus castellanus, altostratus nimbostratus
  • Low clouds – stratus, stratocumulus, cumulus
  • Extensive Vertical Dev. – cumulus, cumulonimbus, cumulo congestus
  • The Ten Basic Clouds
  • There are 10 modifications or combinations to the four core clouds
  • Often there are features of two or more categories
  • We can divide these by height starting with the highest first
  • High Level Clouds
  • Cirrus (Ci)
  • Cirrocumulus (Cc)
  • Cirrostratus (Cs)
  • Mid Level Clouds
  • Altocumulus (Ac)
  • Altostratus (As)
  • Nimbostratus (Ns)
  • Low Level Clouds
  • Cumulus (Cu
  • Cumulonimbus (Cb)
  • Stratocumulus (Sc)
  • Stratus (St)
  • CLOUD CLASSIFICATIONS
  • nimbo or nimbus = rain
  • 2 classifications of rain clouds:
  • stratus and cumulus
  • Nimbostratus and cumulonimbus
  • http://www.srh.noaa.gov/srh/jetstream/clouds/cloudchart.html
  • CIRRUS
  • CIRROCUMULUS
  • CIRROSTRATUS
  • ALTOCUMULUS
  • ALTOCUMULUS LENTICULAR
  • STACKED LENTICULAR
  • ALTOCUMULUS MACKERAL
  • ALTOCUMULUS FLOCCUS
  • ALTOCUMULUS UNDULATUS
  • KELVIN-HELMHOLTZ BILLOWS
  • ALTOCUMULUS CASTELLANUS
  • ALTOSTRATUS
  • CUMULUS
  • CUMULUS HUMILIS
  • CUMULUS MEDIOCRIS
  • TOWERING CUMULUS
  • CUMULONIMBUS
  • CUMULONIMBUS MAMATUS
  • CUMULUS CONGESTUS
  • CUMULUS CALVUS
  • CUMULUS PILEUS
  • CUMULONIMBUS INCUS
  • STRATOCUMULUS
  • STRATUS
  • CUMULUS FRACTUS
  • STRATUS FRACTUS OR PANNUS
  • NIMBOSTRATUS
  • ROLL CLOUD
  • SHELF CLOUD
  • Occurs around a T storm
  • WALL CLOUD
  • Occurs around T Storms
  • CUMULONIMBUS CAPILLATUS INCUS
  • http://m.youtube.com/watch?v=xRTZzPyGYsA
  • FALL STREAK OR HOLE PUNCH CLOUD
  • Asperatus Clouds
  • NOCTILUCENT CLOUDS
  • Much is not know about these
  • Thought to be ice crystals in the thermosphere
  • Possibly caused by Space Shuttle exhaust which is 95% water
  • Look for the long blue streaks at sunset
  • NACREOUS CLOUDS
  • McMurdo station Antarctica
  • High polar stratospheric clouds
  • Composed of water, nitric acid and sulfuric acid

CHAPTER 14

  • precipitation
  • Precipitation types
  • From the book:
  • Drizzle
  • Rain
  • Snow
  • Snow grains
  • Ice crystals
  • Ice pellets
  • Hail
  • Snow pellets
  • Variations of the list from the book
  • Freezing drizzle
  • Freezing rain
  • Graupel
  • Sleet
  • Ice needles
  • Snow grains

 

  • What’s the dif?
  • Rain is larger than drizzle
  • .02 inch or bigger is rain and is widely separated
  • Ice pellets often referred to as sleet are frozen rain droplets
  • Hail is frozen ice balls, graupel is frozen ice balls covered with a layer of water
  • ¼ inch or bigger
  • Snow pellets are actually small hail white in color
  • ¼ inch or smaller
  • Snow grains basically frozen drizzle
  • http://www.srh.noaa.gov/jetstream/global/preciptypes.html

 

  • The growth process
  • Two growth processes
  • Collision-coalescence
  • Ice crystal process also known as the Bergeron process

 

  • Collision/coalescence process
  • Sometimes referred to the warm rain process
  • Warm is relative here as the temp needs to be above -15°C
  • As the droplet falls, air retards the falling drop
  • When the air resistance reaches an amount equal to the pull of gravity it has reached its terminal velocity
  • Larger droplets overtake smaller droplets collide and coalescence is achieved
  • Collision does not always result in coalescence
  • Sometimes they just bounce

 

  • Droplet size
  • Just to give you an idea of how big a droplet we are talking about here
  • Normal human hair is about 60 to 80 microns in diameter
  • A micron is a millionth of a meter

 

  • Collision/coalescence process
  • It takes about 4,000 feet of cloud to produce droplets big enough to fall
  • In cumulus clouds the largest droplets will be first on the scene because they fall the fastest and have had the most collisions
  • Droplets tend to break apart if larger than 5,000 microns so they seldom get any larger
  • The most important factors in droplet growth are
  • liquid water content (the most important)
  • The range of droplet sizes
  • The cloud thickness
  • The updrafts
  • The electric charge of the droplets and of the electrical field within the cloud

 

  • Ice crystal or Bergeron process
  • Extremely important in the middle and high latitudes (like where we live)
  • In this process the temps are well below freezing
  • However water may still exist in temps down to -40°c
  • This is referred to as supercooled water (more in the icing chapter)
  • So we’ve got supercooled water alongside ice crystals
  • Since water escapes easier in a liquid state than frozen, the ice crystals take water molecules from the droplet
  • Saturation vapor pressure just above the droplet is greater than the saturation vapor pressure above the crystal
  • This causes the molecules to diffuse to the crystal
  • This process will continue as long as there is supercooled water available

 

  • Ice crystal or Bergeron process
  • Other processes in play here are:
  • Accretion or riming (rime ice)
  • Where ice crystals collide with supercooled droplets and freeze together on contact creating graupel
  • If the graupel splinters it may provide nuclei for other supercooled droplet to freeze and a rapid chain reaction occurs
  • Like in the fall streak cloud example
  • Aggregation
  • This happens when the ice crystals collide and stick to one another

 

  • Precipitation types
  • Snow when the temp is freezing all the way to the surface
  • Precipitation types
  • Ice pellets or sleet occur when snow passes through a warm layer causing partial melting then refreezing

 

  • Precipitation types
  • Freezing rain requires a relatively deep layer of above freezing temps on top of a freezing layer
  • The lower freezing layer must be shallow enough to supercool the droplets without freezing them
  • If the lower layer is too thick the result is ice pellets (frozen rain droplets)
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