Precipitation Types Drizzle: diameter < 0.5 mm Rain: diameter >= 0.5 mm Where are the raindrops larger, Miami or Seattle? Raindrops almost always < 6 mm because collision among raindrops too large raindrops tend to break up Start as rain, but may fall as drizzle. Does rain always reach the surface? 8
Falling precipitation that evaporate before reaching surface - -- Virga Low humidity - - evaporation - - drops become smaller - - rate of fall decreases Strong updraft Sudden rain shower and cloudburst (Cumuliform) Rain and no-rain in places next to each other (Cumulonimbus) Continuous rain at small vertical currents (layered cloud or Nimbostratus) How does “Virga” look like and where does it happen? 9
Fig. 7-14, p. 174 Virga (Rain not reaching the surface) 10
Fig. 2, p. 175 Shape of Raindrops Spherical 2 mm (surface area / volume) is minimum for 2. 11
Fig. 7-15, p. 176 14 Falling ice-crystals and snowflakes (Fallstreaks) from cirrus clouds. Snow
Fig. 7-16, p. 176 Fig. 7-17, p. 178 15 Common forms of ice crystal Dendrite Why is dendrite most common?
Table 7-3, p. 176 16 Why is dendrite most common? Growth = f(temperature, sat. vap. Pr. difference) Maximum growth rate is at -12 to -16 C, when saturation vapor pressure difference between water and ice is maximum.
Table 7-4, p. 178 17 Intensity of Snowfall Is visibility the best way to measure snowfall? Can wind lead to visibility problem?
Fig. 4, p. 179 18 Some more terms related to snowfall Flurries : light and intermittent from developing cumulus cloud Snow squall : brief but more intense than flurries, usually from cumuliform clouds Drifting snow : surface movement Blowing snow : in air movement Ground blizzard : after falling snow has ended Blanket of snow: snow covering the landscape
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