Earth's atmosphere is a mixture of nitrogen (79%), oxygen (20%), and a small fraction of carbon dioxide, water vapours and other gases. This makes the existence of life possible on Earth. However, the atmospheres on Venus and Mars mainly consist of carbon dioxide. The amount of carbon dioxide on these planets can range from 95% to 97%.

The atmosphere acts as a blanket by performing the following functions:
→ It keeps the average temperature of the Earth fairly constant during day time and even during the course of whole year.
→ It prevents a sudden increase in the temperature during day time.
→ It slows down the escape of heat from the surface of the Earth into outer space during night time.

An uneven heating of the Earth's surface causes winds. On being heated, air becomes lighter and rises up. As a result, a region of low pressure is created. Then, air from a high pressure region moves to a low pressure region, causing wind.

During day time, on being heated, a large amount of water evaporates from various water bodies and goes into the air. A part of this water vapour also reaches the atmosphere through biological activities such as transpiration and respiration. This causes the air in the atmosphere to heat up. When this heated air rises, it expands and cools, which results in the condensation of water vapour forming water droplets. The presence of dust and other suspended particles in air also facilitates the process of condensation. The formation of water droplets leads to the formation of clouds.

Three human activities leading to air pollution are:
→ Smoke from industries e.t.c.
→ Burning of fossil fuels like coal and petroleum→ Deforestation.

The organisms need water:
→ For different cellular process.
→ For transportation of substance from one place to another inside the body.

Rivers.

The discharge of waste water from homes, industries, hospitals, etc. into the river pollutes this fresh water source.