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The Universe and the Earth
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Atmosphere and its composition
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Atmospheric Temperature
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Atmospheric Moisture
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Air Mass, Fronts & Cyclones
15 topics
6
Evolution of Earths Crust, Earthquakes and Volcanoes
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Interior of The Earth
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Landforms
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Geomorphic Processes
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Movement of Ocean Water
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Oceans and its Properties
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Climate of a Region
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Indian Geography - introduction, Geology
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Physiography of India
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Indian Climate
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Indian Drainage
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Chapter 5: Air Mass, Fronts & Cyclones
Chapter Test15 topics•Estimated reading: 45 minutes
Air Mass
Key Point
An air mass is a large body of air, extending over 1600 km or more, with uniform temperature and moisture. Its properties depend on the source region, which must have stable pressure and uniform topography.
An air mass is a large body of air, extending over 1600 km or more, with uniform temperature and moisture. Its properties depend on the source region, which must have stable pressure and uniform topography.
Detailed Notes (24 points)
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Definition
A large body of air, usually 1600 km or more in extent, with homogeneous physical properties (temperature, moisture, etc.).
Concept given by Norwegian meteorologists V. Bjerknes and J. Bjerknes during World War I.
Criteria of an Air Mass
Must cover more than 1600 km horizontally and extend several km vertically.
Must have uniform properties across its extent.
Must be distinct from surrounding air and retain its properties while moving.
Source Region of Air Masses
Defined as the surface area of Earth from which the air mass derives its properties.
Conditions for development:
o Large and uniform topography.
o Gentle, divergent airflow with high pressure.
o Air must remain in contact with the surface for a long time.
Classification of Source Regions
Broad division: Continental (c) and Maritime (m).
Six major source regions:
1. Continental interiors of Siberia.
2. Sahara Desert.
3. Continental regions of Canada.
4. Atlantic Ocean.
5. Pacific Ocean.
6. Southern Indian Ocean.
Stable Air Mass (s): Brings dry, settled weather.
Unstable Air Mass (u): Brings precipitation and disturbances.
Air Mass Source Regions
| Type | Region | Nature |
|---|---|---|
| Continental (c) | Siberia, Sahara, Canada | Dry, stable |
| Maritime (m) | Atlantic, Pacific, Indian Ocean | Moist, unstable |
Mains Key Points
Air masses are crucial in understanding global and regional weather patterns.
Their formation depends on source regions with stable high pressure.
They transport moisture, heat, and influence precipitation.
The interaction between contrasting air masses often leads to cyclones and storms.
Prelims Strategy Tips
Air mass concept developed by V. Bjerknes and J. Bjerknes during World War I.
Source regions must have large, uniform topography and high pressure.
Continental air masses are generally dry; maritime air masses are moist.
Stable air mass brings fair weather, unstable air mass brings precipitation.
Modification and Classification of Air Mass
Key Point
Air masses undergo modification when they move from their source regions, changing temperature and moisture. They are classified into maritime tropical, continental tropical, maritime polar, continental polar, and continental arctic types.
Air masses undergo modification when they move from their source regions, changing temperature and moisture. They are classified into maritime tropical, continental tropical, maritime polar, continental polar, and continental arctic types.
Detailed Notes (13 points)
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Modification of Air Mass
Definition: Heating or cooling of an air mass that alters its temperature and moisture content.
Types of Modification:
o Thermodynamic Modification: Occurs when an air mass moves and undergoes heating or cooling as it travels from one source region to another.
o Mechanical Modification: Occurs due to vertical uplift or downward movement caused by orographic barriers, cyclonic or anticyclonic conditions, turbulence, and eddies.
Classification of Air Mass
Based on temperature and humidity: Polar (cold) and Tropical (warm).
Five major types:
a. Maritime Tropical (mT)
b. Continental Tropical (cT)
c. Maritime Polar (mP)
d. Continental Polar (cP)
e. Continental Arctic (cA)
Types of Air Masses and Their Characteristics
| Air Mass | Source Region | Features | Weather |
|---|---|---|---|
| Continental Polar (cP) | Arctic basin, N. America, Eurasia, Antarctica | Dry, cold, stable | Winter: frigid, clear; Summer: less stable, warmer landmasses |
| Maritime Polar (mP) | Oceans between 40°–60° latitudes | Cool, moist, unstable | Winter: humid, cloudy, precipitation; Summer: fair and stable |
| Continental Tropical (cT) | Sahara, West Asia, Australia | Dry, hot, stable | Dry throughout the year, little precipitation |
| Maritime Tropical (mT) | Tropical oceans (Gulf of Mexico, Pacific, Atlantic) | Warm, humid, unstable | Winter: mild, cloudy; Summer: hot, humid, convectional rain |
| Continental Arctic (cA) | High Arctic interiors | Very cold, dry, stable | Extremely cold, clear conditions |
Mains Key Points
Air masses influence weather by transporting heat and moisture.
Modification determines local climate conditions in source and target regions.
Tropical air masses bring warmth and precipitation, while polar air masses bring cold and dryness.
Interaction of different air masses often leads to fronts and cyclonic systems.
Prelims Strategy Tips
Air masses modify as they move away from their source regions.
Thermodynamic modification = heating/cooling; Mechanical modification = uplift or subsidence.
mT = warm & moist; cT = hot & dry; mP = cool & moist; cP = cold & dry; cA = very cold & dry.
Fronts
Key Point
Fronts are transitional zones between contrasting air masses, critical in shaping global weather. They form due to differences in temperature, pressure, and humidity. Fronts influence precipitation, storms, and mid-latitude cyclones, making them vital for weather forecasting.
Fronts are transitional zones between contrasting air masses, critical in shaping global weather. They form due to differences in temperature, pressure, and humidity. Fronts influence precipitation, storms, and mid-latitude cyclones, making them vital for weather forecasting.
Detailed Notes (38 points)
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Definition and Concept
Front = Boundary zone between two different air masses with distinct temperature and moisture.
Concept introduced by Norwegian meteorologists V. Bjerkens and J. Bjerkens during World War I.
Represent transition zones in global circulation.
Characteristics of Fronts
Temperature contrast controls thickness of frontal zone (stronger contrast = sharper front).
Isobars bend near fronts, showing pressure differences.
Fronts generally occur in low-pressure troughs.
Associated with wind shifts, turbulence, clouds, and precipitation.
Often linked to cyclogenesis in mid-latitudes.
Types of Fronts
# Stationary Front
Occurs when two air masses meet but neither displaces the other.
Winds blow parallel but opposite along the boundary.
Weather: prolonged cloudiness, drizzle, fog; can persist for days.
Example: Prolonged winter rains in Eastern USA.
# Warm Front
Warm air advances over cold air, rising gently (slope 1:100–200).
Cloud sequence: Cirrus → Cirrostratus → Altostratus → Nimbostratus.
Produces widespread, light-to-moderate continuous rain/snow.
Associated with halos around Sun/Moon (cirrostratus clouds).
Example: Pre-monsoon rains in North India linked to western disturbances.
# Cold Front
Cold, dense air undercuts warm air, forcing rapid uplift (slope 1:50–100).
Cloud sequence: Cumulus → Cumulonimbus; intense convection.
Produces heavy rainfall, thunderstorms, squalls, hailstorms.
Weather changes: sharp drop in temperature, pressure rise, clear skies after passage.
Example: Tornado-producing fronts in the US Midwest.
# Occluded Front
Forms when a fast-moving cold front overtakes a warm front.
Warm sector shrinks, warm air displaced aloft.
Weather: complex – thunderstorms, irregular rain, variable winds.
Common in mature extratropical cyclones (Europe, North America).
Global Significance
Mid-latitude cyclones are frontal systems transporting heat and moisture poleward.
Fronts affect agriculture (timing of rainfall, frost risk).
Aviation hazards: turbulence, icing, poor visibility.
Climate: long-term patterns of frontal rainfall define ecosystems (e.g., European temperate forests, US prairies).
Types of Fronts and Associated Weather
| Front Type | Slope | Cloud Sequence | Weather |
|---|---|---|---|
| Stationary | Almost flat | Stratus, Nimbostratus | Persistent cloudiness, drizzle, fog |
| Warm | Gentle (1:100–200) | Cirrus → Cirrostratus → Altostratus → Nimbostratus | Steady widespread rain/snow |
| Cold | Steep (1:50–100) | Cumulus → Cumulonimbus | Heavy showers, thunderstorms, hail |
| Occluded | Variable | Mixed (cumulus + nimbus) | Erratic rain, stormy conditions |
Mains Key Points
Fronts represent the dynamic interaction between air masses and shape weather systems.
Stationary fronts create persistent gloomy weather; warm fronts bring stratified clouds and long-lasting rainfall.
Cold fronts cause severe convection, thunderstorms, and heavy precipitation.
Occluded fronts are typical of mature temperate cyclones, causing complex rainfall.
Frontal systems redistribute global heat and moisture, influencing climate zones.
Understanding frontal meteorology is essential for agriculture, aviation safety, and disaster management.
Prelims Strategy Tips
Stationary fronts cause persistent cloudiness and drizzle.
Warm fronts → halos around Sun/Moon due to cirrostratus clouds.
Cold fronts → steep slope, heavy showers, thunderstorms.
Occluded fronts → found in mature mid-latitude cyclones.
Western disturbances affecting India are frontal systems.
Cyclones
Key Point
Cyclones are large-scale low-pressure systems characterized by fast inward air circulation. They occur in both tropical and temperate regions and play a crucial role in global weather, rainfall, and climate. Cyclones can cause widespread destruction but are also vital for redistributing heat and moisture.
Cyclones are large-scale low-pressure systems characterized by fast inward air circulation. They occur in both tropical and temperate regions and play a crucial role in global weather, rainfall, and climate. Cyclones can cause widespread destruction but are also vital for redistributing heat and moisture.
Detailed Notes (35 points)
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General Definition
Cyclone = fast inward moving air circulation around a low-pressure center.
Derived from Greek word 'cyclos' meaning 'coils of a snake'.
Air circulates anticlockwise in the Northern Hemisphere, clockwise in the Southern Hemisphere (Coriolis effect).
Two types: Tropical Cyclones and Temperate (Extratropical) Cyclones.
Temperate (Extratropical) Cyclones
Location: 35°–65° latitudes, both hemispheres (mid-latitudes).
Formed by frontogenesis – interaction of contrasting air masses (polar vs tropical).
Shape: circular, semicircular, elliptical, or elongated.
Size: Large – spread over 500–600 km (sometimes >2000 km).
Velocity: moderate, around 30–40 km/h; may reach 60–80 km/h.
Weather: cloudy skies, variable winds, widespread rain/snow, often lasting several days.
Move from west to east (westerlies).
Important for winter rainfall in North India (western disturbances).
Tropical Cyclones
Location: 5°–30° latitudes, over warm tropical oceans.
Require SST (Sea Surface Temperature) > 26°C, high humidity, and low wind shear.
Size: Smaller than temperate cyclones (100–1000 km).
Wind velocity: Very high, >120 km/h; extreme cases >250 km/h.
Structure: Eye (calm center), Eye wall (intense winds & rain), spiral rain bands.
Movement: Westward and poleward due to trade winds and Coriolis force.
Names: Called 'Hurricanes' in Atlantic, 'Typhoons' in Pacific, 'Cyclones' in Indian Ocean.
Weather: Intense rainfall, strong winds, storm surges, coastal flooding.
Stages of Tropical Cyclones
1. Formation: Warm ocean + evaporation + Coriolis force creates low pressure.
2. Intensification: Latent heat release strengthens convection.
3. Mature Stage: Well-defined eye, maximum intensity.
4. Decay: Weakens when it makes landfall (friction + loss of moisture).
Significance of Cyclones
Positive: Redistribute heat & moisture, bring rainfall to arid areas, recharge groundwater.
Negative: Cause floods, coastal erosion, crop damage, human loss.
Climate link: Increasing intensity linked with global warming.
Examples
Temperate: Nor’easters (USA), Western Disturbances (India), European windstorms.
Tropical: Hurricane Katrina (USA, 2005), Cyclone Fani (India, 2019), Typhoon Haiyan (Philippines, 2013).
Comparison of Tropical and Temperate Cyclones
| Feature | Tropical Cyclone | Temperate Cyclone |
|---|---|---|
| Location | 5°–30° latitude over warm oceans | 35°–65° latitude, over land & sea |
| Size | 100–1000 km | 500–2000 km |
| Wind Speed | 120–250+ km/h | 30–80 km/h |
| Structure | Eye, eyewall, spiral bands | Frontal system (warm & cold fronts) |
| Movement | Westward & poleward | West to East |
| Duration | Few days to a week | Several days to weeks |
| Weather | Intense rain, storms, floods | Widespread rain, snow, cloudy weather |
Mains Key Points
Cyclones are vital in global circulation, redistributing heat and moisture.
Temperate cyclones are large frontal systems controlling mid-latitude weather.
Tropical cyclones are smaller but more violent, with destructive winds and storm surges.
Cyclones affect agriculture, settlements, and disaster preparedness.
Global warming is linked to increasing frequency and intensity of tropical cyclones.
Western disturbances (extratropical cyclones) are critical for Indian winter crops.
Prelims Strategy Tips
Tropical cyclones need sea surface temperature >26°C.
Temperate cyclones are frontal systems common in mid-latitudes.
Cyclones rotate anticlockwise in the Northern Hemisphere, clockwise in Southern Hemisphere.
Western disturbances in India are extra-tropical cyclones.
Hurricanes, typhoons, and cyclones are the same phenomenon but named regionally.
Distribution of Temperate Cyclones
Key Point
Temperate cyclones develop mainly along polar fronts where warm tropical air masses meet cold polar air masses. They are most common in mid-latitudes (35°–65°) across both hemispheres and strongly influence the weather of North America, Europe, Asia, and parts of the Southern Hemisphere.
Temperate cyclones develop mainly along polar fronts where warm tropical air masses meet cold polar air masses. They are most common in mid-latitudes (35°–65°) across both hemispheres and strongly influence the weather of North America, Europe, Asia, and parts of the Southern Hemisphere.
Detailed Notes (15 points)
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Atlantic-Arctic Front
Warm North Atlantic meets cold Arctic air mass, creating strong temperature contrast and low-pressure systems.
Cyclones move northeastward and influence the weather of Western Europe (e.g., Britain, Scandinavia).
North America Polar Front
Convergence of continental polar air mass with Arctic air mass.
Cyclones typically develop around the Great Lakes region.
They then move northeastward, bringing snowstorms and blizzards to Canada and northeastern USA.
Mediterranean-Caspian Front
Cyclones form where continental cold air meets warmer air over the Mediterranean and Caspian seas.
These extra-tropical cyclones follow regional wind patterns.
Western disturbances (a form of these cyclones) bring winter rainfall and light showers to northwestern India and the Indo-Gangetic plains.
Southern Hemisphere
Fewer large landmasses reduce sharp temperature contrasts.
Cyclones generally develop between 35°–65° S latitudes, mostly over oceans.
They typically move southeastward, bringing widespread rainfall and storms to southern Chile, South Africa, Australia, and New Zealand.
Major Regions of Temperate Cyclone Formation
| Region | Air Mass Interaction | Movement | Impact |
|---|---|---|---|
| Atlantic-Arctic Front | Warm Atlantic vs Cold Arctic | NE towards Europe | Rain, storms in Western Europe |
| North America Polar Front | Continental Polar vs Arctic | NE towards Canada & NE USA | Snowstorms, blizzards |
| Mediterranean-Caspian Front | Continental vs Warm Sea Air | East/Northeast | Western disturbances, winter rain in India |
| Southern Hemisphere | Marine Tropical vs Polar Air | SE | Storms in Chile, South Africa, Australia, NZ |
Mains Key Points
Temperate cyclones form along polar fronts and control mid-latitude weather.
Atlantic-Arctic cyclones bring rain and storms to Western Europe.
North American cyclones cause snowstorms and blizzards in NE USA and Canada.
Mediterranean-Caspian front produces western disturbances important for Indian winter crops.
Southern Hemisphere cyclones are more oceanic, impacting coastal nations like Chile, Australia, and NZ.
Prelims Strategy Tips
Temperate cyclones mainly occur between 35°–65° latitude.
Atlantic-Arctic front cyclones influence Western Europe.
North America’s Great Lakes region is a hotspot for extratropical cyclone development.
Western disturbances in India are linked to Mediterranean-Caspian front cyclones.
Southern Hemisphere cyclones form mostly over oceans due to limited landmass.
Tropical Cyclones
Key Point
Tropical cyclones are violent storms originating in tropical oceans between the Tropic of Cancer and Tropic of Capricorn. They derive their energy from latent heat of condensation, move in spiral form, and cause large-scale destruction in coastal regions.
Tropical cyclones are violent storms originating in tropical oceans between the Tropic of Cancer and Tropic of Capricorn. They derive their energy from latent heat of condensation, move in spiral form, and cause large-scale destruction in coastal regions.
Detailed Notes (22 points)
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Definition
Violent storms originating in tropical oceans, confined between the Tropic of Cancer and Tropic of Capricorn.
Responsible for widespread destruction in coastal regions.
Characteristics
Location: confined within the tropics.
Spread: smaller than temperate cyclones, typically 30–300 km in diameter.
Origin: thermal in nature, acting as self-propelling heat engines due to latent heat of condensation.
Shape: spiral, eye at the center with calm conditions.
Velocity: 32 km/hr to >180 km/hr.
Direction: counter-clockwise in the Northern Hemisphere, clockwise in the Southern Hemisphere (Coriolis Effect).
Regional Names
Cyclone: Indian Ocean.
Hurricane: Atlantic Ocean & Eastern Pacific.
Typhoon: Western Pacific & South China Sea.
Willy-Willies: Australia.
Favorable Conditions for Formation
Warm ocean waters with surface temperature ≥ 27°C.
High relative humidity and abundant moisture supply.
Presence of Coriolis force (hence rare near the equator).
Weak low-pressure disturbance to trigger development.
Divergence aloft/upper-air anticyclonic circulation to sustain convection.
Unstable atmosphere to promote continuous uplift and condensation.
Regional Names of Tropical Cyclones
| Region | Name |
|---|---|
| Indian Ocean | Cyclone |
| Atlantic & Eastern Pacific | Hurricane |
| Western Pacific & South China Sea | Typhoon |
| Australia | Willy-Willies |
Mains Key Points
Tropical cyclones act as major heat engines of the tropics by transferring latent heat vertically.
Cause large-scale destruction in coastal regions due to strong winds, storm surges, and flooding.
Play a role in global heat balance by redistributing energy.
Regional naming reflects cultural and geographical contexts of impact.
Climate change has influenced cyclone intensity and frequency.
Prelims Strategy Tips
Tropical cyclones require sea-surface temperature ≥ 27°C.
Do not form near the equator (lack of Coriolis force).
Latent heat of condensation is the main energy source.
Names vary by region: Hurricane, Typhoon, Cyclone, Willy-Willies.
Distribution of Tropical Cyclones
Key Point
Tropical cyclones occur in well-defined ocean basins across the world, especially over warm tropical waters. Their distribution is influenced by sea surface temperatures, Coriolis force, and seasonal variations. The North Pacific has the highest frequency globally, while the Indian Ocean cyclones are notorious for their destructiveness.
Tropical cyclones occur in well-defined ocean basins across the world, especially over warm tropical waters. Their distribution is influenced by sea surface temperatures, Coriolis force, and seasonal variations. The North Pacific has the highest frequency globally, while the Indian Ocean cyclones are notorious for their destructiveness.
Detailed Notes (22 points)
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North Atlantic (Western Tropical Part)
Regions: Caribbean Sea, Gulf of Mexico, and up to the US Atlantic coast.
Peak Season: August to October.
Known as hurricanes.
Indian Ocean
Regions: Bay of Bengal and Arabian Sea.
Peak Seasons: Pre-monsoon (May) and post-monsoon (October–November).
Cyclones here are among the most devastating, affecting South Asia.
South Indian Ocean
Regions: From Madagascar and Réunion Islands to 90°E longitude, and Timor Sea in northwestern Australia.
Peak Season: January to March.
North Pacific Ocean (Eastern Tropical Part)
Regions: Western coasts of Mexico, Central America up to California coast.
Peak Season: August to October.
Hurricanes often move westward into open Pacific or hit Mexico.
North Pacific Ocean (Western Tropical Part)
Regions: Philippines, South China Sea, Taiwan, Japan.
Peak Season: August to September.
Records the maximum number of tropical cyclones worldwide.
South Pacific Ocean (Western Tropical Part)
Regions: East coast of Australia, Samoa, Fiji Islands, Coral Sea region.
Peak Season: January to March.
Global Distribution of Tropical Cyclones
| Region | Peak Season | Key Areas | Notes |
|---|---|---|---|
| North Atlantic | Aug–Oct | Caribbean, Gulf of Mexico, US Atlantic coast | Called Hurricanes |
| Indian Ocean | May, Oct–Nov | Bay of Bengal, Arabian Sea | Very destructive in South Asia |
| South Indian Ocean | Jan–Mar | Madagascar, Réunion, Timor Sea | Affects East Africa, NW Australia |
| North Pacific (East) | Aug–Oct | Mexico, Central America, California | Hurricanes in Eastern Pacific |
| North Pacific (West) | Aug–Sep | Philippines, S. China Sea, Japan | Highest cyclone frequency in the world |
| South Pacific (West) | Jan–Mar | Australia, Samoa, Fiji, Coral Sea | Summer cyclones common |
Mains Key Points
Global distribution of tropical cyclones is uneven, concentrated in six main basins.
North Pacific (west) is the most active basin due to warm waters and favorable conditions.
Indian Ocean cyclones, especially in Bay of Bengal, pose high risk for South Asia.
Seasonality is a key factor, linked to monsoon cycles and hemispheric summers.
Understanding distribution helps in disaster preparedness and climate impact studies.
Prelims Strategy Tips
North Pacific (western) records the highest number of tropical cyclones globally.
Bay of Bengal cyclones are more frequent and destructive than Arabian Sea cyclones.
Hurricane season in the Atlantic: August–October.
Australian 'Willy-Willies' peak during January–March.
Coriolis force absence near equator prevents cyclone formation there.
Structure and Life Cycle of a Tropical Cyclone
Key Point
Tropical cyclones are intense low-pressure systems powered by latent heat of condensation over warm oceans. They consist of a central calm eye, a violent eyewall, and spiral rain bands. Their life cycle includes initiation over warm seas, intensification with a distinct eye, and dissipation after landfall. They play a dual role: redistributing heat and moisture, but also causing large-scale destruction.
Tropical cyclones are intense low-pressure systems powered by latent heat of condensation over warm oceans. They consist of a central calm eye, a violent eyewall, and spiral rain bands. Their life cycle includes initiation over warm seas, intensification with a distinct eye, and dissipation after landfall. They play a dual role: redistributing heat and moisture, but also causing large-scale destruction.
Detailed Notes (45 points)
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Vertical Structure of a Tropical Cyclone
Extends from surface to tropopause (~12–15 km).
Lower levels: intense inflow of moist air spiraling inward.
Middle levels: rising air with deep convection, heavy cloud towers.
Upper levels: strong outflow (divergence) that maintains cyclone strength.
Structure
# Eye
Calm, clear, central core with sinking air.
Diameter ranges from 10–50 km.
Lowest pressure zone (as low as 880 mb in strongest cyclones).
Temperature inside the eye can be 5–10°C warmer than surrounding areas due to subsidence heating.
# Eye Wall
Most dangerous part of the cyclone.
Towering cumulonimbus clouds rising up to 15 km.
Extreme winds (150–300 km/h or more), torrential rainfall.
Frequent lightning and thunder, strongest convection.
# Spiral Rain Bands
Bands of cumulonimbus clouds extending 100s of km outward.
Produce heavy rainfall, gusty winds, and tornado-like vortices.
Separated by 'rain-free' zones in between bands.
Life Cycle
# Early Stage (Formation)
Ocean water temperature >27°C up to depth of 60m supplies continuous moisture.
Coriolis effect initiates rotation (absent near equator within 5° latitudes).
Weak low-pressure disturbance intensifies into depression.
Latent heat of condensation acts as an energy engine.
# Mature Stage (Intensification)
Well-developed eye and organized cloud bands.
Peak wind speed exceeds 118 km/h (hurricane intensity).
Central pressure drops drastically creating steep pressure gradient.
Storm surge generated due to strong winds and low pressure.
Responsible for heaviest damage to life and property.
# Decay Stage (Dissipation)
Landfall cuts moisture supply; friction weakens winds.
Cold waters or unfavorable wind shear can also dissipate cyclone over sea.
Eventually breaks down into a remnant low or merges with other weather systems.
Hazards Associated with Tropical Cyclones
Storm Surge: abnormal rise of sea water, most destructive component.
Torrential Rainfall: leads to flooding and landslides.
High Velocity Winds: uproot trees, damage infrastructure.
Secondary Hazards: outbreaks of disease, agricultural loss, displacement.
Energy Source
Latent heat of condensation released by rising moist air fuels cyclone.
Functions as a heat engine transferring oceanic heat into mechanical energy.
Strongest over warm tropical oceans; weakens over land and cold water.
Structure of Tropical Cyclone
| Part | Key Features | Hazards |
|---|---|---|
| Eye | Calm, clear, lowest pressure, warm core | None, deceptively calm |
| Eye Wall | Most intense winds & rainfall, towering clouds | Catastrophic winds, storm surge |
| Rain Bands | Spiral cloud bands, heavy rainfall zones | Flooding, tornado-like vortices |
Life Cycle of Tropical Cyclone
| Stage | Features | Outcome |
|---|---|---|
| Early Stage | Warm ocean >27°C, rising moist air, Coriolis force | Depression forms |
| Mature Stage | Well-developed eye, eyewall, heavy rainfall, strong winds | Maximum destruction |
| Decay Stage | Landfall cuts moisture, friction increases, wind shear | System weakens & dissipates |
Mains Key Points
Tropical cyclones are thermal-origin storms powered by warm oceans and latent heat release.
Structure consists of calm eye, violent eyewall, and spiral rain bands.
Life cycle passes through formation, intensification, and dissipation.
Cyclones redistribute global heat and moisture but cause devastating hazards like storm surges, floods, and winds.
Forecasting and disaster preparedness depend on detailed knowledge of their structure and energy sources.
Prelims Strategy Tips
Cyclones are heat engines fueled by latent heat of condensation.
Eye is warm and calm, but surrounded by the destructive eyewall.
Storm surge is the deadliest hazard, not winds alone.
Cyclones weaken rapidly after landfall due to cut-off moisture.
Coriolis force is essential: cyclones do not form within 5° of the equator.
Types of Cyclones
Key Point
Cyclones vary in intensity and structure, from weak tropical disturbances to devastating hurricanes. They develop under different oceanic and atmospheric conditions. A special type of cyclone, Medicanes, forms over the Mediterranean and shares similarities with tropical systems but is weaker and smaller.
Cyclones vary in intensity and structure, from weak tropical disturbances to devastating hurricanes. They develop under different oceanic and atmospheric conditions. A special type of cyclone, Medicanes, forms over the Mediterranean and shares similarities with tropical systems but is weaker and smaller.
Detailed Notes (28 points)
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Tropical Disturbance
Migratory, wave-like low-pressure systems associated with easterly trade winds.
Very extensive, can influence weather of both tropical and subtropical areas.
Often precursors to stronger cyclones.
Example: Easterly waves over Atlantic (often seed hurricanes).
Tropical Depression
Small low-pressure centers, usually within ITCZ (Intertropical Convergence Zone).
Seldom develop in trade wind belt due to insufficient rotation (Coriolis effect).
Sustained winds: less than 63 km/h.
Causes moderate rainfall, localized flooding.
Tropical Storm
Organized low-pressure systems with sustained winds between 63–118 km/h.
Can cause heavy rainfall, flooding, and coastal inundation.
Assigned official names by regional meteorological centers.
Example: Tropical Storm Nargis (before intensifying into cyclone).
Hurricanes / Typhoons / Cyclones
Large-scale tropical cyclones with sustained winds exceeding 118 km/h.
Terminology: Hurricane (Atlantic & Eastern Pacific), Typhoon (Western Pacific & South China Sea), Cyclone (Indian Ocean & Australia).
Size: 160–600 km in diameter; can persist for over a week.
Hazards: Storm surge, torrential rain, destructive winds.
Example: Hurricane Katrina (2005), Typhoon Haiyan (2013), Cyclone Fani (2019).
Medicanes
Short for 'Mediterranean Hurricanes'.
Extra-tropical hurricanes observed over the Mediterranean Sea.
Exhibit hybrid features: tropical cyclone-like eye and eyewall but weaker intensity.
Favorable conditions: relatively warm Mediterranean waters in autumn.
Usually last for 1–3 days, with strong winds and flash floods.
Example: Medicane Ianos (Greece, 2020).
Comparison between Tropical Cyclones and Medicanes
| Feature | Tropical Cyclones | Medicanes |
|---|---|---|
| Occurrence | Warm tropical waters (Atlantic, Pacific, Indian Oceans) | Relatively warm Mediterranean waters (temperate zone) |
| Wind Speed | High (118 km/h or more) | Low to moderate (up to ~100 km/h) |
| Size | Large (160–600 km diameter) | Smaller (70–200 km diameter) |
| Duration | Up to 1–2 weeks | Short-lived (1–3 days) |
| Energy Source | Latent heat from deep warm oceans | Shallow warm waters + baroclinic influences |
| Impact | Storm surge, floods, widespread destruction | Localized flooding, coastal wind damage |
Mains Key Points
Cyclones evolve from disturbances to hurricanes depending on sea surface temperature and atmospheric conditions.
They redistribute heat, moisture, and energy across regions but also cause devastation.
Medicanes are weaker, short-lived, and limited to the Mediterranean but significant for Europe.
Climate change is altering cyclone frequency, intensity, and geographic spread.
Preparedness and early warning systems are crucial in reducing cyclone impacts.
Prelims Strategy Tips
Cyclones are classified into disturbances, depressions, storms, and hurricanes based on wind speed.
Official naming starts from Tropical Storm stage.
Medicanes are rare but increasing in frequency due to Mediterranean warming.
Hurricanes, Typhoons, and Cyclones are the same phenomena with different regional names.
Cyclones require Coriolis effect, so none form within 5° latitude of the Equator.
Naming of Tropical Cyclones & Fujiwhara Effect
Key Point
Tropical cyclones are named systematically by WMO regional bodies to avoid confusion and improve communication. An unusual phenomenon, the Fujiwhara Effect, occurs when two cyclones come close enough to interact, sometimes merging into a single powerful storm.
Tropical cyclones are named systematically by WMO regional bodies to avoid confusion and improve communication. An unusual phenomenon, the Fujiwhara Effect, occurs when two cyclones come close enough to interact, sometimes merging into a single powerful storm.
Detailed Notes (20 points)
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Naming of Tropical Cyclones
Started in Indian Ocean region in 2000.
Names proposed by National Meteorological & Hydrological Services (NMHSs) of WMO regions.
8 Indian Ocean countries: India, Bangladesh, Maldives, Myanmar, Oman, Pakistan, Sri Lanka, Thailand contribute names.
Names follow alphabetical order and are used sequentially when a cyclone forms.
Significance:
- Avoids confusion if multiple cyclones occur simultaneously.
- Enhances public awareness and preparedness.
- Helps media and scientists communicate clearly.
Fujiwhara Effect
Defined as the interaction between two cyclones within 1,400 km of each other.
Causes unpredictability: rapid intensification, unusual movement, or merger.
First observed in 1964 (Typhoons Marie & Kathy, Western Pacific).
Named after Japanese meteorologist Sakuhei Fujiwhara.
Five interaction types:
- Elastic Interaction (EI): Storms deflect paths without merging.
- Partial Straining Out (PSO): Smaller storm partly dissipates.
- Complete Straining Out (CSO): Smaller storm fully dissipates (only if unequal strength).
- Partial Merger (PM): Smaller storm partly merges with larger one.
- Complete Merger: Two storms of similar strength merge entirely.
Fujiwhara Effect – Types of Cyclone Interaction
| Interaction Type | Description | Outcome |
|---|---|---|
| Elastic Interaction (EI) | Cyclones deflect each other's paths. | No merger, altered tracks. |
| Partial Straining Out (PSO) | Smaller storm loses part of energy. | Weakened smaller storm. |
| Complete Straining Out (CSO) | Smaller storm completely dissipates (if weaker). | Only stronger storm survives. |
| Partial Merger (PM) | Smaller storm merges partially into larger storm. | Larger storm intensifies slightly. |
| Complete Merger | Two storms of equal strength combine. | Formation of one powerful storm. |
Mains Key Points
Cyclone naming enhances communication, preparedness, and media clarity.
Fujiwhara Effect makes cyclone tracking and forecasting more complex.
Fujiwhara interactions can lead to rapid intensification or unusual tracks.
Climate change may increase frequency of such cyclone interactions.
Understanding naming & Fujiwhara is key for disaster management and warning systems.
Prelims Strategy Tips
Cyclone naming in Indian Ocean started in 2000.
WMO Regional Committees maintain cyclone name lists.
Each region contributes names in advance (alphabetical order).
Fujiwhara Effect occurs if two cyclones are within 1400 km.
Typhoons Marie and Kathy (1964) were first observed Fujiwhara case.
Difference between Tropical and Temperate Cyclones
Key Point
Tropical cyclones and temperate cyclones differ fundamentally in their origin, scale, energy source, and impact. While tropical cyclones are thermal systems fueled by latent heat over warm oceans, temperate cyclones are frontal systems formed by contrasting air masses in mid-latitudes.
Tropical cyclones and temperate cyclones differ fundamentally in their origin, scale, energy source, and impact. While tropical cyclones are thermal systems fueled by latent heat over warm oceans, temperate cyclones are frontal systems formed by contrasting air masses in mid-latitudes.
Detailed Notes (8 points)
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Key Differences
Tropical cyclones are confined to tropics, while temperate cyclones occur beyond tropics.
Origin: tropical = thermal, temperate = frontal.
Formation: tropical cyclones need warm oceans (>27°C), moisture, and latent heat of condensation; temperate cyclones form by interaction of warm and cold air masses.
Location: tropical cyclones mainly over oceans, temperate cyclones over both oceans and continents.
Velocity: tropical cyclones have very high destructive winds, temperate cyclones have moderate winds.
Duration: tropical cyclones last up to 7 days; temperate cyclones may last 15–20 days.
Impact: tropical cyclones bring storm surges and heavy rainfall; temperate cyclones cause widespread frontal rainfall and snow in higher latitudes.
Comparison between Tropical and Temperate Cyclones
| Aspect | Tropical Cyclones | Temperate Cyclones |
|---|---|---|
| Emergence | Confined to tropics; smaller in area | Beyond tropics; larger in area |
| Origin | Thermal origin | Frontal origin |
| Formation | Require warm oceans (>27°C), latent heat of condensation, moisture | Form due to interaction of contrasting air masses; over oceans & land |
| Velocity | High, violent, destructive winds | Moderate wind velocity |
| Duration | Up to 7 days | 15–20 days |
| Impact | Storm surges, flooding, coastal destruction | Widespread rainfall, snow, frontal weather |
Mains Key Points
Cyclones are broadly classified into tropical and temperate based on origin and location.
Tropical cyclones act as heat engines fueled by oceanic evaporation and latent heat.
Temperate cyclones derive energy from contrasting air masses at fronts.
Impact differs: tropical cyclones devastate coasts, temperate cyclones shape mid-latitude weather.
Understanding differences is vital for disaster preparedness and climate studies.
Prelims Strategy Tips
Tropical cyclones = thermal origin; Temperate cyclones = frontal origin.
Tropical cyclones last up to 7 days; Temperate cyclones 15–20 days.
Cyclone movement differs due to Coriolis: counterclockwise (NH) and clockwise (SH).
Anticyclones
Key Point
Anticyclones are extensive high-pressure systems larger than mid-latitude cyclones, characterized by descending and diverging air. They bring stable weather conditions such as clear skies, dry air, and calm winds, but can occasionally lead to stagnation of air and pollution buildup.
Anticyclones are extensive high-pressure systems larger than mid-latitude cyclones, characterized by descending and diverging air. They bring stable weather conditions such as clear skies, dry air, and calm winds, but can occasionally lead to stagnation of air and pollution buildup.
Detailed Notes (8 points)
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Salient Features of Anticyclones
Large migratory high-pressure cells of the mid-latitudes, larger than cyclones.
Air converges aloft and subsides downward, diverging near the surface.
Winds blow clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere.
No air-mass conflict or fronts are associated with anticyclones.
Characterized by clear skies, calm, and dry weather.
Very limited wind movement near the center, but stronger winds outward.
Can stagnate over a region for days, causing prolonged dry conditions or fog and pollution buildup in winter.
Difference between Cyclones and Anticyclones
| Aspect | Cyclones | Anticyclones |
|---|---|---|
| Pressure Center | Low pressure at center, surrounded by high pressure | High pressure at center, surrounded by low pressure |
| Wind Direction | Winds blow inward toward center | Winds radiate outward from center |
| Wind Nature | Violent and destructive | Mild and not destructive |
| Hemisphere Flow | Anticlockwise in NH, clockwise in SH | Clockwise in NH, anticlockwise in SH |
| Weather | Cloudy, thunderstorms, heavy rain | Calm, dry, clear skies |
| Impact | Associated with floods, storm surges | Associated with drought, fog, pollution stagnation |
Mains Key Points
Anticyclones play a crucial role in global circulation by balancing low-pressure systems.
They bring prolonged dry spells and clear skies, influencing agriculture and water resources.
Winter anticyclones may lead to fog, frost, and pollution accumulation in valleys.
Anticyclones over oceans may help stabilize weather and suppress cyclone formation.
Prelims Strategy Tips
Anticyclones = high-pressure systems with descending air; clear skies and dry weather.
Winds in anticyclones blow clockwise in NH and anticlockwise in SH.
Unlike cyclones, anticyclones contain no fronts.
Long-lasting anticyclones may cause smog and fog in winter.
Thunderstorms
Key Point
Thunderstorms are intense atmospheric circulations associated with cumulonimbus clouds, characterized by strong upward air movement, heavy rainfall, lightning, thunder, and sometimes hail. They form rapidly, evolve in distinct stages, and significantly influence local weather patterns.
Thunderstorms are intense atmospheric circulations associated with cumulonimbus clouds, characterized by strong upward air movement, heavy rainfall, lightning, thunder, and sometimes hail. They form rapidly, evolve in distinct stages, and significantly influence local weather patterns.
Detailed Notes (19 points)
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Structure of a Thunderstorm
Associated with large, dense cumulonimbus clouds.
Composed of several convective cells with strong updrafts.
Each cell passes through a three-stage life cycle:
- **Cumulus Stage**: Warm air rises strongly, leading to cloud development.
- **Mature Stage**: Both updrafts and downdrafts occur; heavy rainfall, hail, lightning, and thunder are common.
- **Dissipating Stage**: Downdrafts dominate, spreading winds on the surface and cutting off vertical air movement.
Thunderstorms and Weather
Heavy rainfall: Short-duration, high-intensity downpour.
Hailstorms: Occur if water droplets are lifted above the freezing level multiple times.
Lightning: Produced when electrical charge difference between cloud and ground or within clouds exceeds a threshold.
Thunder: Caused by sudden expansion of air due to lightning heat (30,000°C).
Can trigger flash floods, crop damage, and transportation disruption.
Severe thunderstorms may produce tornadoes and squalls.
Global Distribution
Common in equatorial and tropical regions due to intense convection.
Frequent in monsoon regions (India, Southeast Asia).
USA experiences severe thunderstorms in the Great Plains (Tornado Alley).
Rare in polar regions due to lack of sufficient heat and moisture.
Life Cycle of a Thunderstorm
| Stage | Characteristics | Weather Impact |
|---|---|---|
| Cumulus | Strong updraft, cloud formation begins | Cloud build-up, no rainfall |
| Mature | Updraft + downdraft, intense convection | Heavy rain, hail, lightning, thunder |
| Dissipating | Downdrafts dominate, no vertical air movement | Clouds weaken, rainfall ends, clear skies follow |
Mains Key Points
Thunderstorms are a result of intense convection and latent heat release.
They impact agriculture, transportation, and infrastructure through heavy rain, hail, and lightning.
Severe thunderstorms may evolve into tornadoes or squalls.
They highlight the link between atmospheric instability, convection, and extreme weather events.
Prelims Strategy Tips
Thunderstorms are associated with cumulonimbus clouds.
Thunder is caused by rapid expansion of air due to lightning heat.
USA's Tornado Alley is a global hotspot for severe thunderstorms.
Thunderstorms are most frequent in equatorial and monsoon regions.
Tornadoes
Key Point
Tornadoes are funnel-shaped storms with extremely low pressure at the center and the strongest surface winds on Earth, capable of reaching up to 500 km/h. They occur mostly in middle latitudes during spring and summer, with the USA experiencing the most violent tornadoes.
Tornadoes are funnel-shaped storms with extremely low pressure at the center and the strongest surface winds on Earth, capable of reaching up to 500 km/h. They occur mostly in middle latitudes during spring and summer, with the USA experiencing the most violent tornadoes.
Detailed Notes (19 points)
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Definition
Funnel-shaped violent storm with intense low pressure at the center.
Strong upward suction with rotating winds creates destruction on surface.
Geographical Distribution
Occur in middle latitudes (25°–50° N/S).
Absent beyond 50° N/S.
Present on all continents except Antarctica.
USA (Tornado Alley) experiences the maximum frequency and intensity.
Bangladesh is most susceptible within the Indian subcontinent.
Characteristics
Wind speeds can reach up to 500 km/h.
Duration: Usually short-lived (minutes to an hour).
Path: Narrow, generally a few hundred meters wide but can extend several km.
Accompanied by thunderstorms, lightning, and hail.
Waterspouts
Tornadoes occurring over the sea/oceans.
Common over warm tropical waters.
Appear as rotating water columns with an intense vortex.
Generally weaker than land tornadoes but dangerous to ships and coastal areas.
Tornadoes vs Waterspouts
| Feature | Tornadoes (Land) | Waterspouts (Sea) |
|---|---|---|
| Location | Over land (plains, valleys, storm-prone regions) | Over warm tropical oceans/seas |
| Wind Speed | Can exceed 400–500 km/h | Generally weaker (<150 km/h) |
| Appearance | Funnel-shaped cloud touching the ground | Rotating water column extending from clouds to sea |
| Impact | Destruction of settlements, infrastructure, crops | Dangerous to ships, boats, and coastal regions |
| Frequency | High in USA, Bangladesh, Argentina | Frequent in tropical oceans, Caribbean, Mediterranean |
Mains Key Points
Tornadoes form due to violent convection and vertical wind shear within thunderstorms.
They are short-lived but extremely destructive, causing high casualties and property loss.
Waterspouts are marine counterparts, weaker but significant for maritime hazards.
Study of tornado genesis and prediction helps in disaster preparedness.
Prelims Strategy Tips
Tornado Alley in USA is the world’s most tornado-prone region.
Tornado wind speeds can exceed 500 km/h, making them Earth’s strongest winds.
Bangladesh records the highest tornado-related casualties outside USA.
Waterspouts are tornadoes over warm tropical seas.
Lightning
Key Point
Lightning is a natural electrical discharge caused by charge separation within cumulonimbus clouds. It occurs when negative charges at the bottom of clouds connect with positive charges at the top or on the ground, producing a sudden flash of light and immense energy release.
Lightning is a natural electrical discharge caused by charge separation within cumulonimbus clouds. It occurs when negative charges at the bottom of clouds connect with positive charges at the top or on the ground, producing a sudden flash of light and immense energy release.
Detailed Notes (18 points)
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Formation Process
Raindrops at high altitude freeze into ice crystals.
Collisions between frozen droplets inside cumulonimbus clouds generate electrical charges.
Negative charges (electrons) accumulate at the bottom of the cloud.
Positive charges (protons) and neutral particles gather at the top.
Charge separation creates intense electric fields.
When the attraction between opposite charges overcomes resistance, electrical discharge occurs as lightning.
Features of Lightning
Can reach temperatures up to 30,000°C – hotter than the Sun’s surface.
Can travel at speeds of 200,000 km/h.
Typically lasts only a few microseconds, but the brightness is visible over large distances.
Produces thunder due to rapid heating and expansion of surrounding air.
Types of Lightning
Cloud-to-Ground (CG): Discharge between cloud base and Earth’s surface; most dangerous.
Intra-Cloud (IC): Within a single cloud; most common type.
Cloud-to-Cloud (CC): Discharge between two separate clouds.
Ball Lightning: Rare phenomenon; glowing, spherical lightning moving for several seconds.
Heat Lightning: Visible flashes without sound, usually from distant storms.
Types of Lightning
| Type | Description | Danger Level |
|---|---|---|
| Cloud-to-Ground (CG) | Lightning strike between cloud and ground | Very High |
| Intra-Cloud (IC) | Within one cloud | Low |
| Cloud-to-Cloud (CC) | Between two clouds | Medium |
| Ball Lightning | Rare glowing ball of lightning | Unpredictable |
| Heat Lightning | Silent lightning from distant storms | Negligible |
Mains Key Points
Lightning is a major atmospheric hazard, causing fatalities, wildfires, and power damage.
Understanding its formation helps in disaster preparedness and lightning arrestor design.
India records high lightning fatalities annually, especially in rural regions.
Advanced forecasting and awareness campaigns can reduce risks.
Prelims Strategy Tips
Lightning forms due to charge separation in cumulonimbus clouds.
Cloud-to-Ground lightning is the most dangerous type.
Thunder results from rapid heating and expansion of air after lightning.
Ball lightning is rare but scientifically documented.
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