CBSE Class 4 Science – 12. Air, Water and Weather (Notes)

 

Weather – The condition of the atmosphere in one area at a particular time is called weather.

Example – Rainy, windy, sunny etc.

The weather can change the number of times. 

Weather refers to the short term changes in the atmosphere of a place.

 It could be sunny, rainy, dry or humid.

Changes in the weather Occur due to the sun.

 

Sun and the weather – 

The part which faces the sun has a summer with longer and warmer days. 

The other part which is away from the sun has winter with shorter and colder days. 

Revolution of the earth around the sun causes changes in seasons. 

The sun is also responsible for rain.

 

Air – The mixture of gases which forms the Earth’s atmosphere and which we breathe is called air.

Air is a mixture of gases.

Air contains 78% nitrogen, 21% oxygen, 0.03% carbon dioxide, and remains other gases.

Air contains water vapour and dust particles.

We can only feel air.

 

Atmosphere – The layer of air or other gases around it is called atmosphere.

 

Wind – Air that is moving across the earth’s surface is called wind.

 

Breeze – A light gentle wind is called breeze.

 

Gale – A very strong wind is called gale.

 

Storm – A heavy fall of rain, snow or lightning with strong’s winds are called storm.

 

Hot air and cold air – 

The sun heats the air which expands and becomes lighter and Rises. 

Cold air which is denser moves down towards the surface and takes its place.

 

 

Sea breeze – The cool breeze that blows from the sea to the land is called sea breeze.

During the day, land gets heated up faster than water.

Now, the air above the lands becomes hot and it expands.

It becomes lighter and rises up.

The cooler sea breeze rushes in to take its place.

It blows during the day.

 

Land breeze – A coastal breeze blowing at night from land to sea is called land breeze.

During the night the land cools faster than the sea.

The air above the sea becomes lighter.

This air rises and the cold air from the land rushes into takes its place.

It blows during the night.

 

Water – A clear liquid, without colour or taste, that falls from the sky as rain is called water.

About 70% of the earth made up of water.

Water is found in seas, oceans, rivers, ponds and lakes.

 

Water vapour – Water found in the earth atmosphere is known as water vapour.

 

Humidity – The amount of water vapour in the air is called humidity.

 

 

Evaporation and the Weather

Evaporation – The process by which water change into gas on heating is called evaporation.

The heat of the sun slowly changes the water on the surface of the earth into water vapour.

Evaporation may be slow or fast depending upon the following factors.

Wind:

Evaporation is faster when it is windy. Moving wind absorbs water rapidly.

Temperature:

Evaporation is faster if the temperature is high.

Dry Air:

Evaporation takes place faster when the air is dry, because dry air can take up more water than moist air.

Surface Area:

Evaporation is faster if the exposed surface area of the liquid is large.

 

Condensation and the Weather

   Condensation -The process by which gas change into water on cooling is called condensation.

Rain, fog, dew and frost all are formed by the process of condensation

Rain:

When water evaporates, its vapour rises up.

The water vapour cools as it goes up and changes into tiny droplets of water.

These droplets join together to make clouds.

When clouds become heavy with water, they burst and we get rain.

Fog:

This is formed when water vapour mixes with very cold air and changes into drops of water and forms a blanket of clouds just above the water or land.

Dew:

In winter when water vapour in the air condenses on touching cold surfaces, small drops of water called dew are formed.

Dew drops can be seen on leaves and flowers in winter mornings.

Frost:

When it is very cold, the dew or surface water freezes into tiny white crystals called frost.

Hail When raindrops pass through a very cold region of the atmosphere, they freeze and form hail.

Snow:

When water vapour is suddenly cooled, it freezes into tiny white snowflakes.

 

Water cycle – The continuous movement of water from the earth’s surface to the atmosphere and then back to the ground are called water cycle.

 

Precipitation – The water droplets form clouds which become heavy and fall from the sky in the form of rain is called precipitation.

 

Impurities in water –

Impurities – Substances that are present in small quantities in another substance and make it dirty are called impurities.

 

Soluble impurities- Those impurities which can be dissolving in water are called soluble impurities.

 

Insoluble impurities- Those impurities which cannot dissolve in water are called insoluble impurities.

 

Removal of insoluble impurities:

 

Insoluble impurities can be removed by the following process –

Sedimentation – The process of particles settling to the bottom of a body of water is called sedimentation.

Soluble impurities in water settle down at the bottom leaving the water clear at the top.

Decantation – The clear water on the top is slowly poured out without disturbing the sediments. This process is called decantation.

Filtration – The process of removing the insoluble particles from water by using a filter is called filtration.

 The filter may be a sieve or a filter paper.

 

Removal of soluble impurities:

Soluble impurities can be removed by –

By boiling

By adding Chemicals

By using water purifiers.

 

Boiling- 

Boiling water for at least 10 minutes kill most of the germs.

It is the easiest method of purification of water.

Adding chemicals –

The process in which chlorine is added to water to kill germs is called chlorination.

Small amount of chlorine tablet or bleaching powder tablets can be added to water to kill germs.

Sometimes potassium permanganate is added to well water to purify it.

Using water filter –

Water filter or purifier is used to clean water to make it fit for cooking and drinking.