Natural Phenomena — Weather, Earthquakes, Stars & Climate | CTET Science P2

Weather and Climate

Weather refers to the day-to-day condition of the atmosphere at a particular place with respect to temperature, humidity, rainfall, wind speed, and cloudiness. Climate, on the other hand, is the average weather pattern of a region over many years (typically 25 years or more).

The main elements of weather are: temperature (measured by thermometer), humidity (measured by hygrometer), rainfall (measured by rain gauge), wind speed (measured by anemometer), and wind direction (measured by wind vane).

The maximum and minimum thermometer records the highest and lowest temperatures in a day. The difference between maximum and minimum temperature of a day is called the diurnal range of temperature.

India experiences six seasons — summer, monsoon, autumn, early winter, winter, and spring — though conventionally four are most studied. Seasons change due to the revolution of Earth around the Sun and the tilt of Earth's axis. When the Northern Hemisphere tilts toward the Sun, it experiences summer while the Southern Hemisphere has winter, and vice versa.

• Tropical climate: High temperature throughout the year with heavy rainfall in summer.
• Polar climate: Very cold with snowfall; experienced near poles.
• Temperate climate: Moderate temperature and rainfall; experienced in mid-latitude regions.

The monsoon dominates Indian climate. Southwest monsoon brings rainfall to most parts of India from June to September. Rainfall is measured in millimetres and India's average annual rainfall is about 1170 mm.

Formation of Wind and Rain

Wind is the horizontal movement of air from high-pressure areas to low-pressure areas. The primary cause of wind is the uneven heating of the Earth's surface. Land heats up and cools down faster than water. During the day, land becomes warmer than the sea, causing hot air over land to rise, creating a low-pressure zone. Cool air from the sea rushes in — this is a sea breeze. At night, the process reverses, creating a land breeze.

On a global scale, the equatorial region receives maximum solar radiation and heats up more than the poles. This creates large-scale convection currents in the atmosphere that drive the major wind belts of Earth — trade winds, westerlies, and polar winds.

The water cycle is essential for the formation of rain. Water evaporates from oceans, rivers, and lakes due to solar heat. This water vapour rises, cools at higher altitudes, and condenses around dust particles to form clouds. When water droplets in clouds grow large enough, they fall as precipitation — rain, hail, or snow.

• Evaporation → Water vapour rises
• Condensation → Clouds form
• Precipitation → Rain, snow or hail falls
• Collection → Water returns to oceans and rivers

Monsoon rains in India result from the differential heating between the Indian subcontinent and the Indian Ocean. The ITCZ (Inter-Tropical Convergence Zone) shifts northward in summer, pulling moisture-laden winds inland, causing the southwest monsoon. Humidity measures the amount of water vapour in the air; high humidity leads to discomfort and can indicate impending rainfall.

Thunderstorms and Cyclones

A thunderstorm develops when moisture-laden warm air rises rapidly in a cumulonimbus cloud. As the air rises, it cools and water vapour condenses, releasing latent heat that further fuels the updraft. Inside the storm cloud, ice crystals and water droplets collide, causing separation of electric charge. The top of the cloud becomes positively charged and the bottom negatively charged, creating a huge potential difference. When this discharges suddenly, it produces lightning — a massive spark — and the rapid expansion of air creates the sound of thunder.

Lightning always strikes the tallest and most pointed objects, which is why tall trees and isolated buildings are dangerous during a storm. Lightning conductors (invented by Benjamin Franklin) are metal rods placed on top of buildings that safely conduct the charge to the ground.

A cyclone (called a hurricane in the Atlantic and a typhoon in the Pacific) is a large-scale, rotating storm system that forms over warm tropical oceans. Its structure includes:

• Eye: The calm, clear centre of the cyclone with low pressure.
• Eye wall: The ring of intense storms surrounding the eye.
• Spiral rain bands: Bands of clouds and rain spiralling outward.

Cyclones are rated by wind speed. In India, cyclones are most common on the eastern coast (Bay of Bengal). The Doppler radar and satellite imagery are used to predict and track cyclones. When a cyclone warning is issued, people should: move to higher ground, avoid coastal areas, store food and water, and follow instructions from authorities. The Anemometer measures wind speed, crucial for cyclone monitoring.

Earthquakes — Causes and Measurement

An earthquake is the sudden shaking or trembling of the Earth's surface caused by the movement of tectonic plates. The Earth's outer shell (lithosphere) is divided into about 12 major tectonic plates that float on the semi-molten asthenosphere. These plates constantly move, and where they meet are called plate boundaries. Most earthquakes occur along these boundaries.

The point inside the Earth where the earthquake originates is called the focus or hypocentre. The point on the Earth's surface directly above the focus is the epicentre — this is where the shaking is most intense. Energy released by the earthquake travels outward as seismic waves in all directions.

Types of seismic waves:

• P-waves (Primary/Compressional): The fastest, travel through solids, liquids and gases.
• S-waves (Secondary/Shear): Slower, travel only through solids.
• Surface waves (L-waves): Slowest but most destructive, travel along the Earth's surface.

Seismic waves are detected and recorded by an instrument called a seismograph (seismoscope for simple detection). The record produced is a seismogram.

The Richter scale (developed by Charles F. Richter in 1935) measures the magnitude of an earthquake — the amount of energy released at the focus. It is a logarithmic scale (0 to 9+). A magnitude of 2 is barely felt; 6 causes moderate damage; 8+ is catastrophic. The Mercalli scale measures the intensity — the effect felt at a specific location.

Earthquake-prone zones in India include the Himalayan region, Kutch (Gujarat), and the Andaman & Nicobar Islands. Tsunami (Japanese for 'harbour wave') is triggered by underwater earthquakes and can cause massive coastal flooding.

The Night Sky — Stars and Constellations

On a clear night, thousands of stars are visible to the naked eye. Stars are massive, luminous balls of plasma held together by gravity that generate energy through nuclear fusion in their cores. The Sun is the closest star to Earth, at about 150 million km away. The next nearest star is Proxima Centauri, about 4.2 light-years away.

Stars appear to move from east to west across the night sky due to the Earth's rotation from west to east on its axis. The Pole Star (Polaris or Dhruv Tara) appears stationary because it lies almost exactly above Earth's North Pole; it is extremely useful for navigation.

Stars are grouped into recognisable patterns called constellations. They are not physically close to one another — they just appear to form patterns from Earth. There are 88 officially recognised constellations.

• Ursa Major (Great Bear / Saptarishi): Seven bright stars forming a dipper shape; helps locate the Pole Star.
• Orion (The Hunter): Visible in winter; contains Betelgeuse (red giant) and Rigel (blue supergiant).
• Cassiopeia: W-shaped constellation visible in the Northern Hemisphere.
• Leo (The Lion): Visible in spring.

The Milky Way is our home galaxy — a barred spiral galaxy containing over 200 billion stars. From Earth, it appears as a faint band of light across the night sky. Our Solar System lies in one of the spiral arms of the Milky Way.

Stars differ in colour based on temperature: blue-white stars are the hottest, red stars are the coolest. A star's life cycle progresses from nebula → protostar → main sequence star → red giant / supergiant → white dwarf / neutron star / black hole.

The Solar System

The Solar System consists of the Sun and all celestial objects gravitationally bound to it — eight planets, their moons, dwarf planets, asteroids, comets, and meteoroids. The Sun contains about 99.86% of the total mass of the Solar System.

The eight planets in order from the Sun are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune (My Very Educated Mother Just Served Us Nachos). They are divided into:

• Inner (Terrestrial) planets: Mercury, Venus, Earth, Mars — rocky, dense, smaller.
• Outer (Jovian/Gas Giant) planets: Jupiter, Saturn, Uranus, Neptune — large, gaseous, have rings and many moons.

Key facts for CTET:

• Mercury — smallest planet; no atmosphere; extreme temperature variation.
• Venus — hottest planet (~465°C) due to greenhouse effect; rotates east to west (retrograde).
• Earth — only planet with life; one natural satellite (Moon).
• Mars — Red Planet; has the largest volcano (Olympus Mons) and the longest canyon (Valles Marineris).
• Jupiter — largest planet; Great Red Spot is a giant storm.
• Saturn — has prominent rings made of ice and rock.
• Uranus — rotates on its side (axial tilt ~98°).
• Neptune — farthest planet; has the strongest winds.

Pluto was reclassified as a dwarf planet in 2006. Asteroids are rocky bodies mostly found in the Asteroid Belt between Mars and Jupiter. Comets are icy bodies with long tails that appear when near the Sun (e.g., Halley's Comet appears every ~76 years). Meteors are space rocks that burn up in Earth's atmosphere; those that reach the surface are meteorites.

Acid Rain and Environmental Impact

Acid rain is precipitation (rain, snow, fog) with a pH lower than 5.6. Normal rain is slightly acidic (pH ~5.6) because carbon dioxide dissolves in water to form carbonic acid. When air pollution adds sulphur dioxide (SO₂) and nitrogen oxides (NOₓ) to the atmosphere, they react with water vapour to form sulphuric acid and nitric acid, making rain far more acidic.

Main sources of SO₂ and NOₓ:

• Burning of fossil fuels (coal and oil) in power plants and factories.
• Vehicle exhaust emissions.
• Industrial processes such as metal smelting.
• Volcanic eruptions (natural source).

Effects of acid rain:

• On aquatic ecosystems: Lowers pH of lakes and rivers, killing fish and aquatic organisms.
• On forests: Leaches essential nutrients (calcium, magnesium) from soil; weakens trees, making them susceptible to disease and pests.
• On monuments and buildings: Corrodes marble, limestone, and metals — the Taj Mahal has been visibly damaged by acid rain from nearby industries.
• On human health: Aggravates respiratory diseases like asthma and bronchitis.

Acid rain can travel thousands of kilometres from the source of pollution, making it a transboundary environmental problem. Solutions include using catalytic converters in vehicles, switching to renewable energy, and using scrubbers in factory chimneys to remove SO₂ before it is released into the atmosphere.

The pH scale (0–14) measures acidity: below 7 is acidic, 7 is neutral, above 7 is basic. Acid rain has a pH between 4 and 5.6.

Safety During Natural Disasters

Knowing what to do during natural disasters can save lives. NCERT Class 8 emphasises safety measures for thunderstorms, cyclones, and earthquakes.

During a thunderstorm / lightning:

• Stay indoors or inside a car (metal body acts as a Faraday cage).
• Do NOT stand under tall isolated trees — lightning strikes the tallest point.
• Avoid open fields, hilltops, and open bodies of water.
• Stay away from electrical appliances, telephone lines, and water taps.
• If caught outdoors, crouch low with feet together — do not lie flat.

During a cyclone:

• Listen to weather bulletins and follow evacuation orders.
• Move to a cyclone shelter or a sturdy building well before the storm arrives.
• Store adequate food, drinking water, and medicines.
• Do not venture out during the 'eye' of the cyclone — calm conditions are temporary.
• Stay away from the coast, rivers, and low-lying areas prone to flooding.

During an earthquake:

• If indoors — drop, cover (under a sturdy table), and hold on.
• Stay away from windows, heavy furniture, and exterior walls.
• Do NOT use lifts; use stairs after shaking stops.
• If outdoors — move away from buildings, trees, and power lines; drop to the ground.
• After the quake, be alert for aftershocks and gas leaks.

India's National Disaster Management Authority (NDMA) and state agencies conduct mock drills, issue guidelines, and coordinate relief operations. Early warning systems — such as the Indian Tsunami Early Warning Centre — play a critical role in minimising casualties. Community preparedness, awareness campaigns, and earthquake-resistant building design (base isolators, shear walls) are long-term measures to reduce disaster risk.