1. Rain - liquid deposits falling from the atmosphere to the surface - with a diameter > 0.5 mm - < 0.5 mm: drizzle - max. size: about 5 - 7 mm (too large to remain suspended) - beyond this size, inter-molecular cohesive forces become to weak to be held in the mass of water together as a single drop

1. PrecipitationPrecipitation

2. Precipitation?Precipitation?
 What is precipitation?What is precipitation?

3. What are the forms of precipitation?What are the forms of precipitation?

4. What are the forms of precipitation?What are the forms of precipitation?

5. What are the forms of precipitation?What are the forms of precipitation?

6. Forms of PrecipitationForms of Precipitation
 1.1. RainRain - liquid deposits falling from the atmosphere- liquid deposits falling from the atmosphere
to the surfaceto the surface
- with a diameter > 0.5 mm- with a diameter > 0.5 mm
- < 0.5 mm: drizzle- < 0.5 mm: drizzle
- max. size: about 5 - 7 mm- max. size: about 5 - 7 mm
(too large to remain suspended)(too large to remain suspended)
- beyond this size, inter-molecular cohesive- beyond this size, inter-molecular cohesive
forces become to weak to be held in theforces become to weak to be held in the
mass of water together as a single dropmass of water together as a single drop

7. Forms of PrecipitationForms of Precipitation
 2.2. Freezing rainFreezing rain
- when falling liquid water droplets reaches a surface- when falling liquid water droplets reaches a surface
with a temperature below freezing pointwith a temperature below freezing point
- so, the rain droplets quickly turn into ice- so, the rain droplets quickly turn into ice
- another * condition: where the rain develops, the- another * condition: where the rain develops, the
temperature of rain develops must be abovetemperature of rain develops must be above
freezingfreezing
e.g. within a temperaturee.g. within a temperature
inversioninversion

8. Forms of PrecipitationForms of Precipitation
 3.3. SleetSleet // ice pelletsice pellets
- transparent / translucent spheres of frozen water- transparent / translucent spheres of frozen water
- with a diameter > 5 mm- with a diameter > 5 mm
- develop first as raindrops in relatively warm- develop first as raindrops in relatively warm
atmosphere (Temp: > freezing),atmosphere (Temp: > freezing),
- then raindrops descend into a colder layer of the- then raindrops descend into a colder layer of the
atmosphere (Temp:<0atmosphere (Temp:<0oo
C)C)
- causing the freezing into ice pellets while reaching- causing the freezing into ice pellets while reaching
the ground surfacethe ground surface
- like freezing rain, an air temperature inversion is- like freezing rain, an air temperature inversion is
requiredrequired

9. Forms of PrecipitationForms of Precipitation
 4.4. SnowSnow
- commonly found in the mid- and high- latitudes- commonly found in the mid- and high- latitudes
- it develops when water vapour deposits itself directly- it develops when water vapour deposits itself directly
to a six-sided (to a six-sided (hexagonhexagon) deposition nuclei as a) deposition nuclei as a
solid crystal, at temperature below freezingsolid crystal, at temperature below freezing
(what is this process called?)(what is this process called?)
- why is this unique form?- why is this unique form?
- snow is usually associated- snow is usually associated
frontal upliftingfrontal uplifting withwith mid-mid-
latitude cycloneslatitude cyclones

10. Forms of PrecipitationForms of Precipitation
 5.5. HailHail
- a frozen form of precipitation with a diameter > 5 mm- a frozen form of precipitation with a diameter > 5 mm
- hailstones: concentric shells of ice with alternating- hailstones: concentric shells of ice with alternating
white cloudy appearance & those that arewhite cloudy appearance & those that are
clearclear
- cloudy white: contain partially melted- cloudy white: contain partially melted
snowflakes that freeze on to thesnowflakes that freeze on to the
to the surface of the growingto the surface of the growing
hailstonehailstone
- clear shell: develops when liquid water- clear shell: develops when liquid water
freezes onto the hailstonefreezes onto the hailstone

11. Forms of PrecipitationForms of Precipitation
 5.5. HailHail
- Strong uprising currents in thunderstorm clouds- Strong uprising currents in thunderstorm clouds
provide the mechanism for forming hailprovide the mechanism for forming hail
- the updrafts move hailstone embryos (e.g. large- the updrafts move hailstone embryos (e.g. large
frozen raindrops) upward through the storm cloudfrozen raindrops) upward through the storm cloud
- where they encounter layers of ice crystals, snow &- where they encounter layers of ice crystals, snow &
supercooled rainsupercooled rain
- each encounter causes the hailstone to grow larger- each encounter causes the hailstone to grow larger
- hailstones can grow very large in size with- hailstones can grow very large in size with
repeated updraftsrepeated updrafts
- until being too heavy to by supported by updrafts, it- until being too heavy to by supported by updrafts, it
begins falling under gravitybegins falling under gravity
- on descending, it can lose great mass due to melting- on descending, it can lose great mass due to melting

12. Forms of PrecipitationForms of Precipitation
 5.5. HailHail
A typical hailstone growth path through a thunderstorm cloud.A typical hailstone growth path through a thunderstorm cloud.
Hailstones add most of their mass during updrafts.Hailstones add most of their mass during updrafts.

13. Necessary conditions?Necessary conditions?
 When clouds form in the atmosphere, theyWhen clouds form in the atmosphere, they
are non-precipitating in 99% of cases.are non-precipitating in 99% of cases.

14. Necessary conditions?Necessary conditions?
 1. cooling of air (e.g. convectional / orographic /1. cooling of air (e.g. convectional / orographic /
cyclonic (frontal) uplifting)cyclonic (frontal) uplifting)
 2. condensation and cloud formation2. condensation and cloud formation
 3. an accumulation of moisture3. an accumulation of moisture
 4. the growth of cloud droplets4. the growth of cloud droplets
-- thethe most criticalmost critical stage!!stage!!
 Water dropletsWater droplets andand ice crystalsice crystals of clouds needof clouds need
to beto be transformed intotransformed into heavier particlesheavier particles if theyif they
are to fall out of clouds as precipitation.are to fall out of clouds as precipitation. Why?Why?

15. Necessary conditions?Necessary conditions?
 Water dropletsWater droplets andand ice crystalsice crystals of clouds needof clouds need
to beto be transformed intotransformed into heavier particlesheavier particles if theyif they
are to fall out of clouds as precipitation.are to fall out of clouds as precipitation. Why?Why?
 As we know, water droplets & ice crystalsAs we know, water droplets & ice crystals cancan
only fallonly fall to the Earth’s surface if they grow to ato the Earth’s surface if they grow to a
sufficiently large sizesufficiently large size thatthat cancan overcomeovercome
updraftsupdrafts

16. Necessary conditions?Necessary conditions?
 Raindrop sizeRaindrop size::
drizzle about 0.2 mmdrizzle about 0.2 mm
light rain about 0.5 mmlight rain about 0.5 mm
small rain about 1.0 mmsmall rain about 1.0 mm
rain about 5.0 mmrain about 5.0 mm
rainstorm up to about 7.0 mmrainstorm up to about 7.0 mm
 How about theHow about the condensed sizecondensed size generated fromgenerated from
condensationcondensation? (Make a guess?)? (Make a guess?)

17. Necessary conditions?Necessary conditions?
 Raindrop sizeRaindrop size::
drizzle about 0.2 mmdrizzle about 0.2 mm
light rain about 0.5 mmlight rain about 0.5 mm
small rain about 1.0 mmsmall rain about 1.0 mm
rain about 5.0 mmrain about 5.0 mm
rainstorm up to about 7.0 mmrainstorm up to about 7.0 mm
 From condensation alone:From condensation alone:
 Between 50 – 200Between 50 – 200 µmµm (size of drizzle only)(size of drizzle only)
 (where 1 mm = 1000(where 1 mm = 1000 µmµm))

18. Mechanism ofMechanism of
Precipitation DevelopmentPrecipitation Development
 1. Coalescence theory1. Coalescence theory
 2. Bergeron-Findeisen process2. Bergeron-Findeisen process

19. Mechanism ofMechanism of
Precipitation DevelopmentPrecipitation Development
 1.1. Collision - Coalescence theoryCollision - Coalescence theory
 A droplet may continue to grow byA droplet may continue to grow by
diffusion beyond 20 micrometers indiffusion beyond 20 micrometers in
diameter, however, once a dropletdiameter, however, once a droplet
attains this size, growth is slow andattains this size, growth is slow and
inefficient.inefficient.
 Droplets this large begin to collideDroplets this large begin to collide
and coalesce with other droplets asand coalesce with other droplets as
they fall through the cloud, meaningthey fall through the cloud, meaning
they will bump into and bond to onethey will bump into and bond to one
another and form larger drops.another and form larger drops.

20. Mechanism ofMechanism of
Precipitation DevelopmentPrecipitation Development
 1.1. Collision - Coalescence theoryCollision - Coalescence theory
 Updrafts in a cloud can transportUpdrafts in a cloud can transport
a droplet upward repeatedlya droplet upward repeatedly
allowing it many opportunities toallowing it many opportunities to
fall back down through the cloudfall back down through the cloud
and collide and coalesce with otherand collide and coalesce with other
droplets.droplets.
 Tiny aerosol nuclei grow into largeTiny aerosol nuclei grow into large
water droplets more than 10,000water droplets more than 10,000
times their initial size.times their initial size.

21. Mechanism ofMechanism of
Precipitation DevelopmentPrecipitation Development
 2.2. Bergeron – Findeisen processBergeron – Findeisen process
 also known as thealso known as the cold raincold rain oror ice crystal processice crystal process
 As the formation of precipitation in theAs the formation of precipitation in the
cold clouds of the mid and uppercold clouds of the mid and upper
latitudes by ice crystal growth.latitudes by ice crystal growth.
 The equilibrium vapor pressure overThe equilibrium vapor pressure over
water is greater than the saturationwater is greater than the saturation
vapor pressure over ice, at the samevapor pressure over ice, at the same
temperature.temperature.

22. Mechanism ofMechanism of
Precipitation DevelopmentPrecipitation Development
 2.2. Bergeron – Findeisen processBergeron – Findeisen process
 Therefore in a mixed phase cloud, the liquid water will beTherefore in a mixed phase cloud, the liquid water will be
out of vapor pressure equilibrium and will evaporate toout of vapor pressure equilibrium and will evaporate to
reach equilibrium.reach equilibrium.
 The water droplets will move towardThe water droplets will move toward
the lower pressure over the ice andthe lower pressure over the ice and
diffuse onto the ice crystals.diffuse onto the ice crystals.
 The vapour will be condensed andThe vapour will be condensed and
freeze onto the ice crystal, causing itfreeze onto the ice crystal, causing it
to grow larger.to grow larger.

23. Mechanism ofMechanism of
Precipitation DevelopmentPrecipitation Development
 1. Coalescence theory1. Coalescence theory
 2. Bergeron-Findeisen process2. Bergeron-Findeisen process
* Are these theories contradictory?* Are these theories contradictory?
Why or why not?Why or why not?