AVIATION 120

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AVIATION 120

• Meteorology

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Todays Agenda

• Precipitation
• Precipitation Types
• Rain, drizzle, sleet (ice pellets), freezing
  rain, freezing drizzle, hail
• Virga, Showers
• Snow, snow pellets, snow grains, ice crystals
• Hail
• Measuring precipitation
• Doppler Radar

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Precipitation Types - Drizzle

• Drizzle is very fine, small droplets of liquid
  precipitation that has a diameter of 0.5mm or
  less
• Drizzle usually falls from stratus clouds
• Small raindrops may partially evaporate after
  leaving the bottom of the cloud and arrive at
  ground level as drizzle
• Drizzle droplets fall very slowly and sometimes
  appear as if they are floating in the air if
  they are caught in slight updrafts

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Precipitation Types - Rain

• Rain is considered to be liquid precipitation
  with a diameter greater than 0.5mm
• Large rain drops can reach 5mm or more in
  diameter
• In mid to high latitudes, rain generally forms by
  the ice crystal (Bergeron) process and then melts
  as it falls through warm air
• Continuous rain is usually produced by
  nimbostratus, showery or intermittent rain can be
  produced by stratus, TCU or CB
• As pilots, rain drop size can indicate the
  strength of updrafts, and therefore turbulence
• Visibility usually improves after a rainstorm due
  to the rain removing particles (condensation
  nuclei) from the air
• Rain that combines with pollution in the air,
  acid rain can form

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Precipitation Types - Snow

• Like rain in the mid and high latitudes, snow is
  formed by the ice crystal (Bergeron) process
• Specifically, snow flakes are formed by ice
  crystal aggregation (when ice crystals collide
  and stick together)
• The falling snowflake will reach the ground as a
  solid if the air is cold
• Snow can exist in temperatures warmer than 0C
  (see the Focus section on page 179 of Meteorology
  Today)
• Ice crystals can exist in several different forms
  such as plates, columns, needles, and dendrites
  which are the most common form
• Ice crystal forms depend on temperature and
  moisture content of the air
• Since a snowflake may fall though several
  different temperature and moisture levels on its
  way through a cloud, many complex snowflake
  shapes can be formed as a result of different
  shape ice crystals aggregating
• Showers of snow are known as flurries heavy
  flurries may be referred to as snow squalls
• Strong winds may cause blowing and drifting snow

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Precipitation Types - Snow
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Precipitation Types Sleet and Freezing Rain

• If a snowflake partially melts in an above
  freezing layer and falls into a deep freezing
  layer, the partially melted snowflake or cold
  raindrop freezes into a small ice pellet called
  sleet
• Sleet pellets are usually transparent and have
  diameters of 5mm or less
• If the freezing layer is not deep, freezing rain
  will occur as supercooled water drops form in the
  cold air
• If the supercooled drops are less than 0.5mm in
  diameter, they are referred to as freezing drizzle

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Precipitation Types Sleet and Freezing Rain
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Precipitation Types Snow Grains and Snow Pellets

• Snow grains are elongated or fairly flat, opaque
  grains of ice with diameters of less than about 1
  mm
• Snow grains fall in small quantities from stratus
  cloud as continuous precipitation and never
  showers
• Snow grains dont bounce or shatter when they hit
  hard surfaces
• Snow pellets are similar to snow grains, but
  larger with diameters less than 5 mm
• Snow pellets are light, snow-like pieces of ice
  that are brittle, crunch underfoot and bounce or
  break apart when they hit a hard surface
• Snow grains and pellets form by riming
  (accretion) of ice crystals with supercooled
  water droplets
• A rimed ice crystal is also known as graupel
• In a CB with strong updrafts, graupel may form
  into hail as accretion continues to increase the
  size of the pellet

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Precipitation Types Snow Grains and Snow Pellets
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Precipitation Types - Hail

• Hailstones are pieces of ice that can be clear or
  opaque and are greater than 5 mm in diameter
• Hail can reach very large diameters with the
  largest verified hailstone measuring 14 cm across
  (weighed 1.67 pounds)
• Hail can be extremely destructive to property on
  the ground as well as a major hazard to aviation
  hundreds of millions of dollars of damage are
  done annually due to hail
• Hail if formed in cumulonimbus clouds when
  graupel or any other airborne particle acts as an
  embryo and collides with supercooled
  waterdroplets
• These collisions cause a hailstone to form as the
  supercooled water droplets accrete to the
  hailstone embryo
• For large hail to form, the growing hailstone
  must remain in the cloud for long periods of time
  (5-10 minutes for a golf-ball size hailstone)
• For the hail to remain in the cloud for long
  enough periods, the CB must produce strong
  updrafts

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Precipitation Types - Hail
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Hail Damage to Aircraft
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Measuring Precipitation

• Instruments
• Standard rain gauge
• Tipping bucket rain gauge
• Weighing-type rain gauge
• Water Equivalent of Snow
• The depth of water from a melted column of snow
  is the water equivalent
• The water equivalent is variable depending on the
  type and density of the fallen snow
• Typical water equivalents for freshly fallen snow
  are on the order of 101 (10 cm of snow melts
  into 1 cm of water)
• Can vary from 301 to less than 21

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Measuring Precipitation
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Doppler Radar

• Doppler radar is used extensively to detect
  precipitation in cloud
• When the radar energy strikes a cloud or rain
  droplet, part of the energy is reflected back to
  the transmitter, producing a radar echo
• The time it takes for the energy to be
  transmitted out and reflected back indicates the
  distance an echo is from the radar site
• Using the principle of Doppler shift, new
  technology radars can measure the horizontal
  speed of rain
• Doppler radar is therefore an important tool for
  detecting tornado activity in CBs
• Radar images can be viewed at http//www.weathero
  ffice.ec.gc.ca/radar/index_e.html

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Radar Image