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