Welcome, dedicated aspirants! If you are preparing for UPSC, SSC, Railways, Delhi Police, or any other state-level competitive exam, you know that Physical Geography is a high-weightage section in the General Studies paper. A clear understanding of topics like Volcanoes, Humidity, Rainfall, and Cyclones is not just about memorizing facts; it's about understanding the processes that shape our world and frequently appear in the news.
This guide, crafted with over a decade of teaching experience, breaks down these complex phenomena into simple, digestible parts. We will use clear explanations, relatable examples, tables for quick revision.
Let's begin this journey to master Physical Geography!
Table of Contents
- Introduction
- What is a Volcano?
- Structure of a Volcano
- Types of Volcanoes
- Major Volcanic Zones
- Negative Positive Impacts
- What is Humidity?
- Types of Humidity
- Factors Affecting Humidity
- What is Rainfall?
- Types of Rainfall
- Rainfall Distribution in India
- What is a Cyclone?
- How a Cyclone Forms
- Types of Cyclones
- Why is the East Coast of India more vulnerable?
📌 Volcano
What is a Volcano?
A volcano is an opening in the Earth’s crust through which molten magma, gases, and ash come out from below the surface. When pressure builds up inside the Earth, it pushes the molten rock (called magma) upward. Once it reaches the surface, it is known as lava.
Example:
Mount Fuji (Japan), Mount Etna (Italy), and Barren Island (India) are famous
volcanoes.
Structure of a Volcano
|
Part |
Description |
|
Magma Chamber |
Underground pool of molten rock beneath the surface. |
|
Vent |
Path through which magma travels to reach the surface. |
|
Crater |
Bowl-shaped opening at the top where lava erupts. |
|
Lava Flow |
Molten rock flowing from the crater during eruption. |
|
Ash Cloud |
Cloud of ash and gases released during eruption. |
Types of Volcanoes
Volcanoes are classified based on their period of eruption and shape.
|
Type |
Description |
Example |
|||
|
Active Volcano |
Volcanoes that have erupted in recent history and are likely to erupt again. |
Mount Etna (Italy), Stromboli (Italy), Barren Island (India's only active volcano, in the Andaman Sea). |
|||
|
Dormant Volcano |
"Sleeping" volcanoes that have not erupted in a long time but could erupt in the future. |
Mount Fuji (Japan), Vesuvius (Italy). |
|||
|
Extinct Volcano |
Volcanoes that have not erupted in recorded history and are not expected to erupt again. |
Kilimanjaro (Tanzania), Diamond Head (Hawaii). |
|||
|
|
|
Major Volcanic Zones
Volcanoes are mostly found at the boundaries of tectonic plates — where plates meet, collide, or move apart.
1. The Pacific Ring of Fire: The world's most seismically and volcanically active zone. It is a horseshoe-shaped belt around the Pacific Ocean where about 75% of the world's volcanoes are found. (e.g., Japan, Indonesia, Philippines, USA West Coast).
2. The Mid-Atlantic Ridge: A divergent plate boundary where plates are pulling apart, and magma wells up to create new crust, forming underwater volcanoes and islands like Iceland.
3. The Mediterranean Belt: Includes volcanoes in Italy, Greece, and Turkey, formed by the collision of the African and Eurasian plates.
Negative Impacts:
· Loss of life and property.
· Destruction of agriculture and pollution of water sources.
· Air travel disruption due to ash clouds.
· Climatic impact: Volcanic ash and gases can block sunlight, leading to global cooling (e.g., the "Year Without a Summer" after the 1815 eruption of Mount Tambora).
Positive Impacts:
· Fertile Soils: Volcanic ash breaks down to create extremely fertile soil, ideal for agriculture. (e.g., the Deccan Traps in India).
· Tourism: Geysers, hot springs, and volcanic landscapes attract tourists (e.g., Rotorua in New Zealand).
· Mineral Resources: Volcanoes bring valuable minerals like sulfur, diamonds, and copper closer to the surface.
· New Land: Volcanic eruptions can create new islands (e.g., Surtsey, near Iceland).
📌 Humidity & Rainfall
What is Humidity?
Humidity refers to the amount of water vapor present in the air. It tells us how “moist” or “dry” the air is.
· When humidity is high → air feels sticky.
· When humidity is low → air feels dry.
Types of Humidity
|
Type |
Definition |
Example |
|
Absolute Humidity |
The actual amount of water vapor in a given volume of air. |
10 grams of water vapor per cubic meter. |
|
Relative Humidity |
Ratio of current humidity to maximum possible humidity at that temperature. |
If air can hold 20g and contains 10g → RH = 50%. |
|
Specific Humidity |
Weight of water vapor per unit weight of air. |
Used in meteorology. |
Example:
During Indian monsoon (June–September), relative
humidity in coastal cities like Mumbai or Chennai reaches above
80%,
while in desert areas like Jaisalmer it remains around 20–30%.
Rocks
& Earthquake Explained: Types, Causes.
Factors Affecting Humidity
1. Temperature – Warm air holds more moisture.
2. Water bodies – Nearby seas, rivers, or lakes increase humidity.
3. Winds – Moist winds increase humidity; dry winds reduce it.
4. Vegetation – More plants → more evaporation → more humidity.
📌 What is Rainfall?
Rainfall
occurs when water vapor condenses
into droplets that become heavy and fall due to gravity.
Rainfall is the main
source of freshwater on Earth and supports agriculture and
ecosystems.
Types of Rainfall
|
Type |
Mechanism & Process |
Where it is Common |
|
Convectional Rainfall |
The sun
heats the ground, which heats the air above it. This warm air rises, cools,
and condenses to form cumulonimbus clouds, leading to heavy rainfall, often
with thunderstorms. |
Common in equatorial regions (like the Amazon) and during hot summer afternoons in many parts of India. |
|
Orographic / Relief Rainfall |
When moist
air is forced to rise over a mountain barrier. As it rises, it cools and
condenses, causing rain on the windward side (the side facing the
wind). The dry air then descends on the leeward side, creating a rain
shadow area. |
The Western Ghats in India. The windward side receives heavy rainfall (e.g., Mahabaleshwar), while the leeward side is a rain shadow region (e.g., Bangalore). |
|
Cyclonic / Frontal Rainfall |
Associated
with cyclones and depressions. It occurs when a warm air mass meets a cold
air mass. The lighter warm air is forced to rise over the denser cold air. As
it rises, it cools and condenses, leading to widespread rainfall. |
Common in temperate latitudes (e.g., UK, Europe) and during the monsoon season in India due to tropical depressions in the Bay of Bengal. |
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Rainfall Distribution in India
|
Region |
Rainfall (cm/year) |
Example |
|
Western Ghats, Meghalaya |
200–1200 cm |
Cherrapunji, Mawsynram |
|
Northern Plains |
100–200 cm |
Delhi, Lucknow |
|
Thar Desert |
Below 25 cm |
Jaisalmer |
|
Leh & Ladakh |
Below 10 cm |
Cold desert |
Example:
Cherrapunji and Mawsynram (Meghalaya) receive the highest rainfall in
the world, due to moist winds from the Bay of Bengal being blocked by Khasi
hills.
Importance of Rainfall
· Supports agriculture and crops
· Maintains groundwater levels
· Affects climate and vegetation
· Provides drinking water sources
📌 Cyclone
What is a Cyclone?
A cyclone is a large low-pressure system in which winds rotate inward and upward around a central eye. Cyclones form over warm oceans where air rises, cools, and creates a spiral motion due to the Earth’s rotation.
What's in a Name?
The same phenomenon is called by different names in different parts of the world:
· Cyclone: Indian Ocean and South Pacific.
· Hurricane: Atlantic Ocean and Northeast Pacific.
· Typhoon: Northwest Pacific.
How a Cyclone Forms (Tropical Cyclone)
The formation requires specific conditions over warm tropical oceans.
Stages of Formation:
1. Warm Ocean Waters: Sea surface temperature must be above 26.5°C to a depth of about 50 meters. This provides the necessary heat and moisture.
2. Coriolis Force: The Earth's rotation provides the necessary spin to the system. This is why cyclones do not form near the equator (within 5° latitude), where the Coriolis force is minimal.
3. Low-Pressure System: A pre-existing low-pressure area causes warm, moist air to rise.
4. Formation of Eye: As the air rises, it cools and condenses, releasing massive energy (latent heat of condensation). This fuels the storm further. A central calm area, called the eye, forms. The eye is characterized by clear skies and light winds.
5. The Eye Wall: Surrounding the eye is the eye wall, which is a ring of towering thunderstorms where the most severe weather and highest winds occur.
Types of Cyclones
|
Type |
Region/Name |
Example |
|
Tropical Cyclone |
Over tropical oceans |
Cyclone Fani (2019), Cyclone Tauktae (2021) |
|
Temperate Cyclone |
Mid-latitudes, between 30°–60° |
Common in Europe, North America |
|
Tornado |
Small, violent whirlwinds |
USA (Tornado Alley) |
Why
is the East Coast of India more vulnerable?
India's east coast (Tamil Nadu, Andhra Pradesh, Odisha, West Bengal) is more prone to severe cyclones than the west coast because:
· The Bay of Bengal is warmer than the Arabian Sea, providing more energy.
· The shape of the coast funnels cyclones into the land.
· Input of freshwater from major rivers (Ganga, Brahmaputra) makes the bay's surface less dense, preventing mixing of warm surface water with cooler water below, thus retaining the heat.
Examples of Major Cyclones in India:
· Super Cyclone, 1999 (Odisha): One of the most devastating.
· Cyclone Fani, 2019 (Odisha): A success story of excellent disaster management.
· Cyclone Amphan, 2020 (West Bengal & Bangladesh): A super cyclonic storm causing widespread damage.
· Cyclone Tauktae, 2021 (West Coast): A rare, powerful cyclone on the west coast.
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FAQs
Q1.
What is the difference between lava and magma?
A: Magma is molten rock inside the Earth, while lava is magma that reaches the
surface.
Q2.
Which is the most active volcanic region in the world?
A: The Pacific Ring of Fire.
Q3.
What is relative humidity?
A: It’s the ratio of current moisture in the air to the maximum it can hold at
that temperature.
Q4.
What are the types of rainfall in India?
A: Convectional, Orographic, and Cyclonic rainfall.
Q5.
What is the main cause of cyclones?
A: Warm ocean water and low-pressure conditions.
Q6. Which is the strongest cyclone in
recent Indian history?
A: Cyclone Amphan (2020) in the Bay of Bengal.
Q7: Is Barren Island really India's only active volcano?
Ans: Yes, Barren Island, located in the Andaman Sea, is the only
confirmed active volcano in South Asia. It last erupted in 2022.
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