4th Class Science Weather, Air and Water Air, Water and Weather

Air, Water And Weather

Synopsis

• Weather is the condition of the sun, air, water and clouds of a place at a particular time.

• Air is a mixture of gases like nitrogen, oxygen, carbon dioxide, water vapour etc.

• The percentage of oxygen and carbon dioxide is kept at a constant level by photosynthesis and respiration.

• We cannot see the air as it is invisible. But we can feel the presence of air. Air occupies space and exerts pressure.

• The sun plays an important role in causing changes in the weather.

• The noon is hottest time of a day because sun's rays fall directly over our head.

• Land and water are heated up differently by the sunlight. Land gets warmer faster than water during the day. Land also cools faster than water in the evening.

• During the day, land is hotter than the sea. So the air above the land gets warmer than the air over the sea. Hot air above the land being lighter, rises and the cool air from the sea takes its place. In this way air blows from sea towards the land. This is called sea breeze.

• At night, The sea is hotter than the land and cooler air above the land moves towards the sea, as the warmer air above the sea rises. Thus air starts to move from the land towards the sea. This is called land breeze.

• The change of water into water vapour is called evaporation. It also increases with the temperature and area of exposure to air. Evaporation is higher on a dry day.

• Change of water vapour into water is called condensation. When the temperature is cooled, water vapour turns to water.

• In the morning we can observe small drops of water on grass blades and leaves of plants. These are called dew drops. When dew drops freeze, it is called frost.

• When moist air mixes with dust particles in the air the particles are suspended and form fog.

• When the clouds are cooled to very low temperature, snow falls.

• When speedy winds take rain drops to colder regions, drops freeze and form small ice-balls. This is called hail.

• Impurities which can be seen are called insoluble impurities while those which cannot be seen are called soluble impurities.

• When water is allowed to stand for a long time without any disturbance, all the impurities will settle at the bottom. This is called sedimentation. Pouring clear water above the sediment impurities into another vessel is called decantation.

• Removing impurities by passing the solution through a filter paper is called filtration.

• The water which contains disease-causing germs is called contaminated water. Boiling the water kills all the germs in it.