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- Collector drops collide with smaller drops.
- Due to compressed air beneath falling drop, there is an inverse
relationship between collector drop size and collision efficiency.
- Collisions typically occur between a collector and fairly large cloud
drops.
- Smaller drops are pushed aside.
- Collision is more effective for the droplets that are not very much
smaller than the collect droplet.
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- When collisions occur, drops either bounce apart or coalesce into one
larger drop.
- Coalescence efficiency is very
high indicating that most collisions result in coalescence.
- Collision and coalescence
together form the primary mechanism for precipitation in the tropics,
where warm clouds dominate.
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- Freezing precipitation is rain or drizzle that freezes on surfaces and
leads to the development of an ice glaze.
- Freezing precipitation is responsible for about 20% of all winter
weather-related injuries.
- Freezing precipitation occurs in about a fourth of all winter weather
events in the continental US.
- Ice storm is defined as a freezing precipitation weather event with ice
accumulation of at least 0.25 in (0.64cm).
- Half of the freezing precipitation events qualify as ice storms.
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- Ice melts at 0°C, but water does not necessary freeze to ice at 0°C.
- Ice nuclei is needed to help water to get frozen.
- Certain microscopic particles, such as clay, organic particles, or
bacteria, have a crystalline structure similar to ice that can allow
water molecular to attach to and to build an ice lattice.
- Without enough ice nuclei, water can exist event its temperature is
below between 0°C and -40°C, which are called “supercooled water”.
- Supercooled water can result in freezing precipitation when they come in
contact with a surface that has a temperature below 0°C.
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- Melting Process è
freezing rain
- When snow falls from high in
the clouds into an atmospheric layer where the temperature exceeds 0°C.
- Supercooled Warm Rain Process è freezing drizzle
- This process occurs in low
clouds, where rain is formed through collision and coalescence without
involving snow (that is why it is called warm rain).
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- Vertical soundings of temperature and moisture are crucial.
- It is particularly important to pay attention to the presence of any
above-freezing layers in the lowest few kilometers.
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