New NCERT Exploration · Class 9 Science · Chapter 13: Earth as a System: Energy, Matter and Life
This chapter shows the Earth as one interconnected system where land, water, air, ice and living things constantly interact. It explains how energy from the Sun heats the Earth unevenly and drives winds, ocean currents and climate. It also covers the water, carbon, nitrogen and oxygen cycles, climate change and sustainable practices.
From the Class 9 Science Short Notes, made as per the new NCERT book Exploration · Pages 133–151 of the book.
Key points
- The Earth has five spheres: geosphere, hydrosphere, cryosphere, atmosphere and biosphere; a change in one affects the others.
- The Sun is the main source of energy for the Earth, but different places receive different amounts, causing uneven heating.
- Uneven heating drives winds, ocean currents, the water cycle and climate.
- The Sun's energy reaches Earth as electromagnetic waves, mostly ultraviolet, visible and infrared radiation.
- Insolation is the solar radiation received by the Earth's surface; the solar constant is about 1.4 kW m⁻², but the surface receives about 1 kW m⁻² on a clear day.
- Albedo is the fraction of sunlight reflected by a surface: fresh snow has high albedo, while forests and oceans have low albedo.
- Cities are warmer than nearby villages because of the Urban Heat Island effect.
- The atmosphere has five layers: troposphere, stratosphere, mesosphere, thermosphere and exosphere; the ozone layer is in the stratosphere.
- The greenhouse effect keeps the Earth warm enough for life; greenhouse gases include CO₂, water vapour, N₂O and CH₄.
- Wind moves from high pressure to low pressure; sea breeze blows by day and land breeze by night.
- Planetary winds are trade winds, westerlies and polar easterlies; warm and cold ocean currents carry heat around the Earth.
- Water, carbon, nitrogen and oxygen are continuously recycled through biogeochemical cycles.
Formulas
| Solar constant | ≈ 1.4 kW m⁻² (about 1400 J s⁻¹ m⁻²) At the top of the atmosphere, on 1 m² perpendicular to the Sun's rays |
| Solar energy at the surface | ≈ 1 kW m⁻² Under clear sky conditions |
Key terms
- Insolation: The amount of solar radiation received by the Earth's surface.
- Solar constant: Average solar energy received each second on one square metre at the top of the atmosphere, facing the Sun's rays.
- Albedo: The fraction of incoming solar radiation that a surface reflects back into space.
- Urban Heat Island: The effect that makes cities warmer than nearby rural areas.
- Greenhouse effect: Trapping of part of the Earth's infrared radiation by certain gases, keeping the Earth warm.
- Ozone layer: A layer in the stratosphere that absorbs most harmful UV radiation from the Sun.
- Biogeochemical cycle: The movement of essential elements like water, carbon, nitrogen and oxygen through living things and the environment.
- Nitrogen fixation: Conversion of atmospheric nitrogen into usable compounds by bacteria like Rhizobium and by lightning.
- Global warming: The long-term rise in the average temperature of the Earth's atmosphere and surface.
Diagrams to learn
- Earth's five spheres
- Uneven heating of the Earth
- Electromagnetic spectrum
- Role of UV, visible and infrared radiation
- What happens to incoming solar radiation (percentages)
- Albedo of snow, desert, forest and ocean
- Urban heat island
- Layers of the atmosphere
- Sea breeze, land breeze, mountain and valley breeze
- World map of planetary winds
- World map of ocean currents
- Water, carbon, nitrogen and oxygen cycles
- Air always moves from high pressure to low pressure; the greater the pressure difference, the stronger the wind.
- Sea breeze blows from sea to land during the day; land breeze blows from land to sea at night and is weaker.
- Lighter surfaces with high albedo stay cooler, while darker surfaces with low albedo become warmer.
A real page from the book
Get the full chapter in the book
This page is a summary. The Class 9 Science Short Notes has the complete chapter in colour, with every diagram and worked example.
Frequently asked questions
What are the five spheres of the Earth?
They are the geosphere (rocks and soil), hydrosphere (liquid water), cryosphere (frozen water), atmosphere (air) and biosphere (living organisms). They constantly exchange energy and matter.
What is albedo?
Albedo is the fraction of incoming solar radiation that a surface reflects back into space. Fresh snow has high albedo, while forests and oceans have low albedo because they absorb more sunlight.
What is the difference between sea breeze and land breeze?
During the day land heats faster, so cool air from the sea moves towards the land as sea breeze. At night the sea stays warmer, so cool air from the land moves towards the sea as land breeze.
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