NCERT Solutions for Class 9th Science Chapter 13 Lead-in to 13.1.1 Interaction of solar radiation on the Earth's surface — In-text Questions
Book page 256 Updated on2026-09-08
Q1.
How does the Sun's radiation interact with the Earth's surface and atmosphere, and warm the Earth?
Answer
The incoming radiation is split four ways — part is reflected by clouds and the atmosphere, part is absorbed by the atmosphere, part is reflected by the ground, and the rest is absorbed by the ground. Only the last part warms the surface, and the surface then warms the air from below.
The four fates of incoming sunlight, and the infrared return path that greenhouse gases intercept (compare Fig. 13.5).
The mechanism, step by step:
On the way in. The ozone layer absorbs most of the short-wavelength UV. Clouds, dust and gases reflect and scatter part of the visible light back to space. What is left reaches the ground.
At the surface. A fraction set by the albedo of that surface is reflected. The rest is absorbed and raises the temperature of the soil, rock or water.
On the way out. Every warm body radiates. The Earth is far cooler than the Sun, so it radiates in the infrared, not in visible light.
The trap. Greenhouse gases — CO2, CH4 and water vapour — are transparent to incoming visible light but absorb outgoing infrared. They send part of that heat back down, keeping the surface warm enough for life.
Key point: the troposphere is heated from below, by the surface, not directly by the Sun passing through it. That is exactly why the temperature falls with height in the troposphere (about 6.5 °C per km) — you are moving away from the heater.