precipitation 5

8/10/2019 Precipitation 5

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PRACTICAL 1: LECTURE


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Metrological Variables

Following variables arerecorded in a Weather

Station:

1. Wind velocity/direction

2. Rainfall

3. Relative humidity and

temperature4. Pressure

5. Radiation, etc


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weather station(Instruments)

Anemometer

Radiometer

Tipping bucket raingage

Relative humidity

and temperature


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ANEMOMETER

An anemometer

measures wind speed

and direction.

The anemometer is a

stand-alone structure

with no moving parts.


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THERMOMETER

A thermometer

( from the greek thermo meaning

warm and meter meaning tomeasure).

It is a device to measure temperature ortemperature gradient using different principles.

It has two important elements:1. Temperature sensor (e.g. the bulb on

mercury thermometer) in which

some physical change occur with temperature

2. Some means of converting this physical change

into a value ( e.g. the scale on mercury thermometer).


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THERMOGRAPH

A Thermograph is a chart recorder that measures

and records temperature. It Uses a bi-metallic strip

for temperature reading and includes one red and

blue felttipped pen to record the reading.

A thermograph usually consists of a pen that

records temperature on a revolving cylinder. Thepen is at the end of a lever that is controlled by a bi-

metal strip of temperature-sensitive metal which

bends as the temperature changes.


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THERMOGRAPH

In this instrument the

movement of recording

pen is controlled by

change in curvature of

bi-metallic strip one

end of which is rigidly

fixed to the armattached to the frame.


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BAROMETER

Barometer consists of a long glass

tube sealed at one end and filled

with mercury.

The tube is inverted so that openend is submerged in cistern of

mercury.

The height of the mercury in the tube is measure

of atmospheric pressure acting in the cistern. Readings are taken by first adjusting mercury

level in the cistern to a zero point and thenmoving a vernier to the top of the mercury

column.


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BAROGRAPH

This consists essentially of an

evacuated metal chamber with

flexible metal walls.

As atmospheric pressure

changes, the changing

pressure difference between

the inside and outside causes

the chamber to expandand contract.

These movements are magnified mechanically to move a penacross a recording chart.

The chart is wrapped around a drum which rotates at a rate ofone revolution per week.


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Measurement of Precipitation

Amount of precipitation: vertical depth of

water that would accumulate on a level

surface (inch, ft, cm, mm etc)

Intensity or Rate of Precip: amount per unit

time

This is measured by Rain Gauges: two types

Non-recording or standard rain gages

Recording rain gauge


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RAIN GAUGES

A rain gauge is a type of instrument

used by meteorologists and

hydrologists to gather and measure the

amount of liquid precipitation.

Rain gauges should be placed in an open

area where there is no obstructions,

such as buildings or trees , to block the

rain.

This is also done to prevent water

collected on the roofs of building or the

leaves of trees from dropping into the rain gauge after a rain

resulting in incorrect reading.


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Location of Gages

Gauges measurepoint rainfall

True precipitation unaffected by surroundings-

winds, trees, buildings

Clearance distance 2 times height of object

For large areas multiple gauges are needed for

more accurate estimates


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A rain gauge is a device that captures rainfall and

measures how much of it has fallen. The diameter of

the "catching hole" is an agreed standard and the

measurement is done by pouring the collected rainfall

into a measuring tube of an agreed standard diameter.The depth of rain in this tube is calibrated in

millimeters or inches and this is the published

measurement of how much rain has fallen.

Measurements are usually taken once every 24 hoursusually at or around 9 am in the morning although

some countries measure it at different time of the day.

Most rain gauges generally measure the precipitation in

millimeters. The level of rainfall is sometimes reportedas inchesor centimeters.

Rain gauge amounts are read either manually or by

AWS (Automatic Weather Station). The frequency of

readings will depend on the requirements of the

collection agency.
http://en.wikipedia.org/wiki/Meterhttp://en.wikipedia.org/wiki/Inchhttp://en.wikipedia.org/wiki/Centimeterhttp://en.wikipedia.org/wiki/Automatic_weather_stationhttp://en.wikipedia.org/wiki/Automatic_weather_stationhttp://en.wikipedia.org/wiki/Centimeterhttp://en.wikipedia.org/wiki/Inchhttp://en.wikipedia.org/wiki/Meter


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Standard rain gaugeThe standard rain gauge, developed around the start of

the 20th century, consists of a funnel attached to a

graduated cylinder that fits into a larger container. If the

water overflows from the graduated cylinder the outside

container will catch it. When measurements are taken,

the cylinder will be measured and then the excess willbe put in another cylinder and measured. In most cases

the cylinder is marked in mm and will measure up to

25 mm (0.98 in) of rainfall. Each horizontal line on the

cylinder is 0.2 mm (0.007 in). The larger container

collects any rainfall amounts over 25 mm that flows

from a small hole near the top of the cylinder. A metal

pipe is attached to the container and can be adjusted to

ensure the rain gauge is level. This pipe then fits over a

metal rod that has been placed in the ground.


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Tipping bucket rain gauge The tipping bucket rain gauge consists of a large copper

cylinder set into the ground. At the top of the cylinder is afunnel that collects and channels the precipitation. Theprecipitation falls onto one of two small buckets or leverswhich are balanced in same manner as a scale (or child'sseesaw). After an amount of precipitation equal to 0.2 mm(0.007 in) falls, the lever tips and an electrical signal is sent

to the recorder. The recorder consists of a pen mounted onan arm attached to a geared wheel that moves once witheach signal sent from the collector. When the wheel turnsthe pen arm moves either up or down leaving a trace on thegraph and at the same time making a loud click. Each jumpof the arm is sometimes referred to as a 'click' in reference

to the noise. The chart is measured in 10 minute periods(vertical lines) and 0.4 mm (0.015 in) (horizontal lines) androtates once every 24 hours and is powered by a clockworkmotor that must be manually wound.
http://en.wikipedia.org/wiki/Seesawhttp://en.wikipedia.org/wiki/Seesaw


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Sources of Error in Gauges

Some water is used to wet the surface ofinstrument

Rain recorded may be less than normalhappened due to direction and speed of wind

Dents in the collector may cause error

Some water is absorbed with measuring stick

Losses due to evaporation may also occcur

Volume of stick may also replace some volumeof rainwater


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Recording Rain Gauges

Float type

Weighing type

Tipping bucket type

SOURCES of Error in Recording Gauges

Dents in collector Moistening of inside surface of funnel and tube

Rain drops splashing from collector

For very intense rain some water is still pouring into

already filled bucket

Inclination of gauge may give less or more reading

Error due to wind

R di l M f E


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Remedial Measures for Error

Precipitation Dents need repair

Wetting of surface and sides need somecorrection factor

Inclined instrument need to be reinstalled

Correction factor for inclination angle can becalculated: P (actual) = P(measured)/ Cos

Example: P (measured) = 125 mm rainfall

P(act) = ???

Angle of inclination = 20 degreeP(act) = P(measurd) / Cos= 125/ Cos (20) =

133mm


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Installation of Rain Gauge Network

More no of gauges more representative data but we

need to observe economy, accessibility of certain area,topography so optimum solution is needed.

WMO ( world meteorological organization) hasrecommended following for minimum for a cathment:

For flat regionat least one in 230 345 sqmiles. One station in 345

1155 sq mile is also

acceptable

Mountainous regions of Temperate,Mediterranean and Tropical Zones, ideal is

atleast one in 35

95 sq miles, However one in95385 sq miles also acceptd.

In arid and polar regions one station in 5753860 sq miles acceptable.


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Symon's rain gauge (non-recording)

It consists of funnel with a circular rim of 12.7 cmdiameter and a glass bottle as a receiver.

The cylindrical metal casing is fixed vertically to the

masonary foundation with the level rim 30.5 cmabove the ground surface.

The rain falling into the funnel is collected in the

receiver and is measured in a special measuringglass graduated in mm of rain fall when full it can

measure 1.25 cm of rain .


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Tipping Bucket Rainguage (recording)

The gauge registers precipitation(rainfall) by counting smallincrements of rain collected.

When rain falls into the funnel it

runs into a container divided intotwo equal compartments by apartition

When a specified amount of rain hasdrained from the funnel the bucket

tilts the opposite way. The number and rate of bucket

movements are counted and loggedelectronically.


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Tipping bucket rain gage


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Weighing type rain gauge (recording)

It consists of funnel, catch bucket, spring

balance , open and cock mechanism.

When a certain weight of rainfall is collected

in a tank , which rests on a spring spring-lever-

balance , it makes a pen to move on a chart

wrapped on a drum.

The rotation of drum sets the time scale while

the vertical motion of the pen records the

commulative precipitation .


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Float type rain gauge (recording)

It consists of funnel, filter chamber, float, spin

chamber, siphon, pen and clock mechanism.

In this type rain is collected in a float chamber, the

float moves up which makes a pen to move on a

chart wrapped round a clock driven drum.

When the float chamber fills up, the water siphons

out automatically through siphon chamber.

The clock revolves the drum once in 24 hours. This

type of rain gauge needs rewinding once in a week.


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PSYCHROMETER

Psychrometers are used for the determination of

relative humidity.

Here, dry and wet bulbs thermometers are used.

The dry bulb measures the actual temperature at

any instant whereas the wet bulb measures the

air temperature as it may exit when the moist

wick on the bulb gets cooled as a result ofevaporation of moisture from the which to

maintain moisture equilibrium with the air.


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Evaporation pan


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Pan Evaporation Method for ET

Evaporation from pans and ET are governed by the

same climatic factors. There is a strong correlation

between them.

The pan evaporation is related to grass ET by the

relation: ET = Kp .E

where,

E = pan evaporation in mm/day.

Kp= Pan coefficient.


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precipitation 5

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