SolveItClass 9 · NCERT

NCERT Solutions · Class 9 Science Earth as a System: Energy, Matter, and Life

21 questions · 21 still being checked

Revise, Reflect, Refine 13.11–13.15 (part 3 of 3)

  1. Exercise 13.11

    How is heat lost from the surface of the Earth? What is its significance?

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    NCERT prints no numerical answer for this exercise, so this working has not been cross-checked against the book.

    The Earth's surface loses heat mainly by re-radiating it as infrared radiation into the atmosphere and space.
    NCERT_Solution_Class9_Science_Ch13_RRR_Q13-11
    By day the surface absorbs incoming solar radiation (UV, visible and infrared), warms up, and then radiates that energy back at longer, infrared wavelengths — because all objects radiate heat.
    Part of this outgoing infrared is absorbed by the greenhouse gases \(\displaystyle CO_{2} \), \(\displaystyle CH_{4} \) and water vapour and sent back down; the rest escapes into space.
    A further part of the incoming radiation never heats the surface at all — it is reflected straight back by the surface (its albedo) and by clouds and the atmosphere.
    Significance — energy balance: this outgoing loss balances the energy arriving from the Sun. If heat were not lost, the Earth would go on getting hotter without limit.
    Significance — habitable temperature: the partial trapping of the outgoing heat is what keeps the planet warm enough for life; without the atmosphere the Earth would be too cold to live on.
    Everyday evidence: a concrete house feels hot at night because the concrete re-radiates the heat it absorbed during the day, while a thick mud-and-wood house stays cool because it re-radiates less.

  2. Exercise 13.12

    If the Earth were a flat disc instead of a sphere, how would the patterns of solar radiation and temperature be different?

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    NCERT prints no numerical answer for this exercise, so this working has not been cross-checked against the book.

    On a flat disc facing the Sun, every point would receive the rays at the same angle, so insolation — and therefore temperature — would be nearly the same all over the lit face.
    NCERT_Solution_Class9_Science_Ch13_RRR_Q13-12
    On the actual spherical Earth, the rays strike different latitudes at different angles: near the equator the beam is concentrated over a small area, while near the poles the same beam is spread over a much larger area.
    That is exactly why equatorial regions stay relatively warm all year and polar regions stay much colder — the equator-to-pole temperature difference.
    On a disc that difference would vanish: no hot equator, no cold poles, and no distinct climate zones by latitude.
    With uniform heating there would be no equatorial low, sub-tropical high, sub-polar low or polar high pressure belts, so the planetary winds would not form.
    With no planetary winds dragging the surface water, the wind-driven ocean currents and gyres would not form either, and heat would not be carried from equator to poles.
    Some uneven heating would remain — land heats faster than water, and dark low-albedo surfaces heat faster than bright ones — but only as local effects, not the global circulation the chapter describes.
    (This assumes the disc's flat face is turned towards the Sun. If it were tilted, the whole face would simply be heated less, uniformly, rather than being heated unevenly by latitude.)

  3. Exercise 13.13

    Suppose there is a rise in atmospheric temperature on Earth. How would this affect the cryosphere, hydrosphere and biosphere?

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    NCERT prints no numerical answer for this exercise, so this working has not been cross-checked against the book.

    A rise in atmospheric temperature disturbs all three spheres, and each disturbance feeds the next.
    NCERT_Solution_Class9_Science_Ch13_RRR_Q13-13
    Cryosphere — it shrinks. Glaciers, Arctic sea ice and polar ice caps melt faster and snow cover is reduced.
    Bright ice and snow have a high albedo (\(\displaystyle 0.50-0.90 \)); as they melt, darker land and sea water are exposed, which absorb more radiation and warm the region further.
    Hydrosphere — more water, moved about more violently. A warmer atmosphere holds more moisture, so evaporation increases, giving heavier rain in some regions and drought in others.
    Meltwater adds to rivers and, in the long run, raises sea level, flooding low-lying regions and threatening coastal cities such as Mumbai and Chennai.
    Warmer sea water absorbs less \(\displaystyle CO_{2} \), weakening the ocean as a carbon sink, while the \(\displaystyle CO_{2} \) it does take up makes it more acidic.
    Biosphere — habitat loss and falling biodiversity. Acidic, warmer seas threaten plankton and coral reefs, damaging the base of marine food chains and coastal fisheries.
    On land, changed rainfall patterns and intensified monsoons hurt agriculture, and species that cannot move or adapt to the new temperature lose their habitats and decline.

  4. Exercise 13.14

    Explain how the Earth’s atmosphere helps in maintaining a suitable temperature for life to survive on the Earth.

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    NCERT prints no numerical answer for this exercise, so this working has not been cross-checked against the book.

    The atmosphere keeps the Earth's temperature suitable for life by playing two roles at once — it filters what comes in, and it traps part of what goes out.
    NCERT_Solution_Class9_Science_Ch13_RRR_Q13-14
    Role $\displaystyle 1$, absorbing incoming radiation: the ozone layer in the stratosphere absorbs the harmful short-wavelength UV rays (\(\displaystyle 100 \) nm to \(\displaystyle 400 \) nm), and clouds, gases and dust particles absorb or scatter some sunlight before it reaches the ground, so the surface is not scorched.
    Because of this, the maximum insolation reaching the surface is only about \(\displaystyle 1\ kWm^{-2} \), lower than the solar constant of about \(\displaystyle 1.4\ kWm^{-2} \) at the top of the atmosphere.
    Role $\displaystyle 2$, trapping outgoing heat: the surface absorbs sunlight and re-radiates it in the infrared, and the greenhouse gases \(\displaystyle CO_{2} \), \(\displaystyle CH_{4} \) and water vapour absorb this heat, preventing it from escaping straight into space — the greenhouse effect.
    Together these two roles keep both daytime heating and night-time cooling within the range that life can survive; without the atmosphere the Earth would be too cold for life.
    The balance is delicate. Venus is hotter than Mercury even though Mercury is nearer the Sun, because Venus has an atmosphere with an uncontrolled greenhouse effect.
    Excess \(\displaystyle CO_{2} \) from human activities enhances the greenhouse effect and causes global warming, which if left unchecked could make the Earth uninhabitable.

  5. Exercise 13.15

    Describe the interrelationship between different spheres of the Earth. Illustrate with example how these spheres function in a delicate balance.

    Not cross-checked

    NCERT prints no numerical answer for this exercise, so this working has not been cross-checked against the book.

    The Earth's five spheres are joined by continuous flows of energy and matter, so a disturbance in any one sphere produces changes in all the others.
    NCERT_Solution_Class9_Science_Ch13_RRR_Q13-15
    Geosphere — solid rock, soil, landforms such as the Deccan plateau and the Thar desert, and the Earth's interior.
    Hydrosphere — liquid water: oceans, rivers such as the Ganga–Brahmaputra system, lakes and groundwater.
    Cryosphere — water as ice and snow: Himalayan glaciers, snow in Ladakh, polar ice caps.
    Atmosphere — the layer of air held by the Earth's gravity, \(\displaystyle 78\% \) nitrogen and \(\displaystyle 21\% \) oxygen with argon, \(\displaystyle CO_{2} \) and water vapour.
    Biosphere — all living organisms and their habitats: mangroves, forests, farms, ocean plankton and coral reefs.
    Example $\displaystyle 1$ — snow, lake and grass (Activity $\displaystyle 13.1$): snow (cryosphere) melts and feeds a lake (hydrosphere); the lake waters the grass (biosphere) that sheep graze. A few winters of poor snowfall lower the lake in summer, so less water reaches the grass and the sheep have less to eat — one small change travels through three spheres.
    Example $\displaystyle 2$ — the monsoon: a warmer atmosphere heats the Arabian Sea (hydrosphere), causing more evaporation and fluctuations in the southwest monsoon; the result is floods in some parts of India and drought in others. The same warming melts glaciers and polar ice (cryosphere), and rising sea level threatens coastal cities and destroys habitats (biosphere).
    Example $\displaystyle 3$ — deforestation: clearing trees (biosphere) reduces transpiration, so local rainfall declines (atmosphere and hydrosphere); without roots the soil erodes (geosphere); and with less photosynthesis, more \(\displaystyle CO_{2} \) stays in the air, warming the planet further.
    Why the balance is delicate: the connecting flows are cyclic — water, carbon, nitrogen and oxygen must keep returning through the biogeochemical cycles. A break anywhere in a cycle does not stay local; it propagates across every sphere.