Precipitation II
Brief review
• Cloud droplets are very small, much too small to fall as rain.
• The
smaller the cloud droplet, the greater its curvature, and the
more likely it will evaporate (curvature effect).
• Cloud
droplets form on cloud condensation nuclei. Hygroscopic
nuclei, such as salt, allow condensation to begin when the
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relative humidity is less than 100%. Also the salt dissolves in the
condensed water, inhibiting evaporation and stimulating growth
(solute effect).
Droplets fall at constant speed (terminal velocity).
Bigger droplets fall faster than smaller ones.
Cloud droplets, in above-freezing air (warm clouds), can grow
larger as faster-falling, bigger droplets collide and coalesce with
smaller droplets in their path
Today:
♦ Ice crystal (Bergeron) process
♦ Precipitation types
♦ Measuring precipitation
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Rain
Drizzle: the size of the rain drops is very small (<0.5mm)
the terminal velocity is very small. It is produced by thin
stratus clouds (why?).
Shower: Intensive precipitation. Sudden showers are
associated with strong upward winds and are very
localized.
Acid rain: atmospheric pollutants dissolve into the water
drops and form an acidic solution.
Virga: the rain drops evaporate before they reach the
ground forming “virga” beneath the clouds. It can occur
when the lower atmosphere is dry and warm.
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Evaporating rain
Small rain drops can evaporate before they reach the
ground, forming “virga” below the clouds.
VIRGA
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Rainfall intensity
The amount of rain (measured in inches) per unit time
(usually 1 hour).
Snow
• Most
of the precipitation starts as a snow but eventually
transitions to rain (why?).
• Snow flurries: light snow showers, do not accumulate.
• Snow squalls: intense snow showers with a short duration.
• Blizzard:
low temperatures and strong winds. It results in
very low visibility.
• Drifting and blowing snow.
• APrevents
snow blanket is a very good thermal insulator (why?).
the roots of the plants from freezing.
Fallstreaks
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Small ice crystals can sublimate before they reach
the ground.
Snowflakes
• Ice crystals have a hexagonal symmetry.
• The exact shape and the growth rate of the ice crystals depend on:
♦ Temperature (dendrite snowflakes form at about –12 to -20 C)
♦ Humidity: simpler forms tend to be formed at lower humidity and
more elaborate forms at higher humidity.
H O
H
Most of the images that follow are taken from the spectacular webpage about
the physics and the making of of snowflakes SnowCrystals.com
The photographic material is published as a collection in the book
The Snowflake: Winter's Secret Beauty
written by Kenneth Libbrecht
photography by Patricia Rasmussen
www.SnowCrystals.com
www.SnowCrystals.com
Stellar Dendrites
www.SnowCrystals.com
www.SnowCrystals.com
Sectored Plates
www.SnowCrystals.com
Hollow Columns
Needles
www.SnowCrystals.com
Spatial Dendrites
www.SnowCrystals.com
www.SnowCrystals.com
Capped Columns
www.SnowCrystals.com
Rimed Crystals
Irregular Crystals
www.SnowCrystals.com
www.SnowCrystals.com
a rare 12-sided snowflake
www.SnowCrystals.com
Growing a Synthetic Snowflake
Annual Snow Precipitation Map
From Snow to Rain (and back?)
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From Snow to Rain
Snow: no melting of the ice crystals.
Sleet: Ice crystals melt and refreeze in a deep layer of
subfreezing temperatures and form small translucent ice
pellets. There is a tapping sound as they hit the ground.
Freezing rain: shallow freezing boundary layer.
Supercooled raindrops freeze on contact with cold objects.
Rain: it might start as snow at high altitudes but the
crystals melt and fall as raindrops on the ground.
Rime
Sleet
Freezing rain
Freezing rain
Average annual number of days with
freezing rain and freezing drizzle
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• Sizes: 0.5 cm – 14 cm
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Hail
Pieces of translucent or opaque ice of spherical or irregular
shape.
Frozen raindrops act as embryos to accumulate smaller
supercooled raindrops.
The drops spend ~5-10
min in the cloud
(the largest in US).
Terminal velocity of
hailstone
♦ 0.5 cm: ~30 mi/h
♦ 14 cm: ~160 mi/h
Hail grains
Measuring precipitation
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measure rainfall
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Rain gauge: any instrument
which can collect and
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Standard rain gauge
♦ Measuring tube
♦ Funnel-shaped collector
(area much (~10) larger
than the area of the tube)
Tipping bucket rain gauge
♦ Used in automated
weather stations
Weighing-type rain gauge
♦ Measures the weight of
the collected rainfall