Clouds and Precipitation 13&14

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BBCC Aviation Meteorology

Chapter 13

Clouds “Signposts In The Sky”

Cloud Forms (The Core Four)

The Height of Clouds

FAA Version

  • Note the high clouds in the temperate regions only go to 40,000 on their chart, memorize the temperate regions

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

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

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 rocket engine 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)

Precipitation types

  • Rain occurs when there is a deep layer of above freezing air based at the surface

 

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