precipitation. precipitation? what is precipitation?

PrecipitationPrecipitation

Precipitation?Precipitation?

What is precipitation? What is precipitation?

What are the forms of precipitation?What are the forms of precipitation?

Forms of PrecipitationForms of Precipitation 1. 1. RainRain - liquid deposits falling from the atmosphere - liquid deposits falling from the atmosphere

to the surface to 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 the forces become to weak to be held in the

mass of water together as a single dropmass of water together as a single drop

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 point with 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 above temperature of rain develops must be above freezing freezing

e.g. within a temperature e.g. within a temperature inversioninversion

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:<0ooC)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

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 with with mid-mid- latitude cyclones latitude cyclones

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 are white 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

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

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.

Necessary conditions? Necessary conditions?

When clouds form in the atmosphere, they When clouds form in the atmosphere, they are non-precipitating in 99% of cases.are non-precipitating in 99% of cases.

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 droplets 4. the growth of cloud droplets - - the the most criticalmost critical stage!! stage!!

Water dropletsWater droplets and and ice crystalsice crystals of clouds need of clouds need to be to be transformed into transformed into heavier particlesheavier particles if they if they are to fall out of clouds as precipitation.are to fall out of clouds as precipitation. Why? Why?


Water dropletsWater droplets and and ice crystalsice crystals of clouds need of clouds need to be to be transformed into transformed into heavier particlesheavier particles if they if 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 crystals As we know, water droplets & ice crystals can can only fallonly fall to the Earth’s surface if they grow to a to the Earth’s surface if they grow to a sufficiently large sizesufficiently large size that that can can overcome overcome updraftsupdrafts


Raindrop sizeRaindrop size::

drizzle about 0.2 mmdrizzle about 0.2 mm

light rain about 0.5 mm light 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 the How about the condensed sizecondensed size generated from generated from

condensationcondensation? (Make a guess?)? (Make a guess?)


Raindrop sizeRaindrop size::

drizzle about 0.2 mmdrizzle about 0.2 mm

light rain about 0.5 mm light 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 – 200 Between 50 – 200 µm µm (size of drizzle only)(size of drizzle only) (where 1 mm = 1000 (where 1 mm = 1000 µmµm))

Mechanism of Mechanism of Precipitation DevelopmentPrecipitation Development

1. Coalescence theory1. Coalescence theory

2. Bergeron-Findeisen process2. Bergeron-Findeisen process


1. 1. Collision - Coalescence theoryCollision - Coalescence theory A droplet may continue to grow by A droplet may continue to grow by

diffusion beyond 20 micrometers indiffusion beyond 20 micrometers in

diameter, however, once a droplet diameter, however, once a droplet

attains this size, growth is slow and attains this size, growth is slow and

inefficient. inefficient. Droplets this large begin to collide Droplets this large begin to collide

and coalesce with other droplets as and coalesce with other droplets as

they fall through the cloud, meaningthey fall through the cloud, meaning

they will bump into and bond to one they will bump into and bond to one

another and form larger drops. another and form larger drops.


1. 1. Collision - Coalescence theoryCollision - Coalescence theory Updrafts in a cloud can transport Updrafts in a cloud can transport

a droplet upward repeatedly a droplet upward repeatedly

allowing it many opportunities to allowing it many opportunities to

fall back down through the cloud fall back down through the cloud

and collide and coalesce with otherand collide and coalesce with other

droplets. droplets. Tiny aerosol nuclei grow into large Tiny aerosol nuclei grow into large

water droplets more than 10,000 water droplets more than 10,000

times their initial size. times their initial size.


2. 2. Bergeron – Findeisen processBergeron – Findeisen process also known as the also known as the cold raincold rain or or ice crystal processice crystal process As the formation of precipitation in the As the formation of precipitation in the

cold clouds of the mid and upper cold clouds of the mid and upper

latitudes by ice crystal growth. latitudes by ice crystal growth. The equilibrium vapor pressure over The equilibrium vapor pressure over

water is greater than the saturation water is greater than the saturation

vapor pressure over ice, at the same vapor pressure over ice, at the same

temperature. temperature.


2. 2. Bergeron – Findeisen processBergeron – Findeisen process Therefore in a mixed phase cloud, the liquid water will be Therefore in a mixed phase cloud, the liquid water will be

out of vapor pressure equilibrium and will evaporate to reout of vapor pressure equilibrium and will evaporate to reach equilibrium. ach equilibrium.

The water droplets will move toward The water droplets will move toward

the lower pressure over the ice and the lower pressure over the ice and

diffuse onto the ice crystals. diffuse onto the ice crystals. The vapour will be condensed and The vapour will be condensed and

freeze onto the ice crystal, causing itfreeze onto the ice crystal, causing it

to grow larger. to grow larger.


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?

precipitation. precipitation? what is precipitation?

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