PRECIPITATION Flashcards
Define Precipitation
Forms?
liquid water droplets or ice particles falling from a cloud to the ground below.
Drizzle, rain, sleet, snow, freezing drizzle, hail
Define virga
Any type of precipitation which evaporates before reaching the ground.
Define rain
How does rain start in NZ?
The two forms & associated cloud types?
Liquid precip, starts life as snow before melting once through FZL.
Continuous or intermittent (stratiform)
Showers (convective)
Define drizzle
Associated cloud type?
Uniform precip made up of small droplets very close together.
Falls from continuous & dense layer of STRATUS cloud with low bases.
How is snow formed
When does it occur (FZL)
consists of ice crystals that have coalesced to form a snowflake.
Occurs when FZL is so close to SFC (<1000ft) that there is no time to melt before reaching the ground.
How would you see sleet annotated in wx products?
SNRA, RASN
What is sleet, and when would you expect it?
A mixture of rain & snow
likely when air temp at SFC is 1-2deg C (rarely occurs at temps >4deg C)
How is Hail formed?
Starts as ice embryo high up in CB.
Gets caught in cycles of up & down drafts, gaining a layer of rime in the higher (colder) levels of the CB, and a layer of glaze when it reaches the lower (warmer) level of CB.
Eventually becomes too heavy & get spat out of the cycle at base/sides/anvil of CB.
Drizzle vs. rain?
Drizzle <0.5mm (or doesn’t create a ripple when it hits a puddle)
What clouds would you expect continuous rain from?
Stratiform or layer cloud (Nimbostratus, altostratus or stratocu)
How is continuous rain defined
characterised by gradual onset & cease & steady rate of fall.
breaks can occur for a short period.
How is intermittent rain characterised
Falls from layer clouds, difference is that the clouds are either thinning or thickening, causing the intermittent showers
How are showers of rain characterised
Fall only from CBs & sometimes well developed TCu
Characterised by abrupt beginning & end, as well as rapid variation in rate of fall.
Define precip rates;
Light
Moderate
Heavy
Light: <2.5mm/hr
Moderate: 2.5-10mm/hr
Heavy: >10mm/hr
What are the two processes by which cloud droplets turns into rain droplets?
Bergeron & coalesence
Describe the Bergeron process
- H2O molecules & SCWDs at equilibrium. Air is saturated wrt liquid H2O molecules
- Ice crystal introduced. Air now super saturated wrt ice.
- H20 molecules deposit onto ice crystal. Air becomes unsaturated wrt liquid H20 & SCWDs start to evap.
- Air is now saturated wrt ice crystal, SCWD totally evaporated
Ice crystals combine to create snowflakes, which thn get too heavy & falls as snow, then rain through FZL
Describe the coalescence process
What encourages it?
Requires drops of water to be big enough to fall within the cloud.
Coalescense is effectively the collision & combining of waterdroplets as they falls & “sweep” through the cloud.
Turbulence encourages & allows for more rapid growth (this process is therefore more prevalent in Cu clouds)
Out of the coalesence and bergeron processes, which is more common in NZ?
Bergeron
