Hygrometer and Weather Phenomenon

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.

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 0^o 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 0^oC, 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.