formation of precipitation

7/29/2019 Formation of Precipitation

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Formation ofPrecipitationEngr. Dielene Fajutagana De

Guzman


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NECESSARY CONDITIONS FOR

THE OCCURRENCE OF

PRECIPITATION

creation of saturation conditions

formation of water droplets or icecrystals

growth of water droplets or ice

crystals


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Creation of saturation conditions

Cyclonic

It is the lifting of the air converging into a low pressurearea

Frontal cyclones are called extratropical because theygenerally occur beyond the tropics while non-frontalcyclones generally occur in tropical regions of theworld.

Examples of non-frontal cyclones are the hurricanes(typhoons)


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Other names associated with the

formation of non-frontal cyclones:Tropical depressions: 40 v < mph

Tropical storms: 40 v < 75 mph

Hurricanes (typhoons): 75v mph


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schematic of the air flow for non-

frontal cyclones

Convective. Lifting of the air is

produced by uneven heating of theearths surface. Convective coolinggenerally produces thunderstorms


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schematic of the air flow for non-frontal

cyclones

Orographic. This may occur

when the air flow is deflectedupward by orographic barrierssuch as mountains.


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Formation of Water Droplets or

Ice CrystalsWhen the air is close to saturation it

requires the presence of certain nuclei for

water droplets or ice crystals to form.Nuclei are small particles of varioussubstances of the order of 0.1 10 m(10-

6 m). They are called condensation nucleiwhen they lead to the formation of waterdroplets and freezing nuclei when theylead to ice crystals.


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Growth of water droplets or ice

crystals

Upon nucleation the water droplet or ice crystalgrows to visible size in a fraction of a secondthrough diffusion of water vapor into it, butgrowth thereafter is slow. Generally the growthof water droplets and ice crystals occur due to:

diffusion of water vapor collision of droplets or ice crystals

coalescence (molecular attraction of particles that areclose to each other)


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General Characteristics of Storm

Types Storms caused by frontal cyclones (movement of

extratropical cyclones generally at latitudes of 35 to 50

degrees North) They generally cover large areas

Low to moderate intensity

Highest intensity associated with cold fronts

because the steeper frontal surface gives astronger lift

It travels in the direction west to east

It travels with a speed of about 30 mph


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Storms caused by non-frontal cyclones

(tropical cyclones)

Cooling and lifting of the air caused by convergence in thecentral core.

It originates with a weak disturbance along the thermalEquator

The air movement towards disturbance is deflected bythe Coriolis force which forms a

vortex. The water vapor from a warm ocean is condensed by

rising air at the center

The storm system generally moves westward due to thewesterly wind system


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Storms caused by non-frontal cyclones

(tropical cyclones)

The storm system decays when reaching landbecause of the greater ground friction and thefact that the moisture (energy) supply is cutoff .

Also it dissipates when reaching colder oceans.

They can be design storms for 1,000 to 2,000

sq.mi areas in coastal regions such as southernand southeastern USA, Philippines, Taiwan, andJapan.


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Storms Caused by Thermal

Convection (convective storms)

Cooling and lifting caused by

thermal heating at the groundGenerally occurs in the afternoon

and early evening after the earth

has been thoroughly heated by thesun.


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Typically covers small areas of the order of

5-10 sq-mi. although they may about 300sq.mi.

High intensity over a small area and a

large spatial and temporal variation ofintensity.

Sometimes accompanied by hail.

They serve to construct design storms forsmall basins


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Storms Caused by Orographic

Lifting

Cooling and condensation caused by lifting ofthe moist air mass over a topographic divide.

Precipitation is a function of elevation, rate ofrise and direction or moist air movement.

Generally widespread in a transverse direction

along the orographic barrier.


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Storms Caused by Orographic

Lifting

Moderate to low intensity.

Duration of the storm persists as longas the moist air flow continues.

Orographic patterns oftensuperimposed on other types of storms

(e.g. a hurricane reaching an mountainrange).


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formation of precipitation

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