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)

SnowSnow

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 Change of falling ice crystal: dependent on atmospheric temperature and windsatmospheric temperature and winds

SnowSnow

• 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 snowfallMap of average annual snowfall

Lake effectLake effect

• Heat and moisture fluxes from warm lake enhance snowfall in downstream regions

Melted snowflake Melted snowflake rain 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 A thunderstorm thunderstorm

in the in the summersummer

0 oC

Water

Mixed ice/water

Ice

Video: Ice stormVideo: Ice storm

SleetSleet• 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 forms similarly to sleet, however, the drop does not completely solidify before striking the surface

Freezing rain

Video: Hail stormVideo: Hail storm

• http://www.youtube.com/watch?v=-eZuqeYlLDo

• 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

GraupelGraupel

• 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

HailHail

Map of hail probabilitiesMap of hail probabilities

• http://www.spc.noaa.gov/new/SVRclimo/climo.php?parm=allHail

Summary: Different types of precipitationSummary: Different types of precipitationDepends on atmospheric temperature and winds Depends on atmospheric temperature and winds

Snow Rain Sleet Freezing rain Graupel/hail

Summary

Condensation

Collision-coalescence

Bergeron Process

Warm

clouds

Cool/cold clouds

Rain Snow(can change to rain, sleet, freezing rain, graupel, hail depending on underlying atmosphere

Riming/Aggregation

• Forces acting on a cloud/rain droplet. Terminal velocity. How does it change with cloud drop radius?• Growth mechanisms for rain and snow