presentation on precipitation

8/7/2019 presentation on precipitation.

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Presentation

on Precipitation

By Veera Satya Mallesh

07011A0148

4/10/2011


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Objectives

Formation of precipitation

Different forms of precipitation.

measurement of Precipitation.

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General concept

There are several forces acting on a water dropletor ice crystal in a cloud.

Winds

Atmospheric stability Gravity

Drag (friction)

When a droplet reaches a certain critical mass

the force of gravity will exceed the other forcesand precipitation will fall.

Rain drops are 100X larger than cloud droplets

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Formation of precipitation

Formation in the Warm air

1. Condensation and deposition

2. Collision and coalescence

Formation in the Cold air

1. Bergeron process

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Condensation and deposition

As long as the rate of condensation exceeds

the rate of evaporation water will be

deposited and accumulate on condensation

nuclei

Droplets grow very slowly

Rarely produces rain drops

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Collision and coalescence

As water droplets fall through a cloud, the

larger droplets fall faster than the smaller

ones, thus they collide

If the droplets coalesce a larger drop is

formed

If it gets big enough rain will fall

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The Bergeron process

Vapour pressure over ice is less than vapourpressure over water of the

same temperature

Thus, water molecules move from

water to ice and freeze on the ice

If the crystals get large enough they

will fall

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Forms of precipitation

Rain

Drizzle

G

laze Sleet

Snow

Snow flakes

Hail

Graupel

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Rain

When the size of the drops is more than 0.5

mm and not greater than 6.25 mm, as drops

greater than this tend to break up as they fall

through air

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Drizzle

When the size of water droplets is under 0.5

mm, and its intensity is less than 0.01 mm per

hour

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Glaze

When the drizzle or rain freezes as it comes in

contact with cold objects, it is known as glaze.

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Sleet

It is frozen rain drops cooled to the ice stage

while falling through air at sub-freezing

temperature.

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Sleet

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Snow

Precipitation in the form of ice crystals resulting

from sublimation. (water vapour changed

directly to ice).

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Snow flakes

Number of ice crystals fused together form

snow flakes.

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Hail

Hail is lumps or bulbs of ice over 5mm

diameter formed by alternate freezing or

melting as they are carried up and down in

highly turbulent air currents.

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Graupel

Graupel (also called small hail) is formed when

super cooled water droplets of water

condense on a snow flake to form a ball of size

2-5mm

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Rainfall measurement

Precipitation gauge

1 - pole

2 - collector3 - support- galvanized metal sheet

4 funnel

5 - steel ring

1. Non recording gauge

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2. Recording gauge / graphic raingauge

The instrument records the graphicalvariation of the fallen precipitation, the totalfallen quantity in a certain time interval and

the intensity of the rainfall (mm/hour). It allows continuous measurement of the

rainfall.The graphic rain gauge

1-receiver

2-floater

3-siphon

4-recording needle

5-drum with diagram

6-clock mechanism

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3. Tele-rain gauge with tilting baskets

The tele-rain gauge is used to transmitmeasurements of precipitation throughelectric or radio signals.

The sensor device consists of a system withtwo tilting baskets, which fill alternatively withwater from the collecting funnel, establishingthe electric contact.

The number of tilting is proportional to thequantity of precipitation

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Tele-rain gauge

The tele-rain-gauge

1 - collecting funnel

2 - tilting baskets

3 - electric signal

4 - evacuation

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Raingauge Network

Since the catching area of the raingauge is verysmall as compared to the areal extent of thestorm, to get representative picture of a storm

over a catchment the number of raingaugesshould be as large as possible, i.e. the catchmentarea per gauge should be small.

There are several factors to be considered to

restrict the number of gauge: Like economic considerations to a large extent

Topographic & accessibility to some extent.

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Mean Precipitation over an area

Raingauges rainfall represent only point sampling ofthe areal distribution of a storm

The important rainfall for hydrological analysis is a

rainfall over an area, such as over the catchment To convert the point rainfall values at various stations

to in to average value over a catchment, thefollowing methods are used:

arithmetic mean the method of the Thiessen polygons

the isohyets method

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Arithmetic Mean Method

!

!!N

i

i

niP

NN

PPPPP

1

21 1..........

When the area is physically and climatically homogenous and the required

accuracy is small, the average rainfall for a basin can be obtained as the

arithmetic mean of the hivalues recorded at various stations.

Applicable rarely for practical purpose

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Method of Thiessen polygons

The method of Thiessen polygons consists ofattributing to each station an influence zone inwhich it is considered that the rainfall is

equivalent to that of the station. The influence zones are represented by

convex polygons.

These polygons are obtained using themediators of the segments which link eachstation to the closest neighbouring stations

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Thiessen polygons .

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A1

A2

A3

A4

A5

A6

A7

A8P1

P2

P3

P4

P5

P6

P7

P8

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Thiessen polygons .

m

mm

AAA

APAPAP

P

!

.....

.....

21

2211

Generally for M station

!

! !!M

i

i

i

total

i

M

i

i

A

AP

A

AP

P

1

1

The ratio is called the weightage factor of station iA

Ai

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An isohyet is a line joining points of equal

rainfall magnitude.

Isohyetal Method

F

B

EA

C

D

129.2

4.0

7.0

7.2

9.110.0

12

8

8

6

6

4

4

a1a1

a2

a3

a4

a5

6

4

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P1, P2, P3, . , Pn the values of the isohytes

a1, a2, a3, ., a4 are the inter isohytes arearespectively

A the total catchment area

- the mean precipitation over the catchment

Isohyetal Method

A

PPa

PPa

PPa

P

nn

n

!

2

...22

1

1

32

2

21

1

The isohyetal method is superior to the other two methodsespecially when the stations are large in number.

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Thank you

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Documents

presentation on precipitation