Formation of snow and hails
Review of last lecture
• Forces acting on a cloud/rain droplet. Terminal velocity.
How does it change with cloud drop radius?
• Growth mechanisms for rain and snow (Warm clouds,
cool clouds, cold clouds)
• Formation of rain: coalescence process (the collector is
larger than the cloud droplets but not too large
• Bergeron process: happens with coexistence of ice
and super-cooled water. Key: Saturation vapor pressure
of ice < that of super-cooled water at the same
temperature.
• Further growth of ice crystals (riming and aggregation)
Snow
Monet: Magpie (1869)
How does human life look alike?
It's like wild goose walking on snow and mud.
Left by chance is some footprint,
But the goose has flown away, not caring to the east or the west.
By Su Shi (1037-1101)
人生到处知何似，
应似飞鸿踏雪泥。
雪上偶然留指爪，
鸿飞哪复计东西。
［宋］苏轼
Change of falling ice crystal: dependent on
atmospheric temperature and winds
Snow
• Snow is precipitation
that forms by the
Bergeron process,
riming, and aggregation,
and reaches the surface
without melting
• Crystal form (habit)
varies with T and RH
• Large, soggy
snowflakes associated
with moist air near
freezing
Map of average annual snowfall
Lake effect
• Heat and moisture fluxes from warm lake enhance snowfall
in downstream regions
Melted snowflake  rain
• Much of the rain in mid-lats (even in summer) begins as
snow!
• The falling snow begins to melt around the freezing level
A
thunderstorm
in the
summer
Ice
Mixed
ice/water
0 oC
Water
Video: Ice storm
Sleet
• Sleet begins as ice crystals which melt into rain as they
fall through the atmosphere. Before reaching the
surface they solidify into a frozen state.
Freezing rain
• Freezing Rain forms similarly to sleet, however, the drop does
not completely solidify before striking the surface
Video: Hail storm
• http://www.youtube.com/watch?v=-eZuqeYlLDo
Graupel
• Graupel – ice crystals that undergo
extensive riming
• Lose six sided shape and smooth out
• Either falls to the ground or provides
a nucleus for hail
Hail
• Hail – concentric layers of ice build around graupel
– Requires very strong updrafts
– graupel carried aloft in updrafts  high altitudes freezing
temperatures
– water accreting to graupel freezes, forming a layer
– Hail begins to fall, carried aloft again by updrafts, process
repeats
– Hailstones are very heavy – high density
– Capable of tremendous amounts of damage
– Great Plains = highest frequency of hail events
Map of hail probabilities
• http://www.spc.noaa.gov/new/SVRclimo/climo.php?parm
=allHail
Summary: Different types of precipitation
Depends on atmospheric temperature and winds
Snow
Rain
Sleet
Freezing rain
Graupel/hail
Summary
• Forces acting on a cloud/rain droplet. Terminal velocity. How does it change with
cloud drop radius?
• Growth mechanisms for rain and snow
Condensation
Collisioncoalescence
Bergeron
Process
Riming/
Aggregation
Rain
Snow
(can change to rain, sleet,
freezing rain, graupel, hail
depending on underlying
atmosphere