Pagasa Weather Insturments

8/10/2019 Pagasa Weather Insturments

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INTRODUCTION

The weatherman just like any professional requires certain instruments

to assist him in the conduct of his calling. He uses sophisticated instruments

developed through the years. Little by little, due to advances in the science ofmeteorology and the advent of more sophisticated instruments, a forecaster

is approaching the threshold where he can forecasts with confidence the

weather for the following day and optimistically, a year later.

The Philippines is not far behind developed countries in instrumentations,

specially when one speaks of basic weather instruments. They are all the

same the world over with slight differences in construction and gradation

as dictated by geographical requirements.

BASIC WEATHER PARAMETERS AND THE INSTRUMENTS USED

The following weather parameters are the minimum requirements

to effectively forecast weather. A brief description of the instruments that

PAGASA uses accompanies the discussion of these weather parameters.

Simplified illustrations given are practicable.

TEMPERATURE

The temperature is the degree of hotness or coldness of a certain body.

In the Philippines, it is measured in degrees Celsius (0C).

In weather forecasting, temperature (actual, surface and temperature

ranges) are important as they give indications, to a certain extent, of the

development and changes of weather conditions.

Temperature change is one of the principal causes in changes of other

basic weather elements. Temperature variations over lands and ocean

result to a range of weather conditions from the gentlest breeze to the most

violent storms. Temperature also affects the development and formation of

clouds, the source of our precious water, when these clouds eventually fall

as rain. It is, then, imperative that variations in temperature be considered

in weather forecasting as they play an important part in the improvement

or deterioration of weather conditions.

Through modern instruments, actual temperature in the atmosphere andsurface temperatures are obtained. Surface temperature is the temperature

of free air at a height between 1.25 and 2.00 meters above the ground.


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The following instruments that measure temperature are commonly

used:

a.) Thermometer

A thermometer (Fig. 1a) measures the degree of hotness or coldness

of a given substance.

It operates on the principle of thermal expansion of the material used,

e.g. liquids like mercury and alcohol, metallic materials etc. Mercury is one

of the liquids very sensitive to changes of temperature. When the substance

to be measures is warm, mercury expands and rises in the capillary tube.

When it cools, mercury contracts.

b.) Maximum Minimum thermometer

In order to measure the temperature range, a set of maximum and

minimum thermometer (Fig. 1b) are used. A maximum thermometer

has a constriction above the bulb that permits the mercury to rise in the

capillary tube but does not allow it to descend the capillary tube unless the

thermometer is reset. The highest point that the mercury reaches indicatesthe maximum temperature for the period. The minimum thermometer,

on the other hand, gives the lowest temperature. It uses colored alcohol

(because of its low freezing point). It is placed at an angle of about 20 .

The black float B called index (Fig. 1c) is pulled down slope to the lowest

temperature of the day by two forces; a) the surface tension at the top of

the alcohol column and b) the force of gravity.

c.) Thermograph

A thermograph (Fig 2) is an instrument that records air temperature

continuously on graphing paper. It usually consists of a cylinder made to

revolve once each week by means of clockworks inside. A sheet of graph

paper is fastened on the outside. A pen point that rests on the paper

traces the temperature curve, according to the expansion and contraction

of a sensitive metallic coil or strip corresponding to the reading of a

thermometer.

These instruments are housed in a thermometer shelter (Fig. 3) which

has double-louvered sides and double-top roofing designed to permit air

to circulated freely through the shelter.


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ig. . Thermograp

Fig. 3. Thermometer Shelter or Screen. In the Northern Hemisphere its door

faces north to prevent the suns rays from directly affecting the intrument

eadings whenever it is opened


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ATMOSPHERIC PRESSURE

Gas molecules exert forces on each other and their environmental they

collide. The magnitude of these forces depends upon the temperature

of the gas and the number of molecules involved. These collision forcesare expressed in terms of quantity called pressure. Pressure difference

is principally related to temperature differences and to the number of

molecules exerting pressure forces. Atmospheric motion results from

pressure variations. The atmospheric pressure on a given surface is the

force exerted by an overlying column of air extending to the outer limit of

the atmosphere per unit area.

To measure atmospheric pressure, a barometer is used, which is

commonly of two types. These are:

a.) Mercurial Barometer Mercurial barometer

A mercury barometer (Fig.4a) is a simple barometer made by filling a

glass tube 32 inches long with mercury and inverting it so that the open

end of the tube is below the surface of mercury in a cistern. The height of

the mercury column is measured by sliding a vernier attached on a scale.

To obtain accurate measurements, corrections are made for temperatureexpansion of the instruments, gravity and latitude. Values are read in

millibars, millimeter or inches of mercury.

b.) Aneroid Barometer

An aneroid barometer (Fig. 4b) is made by exhausting the air from a

thin, circular, metallic box, with practically no air on the inside and an air

pressure of 14.6 pounds per square inch on the outside, the box would

collapse except for a strong spring inside. If one side of the box is fixed,

the other side will move due to changes in atmospheric pressure. The

surface of the metallic box is corrugated to increase the area exposed to

the air. The movement of the spring causes a pointer to move over a scale

of figures corresponding to the readings of a mercury barometer. Since air

pressure decreases with increase in altitude, the aneroid is used to make

altimeters (Fig. 5) On the altimeter, the scales is marked off in hundred

and thousands of feet or meters above sea level. The altimeter is a basic

instrument in aeronautical stations and on board an aircraft.

c.) Barograph

A barometer (Fig. 5a), on the other hand, is a recording barometer. The

pen point that traces the pressure curve on the paper is made to move up


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Fig. 4a. Mercurial Barometer

Fig. 4b. Aneroid Barometer

or down by means of a series of levers attached to the aneroid cells in

tandem. The aneroid cells in tandem provide a more pronounced response

to changes in atmospheric pressure than would be indicated by a single

aneroid of the same size.

WIND

Wind is measured in terms of its velocity. Wind velocity has a vectorial

notation and (usually) refers to both the speed and direction. Speed is

the distance to which an ob ect travels at a certain instant. Wind speed

is usually expressed in meters per second (mps) and the more popular

kilometers per hour (kph). On the other hand, wind direction refers to thedirection of the compass point from where the wind is coming. Thus, when

we say southwest winds, the wind is coming from the southwest and blowing

towards the northwest.

ATMOSPHERIC PRESSURE MEASURING DEVICES


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Fig. 5. Altimeter

Fig. 5a. Barograph


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SURFACE WIND VELOCITY AND DIRECTION

To accurately measure the wind speed and direction PAGASA uses

several instruments: These are:

a. n ane

A wind vane (Fig.6) is used to indicate wind direction. It consists

basically of an asymmetrically shaped object with its center of gravity about

a vertical axis. The front end of this object (in most cases as arrow) which

officers the greater resistance to the motion of the air points to the direction

from where the winds comes. The direction of the wind is determined by

reference to an attached oriented compass rose.

b. Anemometer

An anemometer (fig. 6a) measures the wind speed and is made of

propeller cups which are rotated by the motion of the wind. The essential

parts of the cup anemometer are the cup wheel, a vertical shaft, thenecessary mechanism for counting the revolution of the shaft or indicating

its instantaneous speed of rotation.

c. Aerovane

An aerovane (Fig. 6b) indicates both the wind direction and wind speed

or simply the wind velocity. It is shaped like an airplane. The nose of the

plane ports to the direction from which the wind comes and the two-bladed

propeller measures the wind speed. The propeller shaft is coupled to a

small dynamo which generates current. The amount of current generated

depends on the rate of rotation of the propeller which depends on the speed

of the wind. The generated current activates a dial which gives the direct

reading of the wind speed.

d. Anemographs

Anemograph gives direct record of the variations of wind velocity.


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Fig. 6a. Rotating up Anemograp

Fig. b. An aerovane with its

component, a wind indicator.

Fig. . Windvane


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ATMOSPHERIC HUMIDITY

Humidity is the amount of water vapor or moisture content of the air.

The amount of water vapor in the air affects human comfort. When the air

s very mo st or as g um ty, evaporat on s very s ow so muc so t atperspiration remains on the surface of the skin. This makes a person feel

warm and uncomfortable. Humidity measurement is a useful parameter

for weather forecasting in determining whether or not it will rain.

To measure humidity, the following instruments are used:

a. Sling Psychrometer

The sling psychrometer (Fig. 7a) consists of a dry and wet-bulb

thermometer. The term bulb refers to that portion of the glass tube where the

mercury is stored. The dry and wet bulbs are exactly alike in construction.

The only difference is that the wet-bulb has a piece of muslin cloth or wick

wrapped around its bulb and which is dipped in water shortly before the

psychrometer is read.

This is how it is done. The weather observer first wets the cloth cladding

the wet-bulb, whirls the psychrometer a few times, then reads the wet bulb.

He reads the dry-bulb last. Normally, the wet-bulb reading will be lower than

the dry-bulbs. The dry-bulb reading is the air temperature. The difference

between the dry and wet-bulb reading will give, with the aid of psychrometric

table, the dew point temperature and the relative humidity. (Dew point

temperature at which the water will condense while relative humidity is

the ratios of the amount of water vapor actually present in the air to the

maximum amount of water vapor the air can hold at a given temperature.

b. Hygrometer

The other instrument used to measure humidity is the hygrometer

(Fig. 7b). The hygrometer is less accurate than the psychrometer. It uses

human air from which the oil has been removed by using ether. The hair

becomes longer as the relative humidity of the air increases. This changecan be made to move an indicator needle which moves over a scale, the

graduations of which reads from 0% to 100%.


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HUMIDITY MEASURING DEVICES

Fig. 7b. Hygrothermograph

Fig. 7a. Sling Psychometer Fig. 7b. Hygrometer


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Fig. . An -inch raingauge and its

parts. a. measuring stick; b. tube; c.

receiver and d. over ow can.

c. Hygrothermograph

The hygrothermograph (Fig. 7c) measures and records relative humidity

and temperature on graph paper in the same manner as the thermograph

an arograp o.

PRECIPITATION

When the water vapor in the air aloft cools, it is transformed into water

droplets that form the cloud we see in the sky. When these water droplets

become large and heavy enough that the air could no longer support them,

the water droplets eventually fall as rain, snow, sleet or hail. Rainfall is one

such results of precipitation process.

PRECIPITATION MEASURING DEVICES

a.) measuring stick


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To measure the amount of rainfall, raingauge is used. There are two types

of raingauge used by PAGASA. The 8-inch raingauge and the tipping

bucket raingauge.

a. 8-inch Raingauge

An 8-inch raingauge (Fig. 8), so called because the inside diameter of

the receiver is exactly 8 inches, is provided with a funnel that conducts rain

into a cylindrical measuring tube. The volume of the receiver is 10 times

the volume of the measuring tube. Therefore the actual depth of rainfall is

increased ten times on being collected in the smaller measuring tube.

To measure the amount of rainfall accumulated in the measuring tube,

a thin measuring stick with the magnified scale printed on its face is used.

The precisely dimensioned measuring tube has a capacity of 2 inches (50.8

millimeters). Rainfall exceeding this amount spills into the overflow but can

be easily measured by pouring it into the measuring tube for total rainfall.

Used this way, the gauge has a total capacity of 20 inches.

b. Tipping Bucket Raingauge

Another type of rainfall recording instrument is the tipping-bucket

raingauge (Fig. 9). It is an upright cylindrical that has a funnel-shaped

receiver. The precipitation collected by the receiver empties into one side of

a tipping bucket, an inverted triangular contraption partitioned transversely

at its center, and is pivoted about a horizontal axis. Once it is filled with rain,

it tips, spilling out water and placing the other half of the bucket under the

funnel. The tipping activates a mercury switch causing an electrical current

to move the pen in the recorder. Each tipping is equal to one millimeter ofrainfall.

CLOUDS

Clouds are either composed of water-droplets or ice-crystals dependent

upon their altitude and temperature conditions.

In observing clouds, an accurate description of both type and size plays

an important part in the analysis and forecasting of weather.


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Fig. 9 Tipping Bucket Raingauge (and parts inside)

Parts of an 80-inch raingauge


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Thus, for this purpose an International Classification of clouds was prepared

and adopted by most countries.

In observing cloudiness (the extent where clouds cover the sky), theobserver uses his eyes to determine the presence of cloud layers and

the lateral extent of cloud coverage. He must also be familiar with the

genus and species of each cloud present. On the basis of knowledge and

experience, he estimates the height of each layer or measure it with the

aid of instruments.

To determine the height of the cloud base, PAGASA uses a ceiling light

projector(Fig. 10) and a ceiling balloon.

a.) Ceiling Light Projector

A ceiling light projector is vertically a narrow beam of light into a cloud

base. The height of the cloud base is determined by using a clinometer

located at a known distance from the projector to measure the angle included

by the illuminated spot on the cloud, the observer, and the projector. Fromtrigonometry, the height of the cloud base is equal to the distance of the

observer from the ceiling light projector multiplied by the tangent of the

elevation angle.

b.) Ceiling Balloon

Another away of determining the height of the cloud base is by using aceiling balloon. A ceiling balloon is a meteorological balloon whose rate of

ascent has been predetermined. It is filled with gas lighter than air, usually

hydrogen, and released. The time of release and the time the balloon

disappears into the cloud are recorded. The time difference multiplied by

the rate of ascent will give the height of the base cloud.

SPECIAL INSTRUMENTS

The instruments described earlier are tools for measuring weather

elements prevailing at the surface or near the surface of the earth at a

height not exceeding 10 meters from wherever the observers stands.


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ig. 10. Ceiling Light Projector

Fig. 10a. Ceiling Balloon

The art of weather forecasting however is never completed if theconditions of the air above us are not known. The weather forecaster

needs to know the humidity, temperature, pressure, and speed direction at

different levels of the atmosphere so that he could obtain a better picture

of what the prevailing weather conditions are from the surface upwards.


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These data are called Upper-Air Data. Most of the marked weather

changes and the resulting effects on our daily lives occur at levels higher

that what we observe or feel near the surface of the earth.

Some of the widely used instruments to obtain upper air data are asfollows:

a. PIBAL/Theodolite = Pilot balloon

Theodolite

b. Radiosonde;

c. Rawinsonde;

d. Rawin;

e. Wind-Finding Radar; and

f. Weather Surveillance Radar

a. Pilot Balloon/Theodolite

A pilot balloon (Fig. 11a) is a meteorological balloon that is filled with

gas lighter than air. When the pilot balloon is used in conjunction with a

theodolite it is used to determine the speed and direction of winds at different

levels of the atmosphere. The theodolite (Fig. 10b) is similar to

The elevation and angles of azimuth of the balloon are recorded and

these data at the end of the flight which may last for more than an hour are

transferred to a plotting board. The wind speed and direction at selected

levels are calculated by trigonometric methods.

Night observation is accomplished by attaching a lit paper lantern to

t e a oon.

b.) Radiosonde

An airborne instrument used for measuring pressure, temperature and

relative humidity in the upper air is the radiosonde (Fig. 11). The instrument

is carried aloft by a meteorological balloon inflated with hydrogen. The

radiosonde has a built-in high frequency transmitter that transmits data from

the radiosonde meter and recorded on the ground by a specially designed

radiosonde receiver.


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Fig. a. Pilot Balloon

Fig. b. Theodolite


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Fig. 12. Radiosonde attached to a Meteorological balloon

c.) Rawinsonde

A more sophisticated version of this instrument is the rawinsonde.

The rawinsonde (Fig. 12) is an electronic device used for measuring wind

velocity, pressure, temperature and humidity aloft. It is also attached to

a balloon and as it rises through the atmosphere, it makes the required

measuremen s.

The data gathered are then converted to radio signals which are

received by a receiving set on the ground where they are decoded and

evaluated.

d.) Rawin

Another special instrument is the Rawin which is short for Radar and

Wind. It is an electronic device that measures pressure, temperature and

humidity.

e.) Wind Finding Radar

Another instrument is the Wind Finding Radar (Fig. 13). It determines the

speed and direction of winds aloft by means of radar echoes. A radar target

is attached to a balloon and it is this target that is tracked by ground radar.


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Fig. 14. Weather Surveillance Radar

Fig. 12. Rawinsonde Antenna

Fig. 13. Wind Finding

a ar ntenna


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The bearing and time of interval of the echoes is evaluated by a receiver.

f.) Weather Surveillance Radar

A weather surveillance radar (Fig. 14) is of the long range type whichdetects and tracks typhoons and clouds masses at distance of 400 kilometers

or less. This radar has a rotating antenna disk preferably mounted on top

of a building free from any physical obstruction. Radio energy emitted by

the transmitter and focused by the antenna shoots outward through the

atmosphere in a narrow beam. The cloud mass, whenever it is part of a

typhoon or not, reflects a small fraction of the energy back to the antenna.

This reflected energy is amplified and displayed visually on a radar scope.

The distance or slant range of the target from the radar is determined

through the elapsed time to signal is transmitted and then received as an

echo. Its direction is determined by the direction at which the focused beam

is pointing at an instant the echo is received.

WEATHER SATELLITE Modern Tool for Weather Analysis

Polar-Orbiting Satellites

The National Oceanic and Atmospheric Administration (NOAA) satellitesystem consists of satellites in polar orbit at 833 and 870 km. above the

earths surface. Each satellite transmits data from a circular area of the

earths surface with diameter of 2,800 kms. Inboth satellites, one of the

sensors is the Advance Very High Resolution Radiometer (AVHRR) which

is sensitive to visible near infrared and infrared radiation. This instrument

is used for measuring cloud distribution and for determining temperature

of radiating surface (clouds or surface).

Another sensor is the TIROS Operational Vertical Sounder (TOVS)

system which is used to calculate the temperature profiles from the surface

to 10 mb, water vapor content at three levels of the atmosphere and total

ozone content.

Geo-stationary Meteorological Satellite

The most valuable feature of Geostationary Meteorological Satellites

(GMS) is that they can globally observe atmospheric phenomena uniformly,

including overlying areas in sea, desert and mountain regions where weather

observation is difficult.


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Fig. 15a. Geostationary Meteorological Satelite Antenna

The GMS of Japan is a spin stabilized satellite that is placed in

geosynchronous orbit about the equator and 140 degree longitude.

The GMS provides a real time digital cloud image (Stretched-VISSR)broadcast to the users, which are the Medium Scale Data Utilization Station

(MSDUS). The S-VISSR data can be processed not only by a high grade

computer system but also by an ordinary personal computer system.

PAGASA has both the GMS AVHRR, the NOAA polar orbiting satellite

ground receiving facility. Both are located in Diliman, Quezon City.

Satellite data coming from both the orbital and geo-stationary satellitesare used for monitoring the development of severe weather systems, locating

tropical cyclones centers, determining the cyclone s present intensity and

future movement and weather forecasting.


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MODIS

MODIS (Moderate Resolution Imaging Spectroradiometer) is a key

instrument aboard the Terra (EOS AM) and Aqua (EOS PM) satellites.

Terras orbit around the Earth is timed so that it passes from north to southover the equator in the morning, while Aqua passes south to north over

the equator in the afternoon. Terra MODIS and Aqua MODIS are viewing

the entire Earths surface every 1 to 2 days, acquiring data in 36 spectral

bands or groups of wavelengths.

These data will improve our understanding of global dynamics and

processes occurring on the land, in the oceans and in the lower atmosphere.

MODIS is playing a vital role in the development of validated, global,

interactive Earth system models able to predict global change accurately

enough to assist policy makers in making sound decisions concerning the

protection of our environment.

The qualitative and quantitative estimates and display of atmospheric

parameters and a few oceanographic elements from newly acquired NOAA

HRPT Receiving Systems of PAGASA enables the agency to monitor,

forecast and predict weather and climate and issue early warning of

associated hazards.

MODIS also provides finer horizontal-scale atmospheric vapor gradient

estimates which is a valuable input in weather forecasting. Data derived

from the system used to monitor flood inundation areas. Acquisition of this

new technology strengthens PAGASA farm weather forecasting using data

from the multi-spectral band imaging instrument.

Fig. 16. MODIS

Satellite Reciever


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Multi-functional Transport Satellite (MTSAT)

To improve meteorological services over a wide field of activity (such

as weather forecasts, natural-disaster countermeasures and securing safetransportation), the MTSAT series replaced the GMS series that had been in

operation since 1977. It has taken over the role of the GMS series, covering

East Asia and the Western Pacific region from 140 degrees east above the

equator.

It also provides information to 27 countries and territories in the region,

including imagery for monitoring the distribution/motion of clouds, sea

surface temperatures, and distribution of water vapor.

The MTSAT series carries a new imager with a new infrared channel

(IR4) in addition to the four channels (VIS, IR1, IR2 and IR3) of the GMS-5. Its

imagery is more effective than GMS-5 imagery in detecting low-level cloud/

fog and estimating sea surface temperatures at night and has enhanced

brightness levels, enabling a whole new level of image imagery.

By further computation of cloud imagery, data obtained by MTSATs

observations can be used to calculate wind data for numerical weather

prediction; make nephanalysis charts and analyze the distribution of cloud

amounts accor ng to area.

The Imager scans the earth by moving an internal scan mirror in an

east-west and north-south direction. The light reflected by the mirror is

converted into a beam and channeled through a system of lenses and filters

and is separated into one visible and four infrared channels.

The beam intensities are converted to electric signals by visible and

infrared detectors and these signals are transmitted to the Meteorological

Satellite Centers Command and Data Acquisition Station (CDAS).

PAGASAs weather forecasting has significantly improved with the

availability of high resolution satellite imageries both from the MTSAT and

MODIS installed at the Weather and Flood Forecasting Center (WFFC)

Building in Quezon City. A redundant Meteorological Satellite High

Resolution Imaging (MTSAT-HRIT) was also installed at Cebu PAGASA

Complex Station.


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Fig. 17. MTSAT Satellite Receiver Facility


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Department of Science and Technology

PHILIPPINE ATMOSPHERIC, GEOPHYSICAL AND

ASTRONOMICAL SERVICES ADMINISTRATION

Science Garden, Agham Road, Diliman, Q.C.

ww.pagasa.dost.gov.phemail: [email protected]

elefax: 434-2696 / 927-9308

tracking the sky... helping the country


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Department of Science and Technology

PHILIPPINE ATMOSPHERIC, GEOPHYSICAL AND ASTRONOMICAL

SERVICES ADMINISTRATION (PAGASA)

P bli I f i d I i l Aff i S ff

PAGASA Synoptic Station

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