Question

Assertion (A): About 90 per cent of the mass of air lies within a height of about 20 km from the surface of the earth.

Reason (R): The air of the lower layer is always heavier or denser, while the upper layers are thinner or less dense.

Options

• Both A and Rare true and R is the correct explanation of A.

• Both A and Rare true but R does not explain A.

• A is true but R is false.

• A is false but R is true.

Answer 1

Both A and Rare true and R is the correct explanation of A.

Explanation:

Most of the air mass lies within the first 20 km because gravity pulls air molecules toward the Earth, making the lower layers denser.
As altitude increases, the air becomes thinner and less dense, which explains why most of the atmospheric mass is concentrated near the surface.