|
|
|
|
|
|
|
|
|
|
|
Collector drops collide with smaller drops. |
|
Due to compressed air beneath falling drop,
there is an inverse relationship between collector drop size and collision
efficiency. |
|
Collisions typically occur between a collector
and fairly large cloud drops. |
|
Smaller drops are pushed aside. |
|
Collision is more effective for the droplets
that are not very much smaller than the collect droplet. |
|
|
|
|
When collisions occur, drops either bounce apart
or coalesce into one larger drop. |
|
Coalescence efficiency is very high indicating that most
collisions result in coalescence. |
|
Collision and coalescence together form the primary mechanism
for precipitation in the tropics, where warm clouds dominate. |
|
|
|
|
|
|
|
|
|
|
Snowflakes have a wide assortment of shapes and
sizes depending on moisture content and temperature of the air. |
|
Snowfall distribution in North America is
related to north-south alignment of mountain ranges and the presence of the
Great Lakes. |
|
Lake effect: snows develop as the warm lake
waters evaporate into cold air. |
|
|
|
|
|
|
Rain is associated with warm clouds exclusively
and cool clouds when surface temperatures are above freezing |
|
Rainshowers are episodic precipitation events
associated with convective activity and cumulus clouds |
|
Drops tend to be large and widely spaced to
begin, then smaller drops become more prolific |
|
Raindrop Shape begins as spherical |
|
As frictional drag increases, changes to a
mushroom shape |
|
Drops eventually flatten |
|
Drops split when frictional drag overcomes the
surface tension of water |
|
Splitting ensures a maximum drop size of about 5
mm and the continuation of the collision-coalescence process |
|
|
|
|
|
|
Graupel are 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 forms as concentric layers of ice build
around graupel |
|
Formed as graupel is carried aloft in updrafts |
|
At high altitudes, water accreting to graupel
freezes, forming a layer |
|
Hail falls but is eventually carried aloft again
by an updraft where the process repeats |
|
The ultimate size of the hailstone is determined
by the intensity of the updraft. |
|
Great Plains = highest frequency of hail events |
|
|
|
|
|
|
|
Sleet begins as ice crystals which melt into
rain through a mid-level inversion before solidifying in colder near
surface air |
|
Freezing Rain forms similarly to sleet, however,
the drop does not completely solidify before striking the surface |
|
|
|
|
|
|
|
|
The objective is to convert some of the
supercooled droplets in a cool clouds to ice and cause precipitation by the
Bergeron process. |
|
Two primary methods are used to trigger the
precipitation process. |
|
Dry ice is used to lower cloud temperature to a
freezing point in order to stimulate ice crystal production leading to the
Bergeron process. |
|
Silver iodide initiates the Bergeron process by
directly acting as freezing nuclei. |
|
Under ideal conditions, seeding may enhance
precipitation by about 10%. |
|
|
|
|
|
|
|
Standard raingages, with a 20.3 cm (8”)
collected surface and 1/10 area collector are used to measure liquid
precipitation |
|
Depth of water level conveys a tenfold increase
in total precipitation |
|
Automated devices provide a record of
precipitation amount and time of the event |
|
|
|
|
|
|
|
Raingages are inadequate for measuring frozen
precipitation |
|
Measurements of accumulated snow are used |
|
Water equivalent of snow, a 10 to 1 ratio is
assumed |
|
Automated snow pillows are common in many
locations |
|
Detect
snow weight and convert directly to water equivalent |
|