Tiny drops of water appear on the outer wall of the tumbler, when we put chilled water or ice cubes in it. Do you know from where the water drops come? Actually, these drops appear because of the condensation of moisture present in the air. Now the question arises what is the source of moisture in the air?In fact, moisture is always present in the air in the form of water vapour but we can not see it. The amount of water vapours present in the air changes from place to place. Some amount of water vapours’ also present in big desserts. We know that 70% of our earth surface is covered with water. The water of the river, seas, ponds, lakes evaporates because of the solar heating and gets mixes with the air. Thus, the regular supply of water vapour to the air is continued. Scientifically, the water vapour present in the air is called humidity. It changes from season to season. In summer season the large amount of water vapours present in air than in winter season. This is due to large amount of water from the rivers, seas, ponds and lakes gets changed into water vapour because of higher atmospheric temperature in summer season. The humidity increases extremely, in rainy season, as a large portion of earth’s surface is covered with water which goes on evaporating into the atmosphere. Dew, snow, fog frost etc. are formed because of the water vapours present in the air. Humidity is measured by an instrument known as ‘hygrometer’. There is possibility of rains in high humidity. In humid conditions the germs of some diseases grow very fast. As the moisture present in the atmosphere avoids the breaking of cotton threads therefore cotton mills require humid atmosphere.