5 Basic Meteorological Instruments & Observational Techniques

R P Samui

Table of Contents

1 Learning outcomes

2 Introduction

3 Site for the observatory

4 Hours of observation

5 Order of observation

6 Instruments and their observations

6.1 Wind instruments

6.2 Raingauge

6.3 Radiation instrument:sunshine recorder

6.4 Thermometers

6.5 Thermograph

6.6 Hair hygrograph

6.7 Barometer

6.8 Psychrometers

6.9 Class A pan evaporimeter

6.10 Duvdevani dew-gauge

6.11 Soil moisture observations

7 Non-instrumental observations

8 Summary

1 Learning outcomes

After reading the module you shall be able to:
know the different type of meteorological observations
know about the various meteorological instruments and their functions know about the observational techniques
learn about the importance of each meteorological observation

2 Introduction

Meteorological observations are taken regularly and simultaneously at standard hours of observations all over the world. These help to use these observations for synoptic, climatological and numerical weather prediction studies. Surface meteorological observations of the India Meteorological Department are divided into six classes – i) Class I, equipped with both eye reading and self recording instruments, ii) Class II, equipped only with eye reading instruments, taking regular observations at least twice daily, iii) Class III having the same equipments as class II, but observations are taken only once daily, iv) Class IV, V and VI, have lesser number of instruments or take non- instrumental observations only. However, agrometeorological observations are taken at 0700 and 1400 hrs LMT (Local Mean Time), when crop canopy level micro-climate is assumed to be the same throughout the country. Agromet observations are categorized in three types viz., Principal, Ordinary and Auxiliary.

3 Site for the observatory

A proper site of the observatory not only helps to generate accurate data but also its easy access for multi-purpose use. A well exposed, bare level plot of 55 m (N-S) by 36 m (E-W), located more or less at the centre of the area, free from water logging and easily accessible even during the wettest weather, representative of the soil of the location is ideal for an observatory. The site should be well away from trees, high buildings, main irrigation channels, drainage etc. It is necessary to enclose the observatory with wire fencing for the safety of the meteorological instruments.Recommended layout of instruments in a Surface/Agromet observatory is shown in Fig. 1.

4 Hours of observation

Regular surface observations are taken at 0530, 0830, 1130, 1430, 1730, 2030, 2330 and 0230hours IST, (i.e. 00, 03, 06, 09, 12, 15, 18 and 21 hours GMT) respectively, at Class I observatories. However, two observations i.e., morning and afternoon refer to 0830 and 1730 hours IST, respectively are recorded from most of the observatories, except agrometeorological observations which are taken at 0700 and 1400 hours LMT corresponding to minimum and maximum epoch, which when converted into IST, varies according to the longitude of the station.

5 Order of observation

The instruments at the observatory should be read in the following order commencing from 10

minutes proceeding the hour of observation – 1) Wind instruments, 2) Raingauge, 3) Thermometers and 4) Barometer. Non-instrumental observations (e.g., clouds, visibility etc.) should be taken in the interval of 3 minutes between the first and second readings of the anemometer or before commencing the instrumental observations.

Instruments and their observations

6.1 Wind instruments

Wind is defined as air in motion which is expressed in terms of direction and speed. Wind direction is from which it blows and speed as the rate of movement of air in its instantaneous direction. Wind direction is determined with reference to true north and is expressed to the nearest 10 degree or to 16 points of the compass (Fig. 2). Wind speed is measured in knots (i.e., nautical miles per hour) or in km/hour (1 knot = 1.69 ft s-1 = 0.515 m s-1 =1.853 km h-1 =1.1515 miles h-1).

6.1.1 Wind vane

The wind direction is recorded using wind vane. It is a balanced lever which turns freely about a vertical axis (Fig. 3). Like an arrow, one end of the lever has pointed narrow shape while the other end is like a flat plate with broad surface area. The narrow end points to the direction from which wind is blowing.

6.1.2 Cup counter anemometer

The wind speed is measured by cup anemometer. The standard exposure of wind instruments over level, open terrain is 3 meters above ground. It consists of 3 large semi-conical cups with beaded edges fixed at the ends of three rods. The cups are mounted symmetrically about a vertical axis. As the force on the concave side of any cup, due to the wind is greater than that on a convex side in a similar position, the cup wheel rotates. The cups are attached to a central spider which is mounted on a spindle carrying a worm. The worm engages with gear wheel and drives a revolution counter mounted in a water proof aluminium housing. The counter is read at the beginning and end of a period and the difference is noted. The mean wind speed during the given period is obtained by dividing the difference in counter readings by the time interval in minutes (Fig. 4).

6.1.3 Dines pressure tube anemograph

The instrument is used for continuously recording the instantaneous values of wind speed and direction on a single chart, so that the wind is completely described as a vector magnitude. The detailed structures of the wind, the maximum and minimum gust speed, occurrence of squall are also recorded. Wind direction is measured and recorded by a wind vane and a mechanical twin pen recorder. Wind speed is measured by a Piotic static tube and recorded by a sensitive float manometer. The pilot head is kept facing the wind by vane and as the wind blows into the mouth of the tube, it produces an increase in pressure in it proportional to the square of the wind speed, hence the name pressure tube. The wind blowing past suction holes below the arm of the vane cause decrease of pressure inside the suction tube. The difference in pressure in the pressure and suction tubes is directly proportional to the square of the wind speed and is recorded by the float manometer of a chart wound on a clock-drum (Fig. 5).

Fig. 2: Wind Direction in 16 points of compass

Fig. 3: Wind vane

6.2 Raingauge

The amount of rainfall at a station is measured by a raingauge. FRP (Fibre Glass Reinforced Polyster) raingauges are standard instrument used at all stations in the country.

6.2.1 Ordinary raingauge

The essential parts of ordinary raingauge are – a collector with a metal rim of truly circular shape (100 or 200 sq. cm area), a base, polythene bottle and a measuring glass. Both the collector and the base are made of fibre glass reinforced polyster (Fig. 6). The rim of the raingauge should be exactly horizontal and remain at a height of 30 cm above ground level. To measure the rainfall, remove the funnel/collector of the rainguage and take out the polythene bottle. Place the measuring glass in an empty basin and slowly pour the content of the receiver into the glass taking care to avoid spilling. Read the amount of rain in the measuring glass.

6.2.2 Self -recording raingauge (S.R.R.G.)

It gives a continuous record of the rainfall. The principle governing it is that rain water entering the gauge at top of the cover is led via the funnel to receiver, consisting of a float chamber and siphon chamber. The pen is mounted on stem of the float and as the water level rises in receiver the float rises and the pen records on a chart placed on a clock drum. The clock drum revolves once in 24 hours or 7 days, so that a continuous record of the movement of the pen is made on chart. Automatic siphoning occurs when the pen reaches top of the chart and as the rain continues, the pen rises again from zero line of the chart. If there is no rain the pen traces a horizontal line from where it leaves off rising (Fig.7). The SRRG is installed with rim of funnel placed horizontally at a height, exactly 75 cm above ground level.

Radiation instrument: Sunshine recorder

Sunshine recorder is an instrument for obtaining daily duration of bright sunshine (Fig. 8). It consists of a glass sphere (diameter about 10 cm), mounted concentrically in a section of a hemi-spherical bowl with three slots to hold cards (specially treated with chemical).

i) long curved summer card (at bottom slot) from 13th April to 31st August, ii) short curved winter card (at top slot) from 13th October to end of February and iii) straight or equinoxial card (middle slot) from 1st March to 12th April and 1st September to 12th The instrument must be exposed on a 10 feet pillar inside the observatory area. For setting a sunshine recorder, the adjustments required are concentricity, latitude, meridian and level.

6.3 Radiation instrument: Sunshine recorder

At the bottom of the bowl, latitudes from 10°-35° are marked. The instrument is placed in such a way that the light from the sun, as it passes through glass sphere is concentrated to a point on the card and burns it right through. The cards have hourly markings according to apparent motion of the sun. The burn mark thus shows the time during which the sun is shining. Three types of cards are used: 1)long curved summer card (at bottom slot) from 13th April to 31st August, ii) short curved winter card (at top slot) from 13th October to end of February and iii) straight or equinoxial card (middle slot) from 1st March to 12th April and 1st September to 12th The instrument must be exposed on a 10 feet pillar inside the observatory area. For setting a sunshine recorder, the adjustments required are concentricity, latitude, meridian and level.

6.4 Thermometers

Four different types of thermometers viz., the maximum, minimum, dry bulb and wet bulb thermometers are used in single Stevenson screen. Thermograph and hair-hygrograph are used in double Stevenson’s screen. The essential conditions for the exposure of thermometers are that air should have free access to the bulbs of thermometers and neither the sun rays nor rain should fall on them. Stevenson’s screen is a rectangular box with its sides and door double louvered and with a double layered roof with air space in between. The door opens to the north (in Northern Hemisphere) and the bulbs of the wet and dry bulb thermometers are placed between 1.30 to 1.40 meters above the ground. The tops of the posts of Stevenson’s screen are 1.25 m above the ground.

6.4.1 Maximum thermometer

It is mercury-in-glass thermometer having a constriction near the bulb end (Fig. 9). When temperature of air rises, the mercury in the thermometer bulb expands and forces its way into the stem past this constriction. But when the bulb cools, none of the mercury above the constriction will get back into the bulb thus the length of the mercury column remains the same. The end of the mercury thread farthest from the bulb registers the highest temperature reached in a day.

6.4.2 Minimum thermometer

The liquid used in minimum thermometer is alcohol in which a dumb-bell shaped index is set (Fig. 10). When the temperature falls, the alcohol column drags the index towards the bulb end and when the temperature rises the alcohol column expands and run past the index without disturbing it. Thus the end of the index farthest from the bulb gives the lowest temperature attained in a day.

6.4.3 Dry-bulb thermometer

It is a mercury-in-glass thermometer having a round bulb (Fig. 11). It is exposed at a height of 4 feet 3 inches inside thermometer screen. The reading of this thermometer gives the free air ambient temperature at a place at the time of reading.

6.4.4 Wet-bulb thermometer

It is precisely similar to dry bulb thermometer except that the bulb of the wet-bulb thermometer is always kept wet by means of a muslin sheath fed by distilled water from a bottle through a wick (Fig. 12). The bottle must be placed a little on one side of the bulb to prevent the thermometer from reading too high. The difference between the dry bulb and wet bulb thermometers reading is a measure of moisture content in the atmosphere.

6.4.5 Soil thermometers

The absorption of incoming solar radiation by earth’s surface causes the diurnal variation of temperature which is the maximum at surface and decreases rapidly downwards. This becomes almost negligible at a depth of 30 cm. Soil thermometer is mercury in glass thermometer with a bend of 120 degrees in the stem just above the bulb. The thermometer has a range from – 5 oC to + 70 °C and can be read to an accuracy of 0.l °C.

It is installed with a triangular iron stand bent at 60 degrees so that the thermometer when mounted, makes an inclination of 120owith the ground. Soil thermometers at different depths (0, 5, 10, 20 and 30 cm) are installed along a line running east-west at a distance of 60 cm from the southern side (Fig. 13). Soil temperature probes with digital meters can also be used to record soil temperature.

6.4.6 Grass minimum thermometer

The grass minimum or terrestrial radiation thermometer is used mainly to obtain information about ground frost at night (Fig. 14). It is a sheathed minimum thermometer in which the graduations of stem are protected by an outer glass jacket. The bulb is link-shaped and provides a larger surface for exposure than a spherical bulb. The instrument is exposed on a plot covered with short grass 2.5 to 5.0 cm high. It is kept on two Y-shaped wooden supports with the bulb just touching tips of the blades of grasses. The instrument should be read at the hour of routine morning observation (viz., 0830 hrs IST). After the reading is over, the thermometer is kept inside the screen, bulb downwards, until the evening when it should be reset.

6.5 Thermograph

It is an instrument for obtaining a continuous and automatic record of the air temperature (Fig. 15). The thermograph consists of a temperature sensitive element connected by a system of linkages to a pen recording on a chart fixed on a drum driven by clock-work. The clockwork with drum is mounted on main base and has either a daily or weekly movement. Standard IMD drum of the ‘fixed clock’ type, 93 mm in diameter and 130 mm high is used in thermographs. The sensitive element of thermograph is either a bimetallic strip (like invar and bronze or steel and brass) welded together. One end of the arc or helix is rigidly attached to the frame of the instrument, while the other end is fixed to a horizontal spindle to which the pen arm is screwed. The coefficient of expansion of invar is negligible in comparison with that of bronze. So, when the temperature changes, as a result of the differential expansion of the two metals, the curvature of the arc increases or decreases. This movement is transmitted to the pen point and is recorded on a chart fixed on a revolving clock drum. It is installed in double Stevenson screen.

6.6 Hair hygrograph

It is used for recording continuously the relative humidity of the air (Fig. 16). The length of a human hair varies considerably with the relative humidity. The length of the hair increases as the humidity increases and vice-versa. The change in the length of the hair is proportional to the logarithm of the change in the relative humidity. A suitably designed cam has therefore been introduced in the hygrograph to modify the magnification so that the movement of the pen is proportional to the relative humidity. The hair should be kept clean and free from dust, regularly (once a week) by washing with distilled water. It is installed in double Stevensons’ screen.

6.7 Barometer

The barometer used at the observatories of the IMD is the Kew Pattern Barometer (Fig. 17). Its essential parts are – a glass tube about 90 cm long closed at the top and open below, a cup or cistern and brass scale. The glass tube is filled with mercury (Hg) and its open end is dipped in Hg in the cistern which prevents air from entering the tube. Above the Hg column in the tube is an empty space. Great care is taken to remove all air from this space as its presence even in very minute quantities, will vitiate the readings of the instrument. The Hg column in the tube is supported by the pressure of the air on the surface of the mercury in the cistern. The Hg-level in the cistern is taken into account in the graduation of the scale itself.

6.8 Psychrometers

A psychrometer consists of two types of thermometers, viz., dry bulb (DB) and wet bulb (WB), for measurement of temperature and humidity of the air. Two main types of psychrometer are described below:

6.8 Psychrometers

6.8.1 Assmann psychrometer

In an Assmann psychrometer (Fig. 18a) two sensitive thermometers are mounted side by side in the air tube through which air is drawn by the clock work fan housed in the casing. The bulb of one thermometer is covered with a thin muslin cap which is moistened with distilled water every time when the instrument is used.

Exposure: Observations should be taken in open situation with instrument either suspended from a clamp or bracket attached to a post.

Operation: Moisten the wet bulb by distilled water using the injector. Wind the clock work motor.

Wait for four to five minutes until the wet bulb reading has become steady. Read the WB and DB.

Maintenance/care: I) Distilled or rain water should be used for moistening the WB. II) Thermometers should read correct to 0.1 oC. III) Special humidity table should be used to obtain relative humidity (RH), dew point (DP) and vapour pressure (VP) of the air. IV) Keep the muslin clean and change it as frequently as required.

6.8.2 Whirling psychrometer

In the Whirling psychrometer (Fig. 18b), the aspiration is provided by whirling or rotating the thermometers, which are mounted side by side on suitable wooden frame. To obtain the desirable speed 5 m s-1 psychrometer should be given about 4-5 revolution.

Operation: I. Moisten the WB wick completely using distilled water. II) Whirl the instrument standing with back to the Sun to avoid direct sunlight falling on the instrument. III) Read the wet bulb after about 15 second of whirling, note this reading but do not record it. IV) Continue whirling and read after 10 seconds. If the reading is still dropping rapidly, continue reading at interval of 10 seconds. V) Finally when continued ventilation causes no further drop in WB temperature the lowest reading correct to 0.1 oC is recorded. VI) Lastly read the DB thermometer.

Precautions: I) Avoid direct sunlight falling on bulbs as bulbs are not protected against radiation. II) Special Humidity table is used to obtain RH, DP and VP. III) Only distilled water or rain water is used for moistening WB. IV) The wick or muslin should be replaced as often as necessary to maintain a covering free from dust and before the use of wick/muslin wash it in pure soap water/distilled water..

6.9 Class A pan evaporimeter

This consists of a large circular pan (diameter 122 cm) with stilling well to provide an undisturbed water surface around the point of a hook gauge by breaking any ripple caused by wind. The pan rests over a white painted wooden stand. The pan (Fig. 19) is covered with wire-netting. A thermometer to measure the surface temperature of water is fixed with the bulb just dips in water.

The amount of water lost by evaporation from pan during any interval of time is measured by adding known quantities of water to the pan from a graduated cylinder (diameter 12.2 cm), till the water level touches the reference point (at a height 19.0 cm). The amount of water added equals the amount of water lost by evaporation from the pan and this divided by the time interval, gives the rate of evaporation. Rain falling into the pan is accounted for by assuming that the catch of a nearby raingauge represents the added depth of water due to the rain. The readings are taken at 0830 and 1430 hours IST. Total evaporation is given by

For light rain days with water level below fixed point

6.10 Duvdevani dew-gauge

Duvdevani dew-gauges are used for measuring dew (Fig. 20). The wooden block of 32.0 x 5.0 x 2.5 cm3 dimension, coated with red oxide (Johnsons and Nicholson red oxide) which gives smooth surface representing leaf surface is used for recording dew. The dew-gauges are kept at 5.0, 25.0, 50.0 and 100.0 cm above the ground level. The instrument is set after sunset and the observation is taken just before sunrise. The amount of dew over the dew-gauges are compared with standard dew photographs in scale ranging from 0 to 9, (0 = no dew, 9 = no observation).

6.11 Soil moisture observation

Soil moisture (SM) is the most important parameter for crop growth. Accurate and systematic collection of soil moisture data is vital. Usually it is measured by Gravimetric method using soil sample. Soil is dried at 1050C for 24 hours until a constant weight is reached.

7 Non- instrumental observations

Cloud observations: The main groups of clouds are a) High clouds (6-18 kms) – Cirrus, Cirrostratus and Cirrocumulus, b) Medium clouds (2-8 kms) – Altocumulus, Altostratus and Nimbostratus, c) Low clouds (0-2 kms) – Stratocumulus, Stratus, Cumulus, Cumulonimbus. The cloud amount is reported in okta (i.e. 1/8th of the sky). When sky is completely cloudless, amount is zero and when it is completely overcast, the amount is eight. Cloud height means the vertical distance of the base of cloud from the ground level. The height can be measured with the help of balloons or by cloud search-light at night. The direction of movement of cloud is visually estimated nearest to the eight points of the compass.

8 Summary

There are six classes of meteorological observatories. Observations on weather elements are used for weather forecasting and day to day agricultural operations.
Meteorological observations are taken at standard hours of observations all over the world.
Proper site of the observatory and maintenance of observatory help to generate accurate data.
The observations are taken in the following order: 1) Wind instruments, 2) Raingauge, 3) Thermometers and 4) Barometer. Clouds, visibility etc should be taken in the interval of 3 minutes between the first and second readings of the anemometer.
The wind direction and speed are recorded using wind vane and cup anemometer, respectively. The maximum, minimum, dry bulb and wet bulb thermometers along with thermograph are
used to record temperature. Soil thermometer (Hg-in-glass type) with a bend of 120 degrees in the stem just above the bulb is used for recording soil temperature at different depths.
Hair hygrograph is used for recording continuously the relative humidity of the air.
A psychrometer consists of dry bulb and wet bulb thermometers. It gives instantaneous measurement of temperature and humidity of the air.
Sunshine recorder is an instrument for measuring daily duration of bright sunshine hours.
The grass minimum or terrestrial radiation thermometer is used mainly to obtain information about ground frost at night.
The information on water loss from pan evaporimeter, dew deposition, soil moisture and bright sun shine hours helps the farming community in their day to day agricultural operations.

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