Note on Hygrometer and Weather Phenomenon

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Wet and Dry Bulb Hydrometer

A hygrometer is a device which is used to measure the relative humidity of the air. A wet and dry hygrometer are the simplest of all the hygrometers.

Wt and dry bulb hygrometer.
Wet and dry bulb hygrometer.

It consists of two mercury thermometers A and B as shown in the figure. Here, thermometer A is dry which is used to measure room temperature while thermometer B is wet which is use to determine the dew point. The bulb of B is wrapped with wet muslin piece which is in contact with water in a beaker. Due to capillary action, water rises up in muslin piece and evaporates. As a result temperature of thethermometer, B falls down. The temperature is noted after every minute. Finally, hygrometer shows constant temperature. The difference in the reading of the dry and wet thermometers is a measure of relative humidity of the air.

Gas and Vapour

A vapour above the critical temperature is gas and a gas below the critical temperature for the substance is a vapour.

A gas cannot liquefy by the application of pressure alone, how large the pressure may be. On the contrary, a vapour can be liquefied under pressure alone. To liquefy a gas, it must be cooled up to or below its critical temperature. Further, both the vapour and gaseous states obey the same gas laws.

Weather Phenomenon

When the temperature of the air falls below a dew point, the condensation, the weather exhibits many phenomena like the formation of dews, hail, cloud, rain, fog, mist etc.

Dew

At night, the surface of the earth radiated heat and its temperature falls. In the morning the temperature of the atmosphere becomes less than a dew point, the water vapour condenses in the form of tiny water drops on the leaves or grasses. Those tiny drops are dews.

Hoarfrost

If the temperature of the temperature falls below 0o C, the dew formed on the leaves and grasses condense directly into solid white grains called hoarfrost.

Fog and Mist

When air gets cooled up to some distance from the ground to a temperature below the dew point, water vapours present in the temperature falls below 0oC, the dew point, water vapours present in the temperature condenses on dust particles in the atmosphere and the mist is formed.

Clouds and Rain

When the temperature of the layers of air at high altitude falls below the dew point, the condensation of water vapour takes place on the dust particles present there and the cloud is formed. When a small drop of water in the cloud combine together, bigger water drops are formed and rainfall occurs due to the gravity.

Snow and Hail

When the cloud passes through a very cold region, the water drops freeze and fall in the form of snow and hail. Snowfall is common in all hill stations because of very low temperature in the winter. Hails are caused due to the freezing of raindrops in the atmosphere. Hails are of different sizes whereas snow is quite soft.

Hygrometer is a device which is used to measure the relative humidity of the air. Wet and dry hygrometer is the simplest of all the hygrometers.

A vapour above the critical temperature is gas and a gas below the critical temperature for the substance is a vapour.

If the temperature of the temperature falls below 0C, the dew formed on the leaves and grasses condense directly into solid white grains called hoarfrost.

When the cloud passes through a very cold region, the water drops freeze and fall in the form of snow and hail. 

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Very Short Questions

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  • ______ is a device which is used to measure the relative humidity of the air.

    Lactometer


    Hygrometer


    Spygmomanometer


    Hydrometer


  • ______ is a device which is used to measure the relative humidity of the air.

    Lactometer


    Hydrometer


    Spygmomanometer


    Hygrometer


  • The vapour state of a substance is obtained through the process of ______.

    sublimation
    ebullition
    all the answers are correct
    evaporation
  • The vapour state of a substance is obtained through the process of ______.

    ebullition
    evaporation
    all the answers are correct
    sublimation
  • Hygrometer is instrument for measuring ______.

    the density of air


    the expansion of water vapour under temperature


    the amount of oxygen in air


    the amount of water vapour in the atmosphere


  • Higher difference between temperatures of the wet bulb and dry bulb indicates ______.

    no information about relative humidity


    greater relative humidity


    higher dew point


    lower relative humidity


  • A liquid boils at a temperature at which its saturated vapour pressure becomes ______.

    none of the answers are correct


    equal to atmospheric pressure


    half the atmospheric pressure


    twice the atmospheric pressure


  • If the atmospheric temperature and dew point are nearly equal ______.

    relative humidity is zero


    relative humidity 50%


    nothing can be said about relative humidity


    relative humidity is nearly 100%


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