6. Atmosphere

Its temperature falls to about −100° C.

Ozone is most concentrated at a height of 25 to 40 km above sea level, which lies within the stratosphere.

The troposphere has the highest air density, and water vapour is found only up to 8 to 12 km above sea level.

True

Water vapour condenses around tiny dust particles to form water droplets. This is how clouds form and rain falls.

False

Correct statement: The meteorites that come towards the Earth are burnt in the mesosphere.

False

Correct statement: The visibility decreases in fog. Because of this, road accidents increase and flights are cancelled.

False

Correct statement: The proportion of nitrogen is maximum in the atmosphere (about 78%). Hydrogen is only a small part of the other gases. It is found in a higher proportion only in the exosphere, the outermost layer.

• A) Troposphere – changes in the atmosphere
• B) Mesosphere – Burning and vanishing of meteorites
• C) Ionosphere – Communication
• ✗ D) Stratosphere – Dominance of Hydrogen and Helium

Wrong pair: D — the dominance of Hydrogen and Helium is a feature of the exosphere. The stratosphere is known for its ozone layer.

• A) Troposphere
• B) Mesosphere
• C) Ionosphere
• ✓ D) Stratosphere

Answer: D) Stratosphere — the ozone gas in this layer absorbs the Sun’s ultraviolet rays.

• Troposphere (0 to 10 km): The temperature decreases with increasing height.
• Stratosphere (10 to 60 km): The temperature is higher than in the troposphere, because the ozone gas in this layer absorbs the Sun’s ultraviolet rays.
• Mesosphere (60 to 100 km): The temperature decreases again and falls to about −100° C. It is the coolest layer.
• Ionosphere (100 to 300 km): The temperature rises rapidly with height and is more than about 1000° C.
• Exosphere (above 400 km): This outermost layer has extreme temperatures.

So the temperature of the atmosphere does not change in one direction. It falls, rises, falls again and then rises sharply as we go higher.

Dust particles are microscopic solid particles suspended in the air — soil particles, pollen, seeds of fungi, dry plant particles, industrial pollution and volcanic ash. Some are harmful to health, but they are important for the balance of the environment:

• Cloud formation: Water vapour condenses around dust particles to form droplets, which make clouds.
• Precipitation: Rain, snowfall and hail are possible because of these droplets.
• Sunlight radiation: Dust particles scatter and reflect sunlight, which also gives the sky its colours at sunrise and sunset.
• Temperature control: By absorbing and scattering the Sun’s heat, they help control the temperature near the Earth.
• Pollination: Pollen carried in the air helps in the pollination of plants.
• Plant growth: Fine soil and other particles carried by the air settle on land and help plants grow.

Many phenomena occur in the atmosphere. Some are beneficial to human life, while others are harmful and disrupt it.

• Fog: Dense fog reduces visibility, so road accidents increase, flights are cancelled and water transport is disrupted. It also affects the respiratory system.
• Extremely heavy rainfall: It causes floods, damages houses, crops and roads, and stops traffic and railways, as happens in cities like Mumbai during heavy monsoon rain.
• Heat waves: Very high temperatures cause sunstroke and water shortage, and affect outdoor work.
• Storms and lightning: Strong winds uproot trees and electric poles. Lightning can kill people and animals and start fires.
• Hailstorms: Hail destroys standing crops and fruit orchards, causing heavy losses to farmers.
• Snowfall: Heavy snowfall blocks roads and cuts off hilly regions.
• Beneficial effects: Timely monsoon rain supports agriculture and fills rivers, lakes and dams, which supply drinking water and electricity.