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This article throws light upon the eight main types of weather instruments. The types are: 1. Barometer 2. Fortin’s Barometer 3. Aneroid Barometer 4. Thermometer 5. Wet and Dry Bulb Thermometer 6. Wind Vane 7. Anemometer 8. Rain-Gauge.
1. Barometer:
The atmospheric pressure is measured with the help of a barometer. There are two types of barometer—Mercurial and Aneroid.
The mercurial barometer—Fortin’s type holds a column of mercury which balances weight of the air. The aneroid type is a vacuum box which measures the atmospheric pressure.
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Remember:
1. Under normal condition the air pressure per square centimetre is nearly 1.17 kg. at the mean sea level.
2. A continuously rising barometer indicates fair weather and steadily falling barometer indicates bad weather or a storm.
2. Fortin’s Barometer:
The Fortin’s Barometer is made of a glass tube nearly one metre long and open at one end. The glass tube is covered by a metal jacket to give protection. The tube is filled with mercury and held inverted on a mercury container.
The lower part of the container is made of leather which can be raised up or down with the help of a screw. An ivory pointer is attached at the lower part of the metal jacket in such a way that it touches the upper surface of the mercury. A vernier scale is attached to the metal jacket.
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Steps to be followed to take reading in the Fortin’s Barometer:
(i) Adjust the ivory pointer with the help of the lower screw.
(ii) With the help of the side screw, the vernier scale is to be slided up or down so that the base line of the vernier coincides with the upper level of the mercury column.
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Important:
Reading should be taken looking at the mercury column, in a horizontal position. Do not look at the column by viewing up or down.
(iii) Take reading from the main scale upto the base line of the vernier. (Suppose it is 76.4 cm).
(iv) Find out vernier constant. For this,
(a) First bring Zero of the vernier to be coincided with a particular division of the main scale.
(b) Look along the vernier and see which division of the vernier coincides with which division of the main scale. Suppose, 9 divisions of main scale coincide with 10 divisions of vernier.
... The difference between 1 vernier division and one division of main scale
This is called vernier constant.
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The primary divisions of the main scale of the Barometer are marked as cm and secondary divisions are in mm.
(v) Count which vernier division coincides with the main scale division.
(vi) Multiply this value by the vernier constant. (Suppose, in taking your reading, 8 vernier divisions coincide with a particular mainscale division)
... Vernier value = 8 x .01 = .08 cms.
(vii) Add Vernier value to main scale reading which you have taken already to get the actual value of the air pressure.
For example, correct reading = 76.4 cm + .08 cm.
= 76.48 cms
(viii) Take at least three readings, repeating the observations.
(ix) Take average to get final reading.
Important:
1. As air pressure has inverse relation with temperature:
2. Take room temperature before taking Barometer reading.
3. Find out vernier constant.
4. Take atleast three readings and find out average.
5. Take room temperature again after completion of taking reading.
3. Aneroid Barometer:
This is another specialised type of Barometer. It is made of elastic metal box and the inside is made airless. It does not contain any mercury or any other liquid. The metal box contains a spring which presses the lid outwards against air pressure. When the pressure outside increases, the lid moves inwards and vice-versa. The movement of the lid makes a needle to rotate over a graduated dial from which reading can be taken.
Important:
1. No calculation is necessary.
2. Reading can be taken directly.
3. Easy to carry.
4. However, it does not give accurate readings.
4. Thermometer:
Thermometer is another important instrument for measuring the air temperature. As the temperature recorded in the weather map relates to average, the use of maximum and minimum thermometer is quite common.
Six’s Maximum and Minimum Thermometer:
This Thermometer records both minimum and maximum temperature at a time. The mean temperature for a day is calculated by taking average.
The Thermometer has two arms and it looks like U. One of the arms is fitted with a scale to record maximum and the other arm has a scale to record minimum temperature. The lower portions of the arms are filled with mercury and the upper portions with alcohol.
There are two smooth steel pointers between mercury and alcohol. These two pointers are such that they move according to the expansion and contraction of mercury, but they will not change their positions, by the expansion and contraction of the alcohol.
By the increase of air temperature outside, the alcohol of the minimum thermometer expands and pushes the mercury. As a result, the mercury level of the maximum thermometer rises along with pointer and indicates maximum temperature of the day.
Similarly, the alcohol of the minimum thermometer contracts when air temperature outside falls. As a result, the pointer along with the mercury column of the minimum thermometer moves up and that of maximum thermometer comes down leaving the pointer at the original position. The pointer in the minimum thermometer indicates lowest temperature of the day.
5. Wet and Dry Bulb Thermometer:
The atmospheric humidity is measured with the help of Wet and Dry Bulb Thermometer. It is also called as Manson’s Hygrometer. In this instrument, two thermometers are fixed on a wooden plank side by side. The bulb of one of the thermometers is rapped with a muslin and is kept wet by dipping one end to the container of a pure water.
The other bulb is kept uncovered and exposed to dry air. The reading of the wet-bulb gives the temperature of the wet air and dry-bulb gives the temperature of dry-air. The amount of humidity can be calculated with the help of a special table which shows percentages of humidity for different temperature differences.
Remember:
1. Greater is the temperature differences, lesser is the humidity and vice-versa.
2. No temperature difference indicates completely saturated air.
3. Amount of humidity in the table is given in percentage.
6. Wind Vane:
The direction of wind is determined by Wind Vane. It is also called as Weather Cock. It consists of a light wooden structure having ji pointed face with an arrow towards the direction from which wind blows and a broad face opposite to it. The structure rotates on a vertical rod freely. The rod may also be fitted with a cross to indicate four cardinal directions.
Important:
The wind is known by the direction from which it blows. East wind means, wind is coming from east and so on.
7. Anemometer:
The speed or velocity of wind is measured with the help of anemometer. It consists of 4 empty cups mounted on horizontal arms attached to a vertical spindle. When wind blows, the cups are pushed and the spindle rotates.
At the bottom of the spindle, a measuring instrument is attached which records velocity of wind in metres per second (m.p.s.). To get average velocity, at least three readings should be taken after an interval of 3-4 hrs.
Remember:
Calm wind – Smoke rises vertically.
Breeze – Leaves and branches of trees in motion.
Gale – Break twigs of trees.
Storm – Widespread damage.
8. Rain-Gauge:
The amount of rainfall is measured by means of Rain-gauge. The rain-gauge consists of a circular funnel fitted at the top of a cylinder. The rainfalls on the funnel is collected in the cylinder. The water so collected is measured with the help of a measuring glass.
Important:
1. Rain-gauge should be placed in open ground and at least 30 cm. above the surface.
2. Care should be taken so that the water collected in the cylinder cannot go out or evaporate.
