Climatology

The science of climate systems, atmospheric processes, and global weather patterns.

Author

Geography Team

Official Syllabus

NEP-2020 Syllabus

NoteCore I Paper XIII — Climatology and Oceanography (Climatology Section)

**(4 Credit, Theory: 45hrs, Practical: 30hrs)*

*Note: The NEP-2020 syllabus combines Climatology and Oceanography as a single paper. The climatology-specific topics are listed here.

  • Composition and structure of the atmosphere
  • Insolation and heat budget of the Earth
  • Distribution of temperature
  • Atmospheric pressure and general circulation of winds
  • Monsoons and jet streams
  • Stability and instability of the atmosphere
  • Air masses and fronts
  • Temperate and tropical cyclones
  • Types and distribution of precipitation
  • Climate classification (Köppen, Thornthwaite)
  • Hydrological cycle
  • Global warming

UGC NET Syllabus

TipUnit II — Climatology
  • Composition and structure of the atmosphere
  • Insolation and heat budget of the Earth
  • Distribution of temperature
  • Atmospheric pressure and general circulation of winds
  • Monsoons and jet streams
  • Stability and instability of the atmosphere
  • Air masses and fronts
  • Temperate and tropical cyclones
  • Types and distribution of precipitation
  • Classification of world climates: Köppen’s scheme, Thornthwaite’s scheme
  • Hydrological cycle
  • Global warming

Climatology (Paper II / Advanced Topics)

  • ENSO Events (El Niño, La Niña and Southern Oscillations)
  • Meteorological Hazards and Disasters (Cyclones, Thunderstorms, Tornadoes, Hailstorms, Heat and Cold Waves, Drought and Cloudburst, GLOF)
  • Climate Change: Evidences and Causes of Climatic Change in the Past, Human Impact on Global Climate

NEP-2020 Programme Structure

NoteUG Geography (Major) — Course Structure Under NEP-2020
Semester Course Name Credit Marks
V Climatology and Oceanography 4 100

Multidisciplinary Courses: Climatology (3 credits, 100 marks)

Evaluation (With Practical): Semester End Theory 50 + Sessional 10 + Mid-Semester 10 + Practical End 20 + Practical Mid 10 = **100 marks*

Welcome to the Climatology module of Geography OpenCourseWare.

Part A: Common Topics (NEP-2020 & UGC NET)

These topics are covered in both the NEP-2020 undergraduate syllabus and the UGC NET syllabus.

Composition and Structure of the Atmosphere

Warning📘 Syllabus Coverage
Syllabus Topic Details
NEP-2020 Climatology section — Composition and structure
UGC NET Unit II — Composition and structure of the atmosphere

Monkhouse: *“The atmosphere is a thin layer of gas held to the earth by gravitational attraction.”

  • CO₂ — 1752: first gas to be studied (0.03%)
  • ‘Mephitic Air’ — 1772: Rutherford discovered Nitrogen gas (N₂)

Gaseous Composition (%)

Gas Extent Percentage
Nitrogen (N₂) Up to 100 km 78.03%
Oxygen (O₂) Up to 120 km; concentration at 16 km 20.99%
Argon (Ar) — (enters via radioactive breakdown) 0.94%
Carbon Dioxide (CO₂) Up to 32 km. Mainly from respiration and decomposition by biota. 0.03%
Hydrogen (H₂) Up to 1100 km 0.01%
Neon (Ne) — 0.0018%
Helium (He) — 0.0005%
Krypton (Kr) — 0.0001%
Xenon (Xe) — 0.000009%
Ozone (O₃) 12 to 50 km; absorbs UV-B wavelength 0.000001%
  • **Other Trace Gases:*
    • Methane (CH₄): Derived from enteric fermentation in animals and decomposition.
    • Nitrous Oxide (N₂O): Derived from microbial activity in soil.
  • Water Vapour — varies from 0–4%; less above 2000 meters
  • Dust Particles — hygroscopic nuclei; Smoke + Fog = **Smog*

Atmospheric Layers by Gas

Layer Altitude
Molecular Nitrogen Layer 90–200 km
Atomic Oxygen Layer 200–1100 km
Helium Layer 1100–3500 km
Atomic Hydrogen Layer 3500 km to outermost

Structure of the Atmosphere

Troposphere

  • Altitude: 0–8 km (poles) to 16 km (equator); Avg = 16 km
  • Lapse Rate: 1°C per 165 m (or 6.4°C per km). A negative lapse rate is synonymous with temperature inversion, where temperature increases with height.
  • Water vapour: 5000 ppm (lower) to 100 ppm (upper, 11–12 km)
  • Pressure: 1013 mb at sea level → 200 mb at 12 km
  • Density: 1 kg/m³ → 0.2 kg/m³ at top
  • Tropopause: 1.5 km thick; acts as an effective lid on convection; it is an isothermal layer.
    • Height varies latitudinally.
    • Average temperature ranges from −70°C to −85°C (at equator).
    • Average pressure is approximately 100 mb.
    • Discovered by WMO (World Meteorological Organization) — 1957.
  • Air Travel: Air travel from London to New York involves longer time than the return journey because of the resistance of the Upper air jet stream (headwinds).

Stratosphere

  • Altitude: up to 50 km; contains 10% of atmosphere mass
  • ‘Mother of Pearls’ or ‘Nacreous’ clouds found here
  • Ozone Layer: 15–35 km

Stratopause

  • At 50–55 km

Mesosphere

  • Temperature reaches −90°C at the top
  • Pressure: 1 mb at 50 km → 0.001 mb at 90 km

Thermosphere

  • Temperature: −90°C at 80 km → 120°C at 350 km

Ionosphere (80–400 km)

  • Aurora Borealis and **Aurora Australis*
Layer Altitude Function
D-layer 80–90 km Reflects low frequency radio waves
E-layer (Kennelly-Heaviside) 90–160 km Reflects medium frequency
F-layer (Appleton Layer) — Reflects medium to high frequency
G-layer Above 480 km —

Exosphere

  • Altitude: 500–750 km

Insolation and Heat Budget

Warning📘 Syllabus Coverage
Syllabus Topic Details
NEP-2020 Climatology section — Insolation and heat budget
UGC NET Unit II — Insolation and heat budget of the Earth

Insolation

  • Insolation = Incoming Solar Radiation
  • Averaged over a year, approximately 342 W of solar energy reaches every m² of Earth.
  • Solar radiation is mostly in the short wave range of < 4 µm. Solar wavelengths shorter than 0.285 µm hardly penetrate below 20 km altitude of the atmosphere.
  • Pyranometer — instrument to measure Albedo
  • 44% of Energy emitted by Visible Light (EMR)
  • Langley — unit to measure solar constant
  • Astronomical effects (Milankovitch cycles): Eccentricity of earth’s orbit = 95,000 years; Axial tilt = 41,000 years; Wobble and shift of axis (precession) = 21,000 years.

Scattering

  • Lord Rayleigh — explained the phenomena = *“Rayleigh Scattering”
  • Peterson: “at altitude and thickness of sun rays”
  • Scattering factors at different latitudes:
    • 0° → 1; 30°N/S → 1.15; 60°N/S → 2.0; 80°N/S → 5.7; 90° → 44.7

Cloud Reflection

  • Overcast Cirrostratus: 44–50%
  • Cumulonimbus: 90%

Cloudiness vs. Radiation

Cloudiness (0–10 scale) 0 1–3 4–7 8–9 10 (Clx)
Radiation (%) 100 93 82 68 41
  • Sunspots — dark areas of high temperature (faculae) on the sun’s surface

Heat Budget

*“Balance between incoming and outgoing radiation”

  • Albedo (Latin: Albus = white) = Earth’s reflectivity = 35% (Möller)
  • Two billion part of solar radiation = 1/2 billion of total energy
  • 38° N/S — after this latitude, outgoing radiation > incoming
  • Ocean currents transfer excessive heat from tropical to mid-latitude (30–50°) regions

Heat Budget by Different Scholars

Scholar Breakdown
George Kimball 42% albedo + 11% absorbed by water + 4% gases + 57% reach atmosphere
Möller 35% albedo (2% surface, 6% atmosphere, 27% clouds); 65% to heating atmosphere (14% atmosphere + 51% Earth surface)
Howard J. Critchfield R = Qs(1−α) + I; 51% reach Earth surface + 49% scattered (26% clouds, 4% Earth surface, 16% air/dust, 4% clouds)
Glenn T. Trewartha 50% reach surface + 20% absorbed atmosphere + 30% scattered by Earth surface
Strahler & Strahler 32% albedo (5% scattered atmosphere, 21% cloud, 6% Earth surface); 68% absorbed
W.H. Diner 15% absorbed at top of atmosphere + 40% reflected by clouds + 45% reach surface

Distribution of Temperature

Warning📘 Syllabus Coverage
Syllabus Topic Details
NEP-2020 Climatology section — Temperature distribution
UGC NET Unit II — Distribution of temperature
  • Isotherms — imaginary lines joining equal temperature
  • Temperature Anomaly — difference between mean temperature and parallel temperature

Climatic Zones

Zone Latitude Features
Tropical / Torrid 23½°N to 23½°S No winter season
Temperate 23½°N/S to 66½°N/S Both summer and winter
Frigid 66½°N/S to 90°N/S —
Minimum Temperature Range — The range of temperatures ever recorded had been the minimum in the continent of Antarctica.

Vertical Temperature

  • Normal / Environmental Lapse Rate = 0.0065°C/m (or 6.4 to 6.5°C/km).
  • Temperature at upper troposphere = −55°C to −60°C
  • Temperature varies at different times of the day because radiation intensity per unit area and the amount of reflection depend on the angle between solar rays and a tangent to the earth’s surface.
  • Mesosphere upper limit = −80°C
  • At 400 km = 1000°C

Temperature Inversion

Conditions Required

  • Long Nights
  • Clear Sky
  • Stable Weather
  • Dry Air

Types of Temperature Inversion

Type Description Region
Radiation Inversion Air near land cools fast during nights (midnight to 4 AM) Snow-covered North America, Europe
Air Drainage Inversion During long winter nights; houses on upper slopes are warmer Mountainous Europe, Brazil, Canada, Himalayas, USA
Advection Inversion Between air masses (cold & warm) Gulf of Mexico
Subsidence Inversion Concentration of pollutants in lower troposphere —
Convection Inversion Cumulonimbus clouds form —
Frontal Inversion Convergence of warm and cold air masses —
Trade Wind Inversion — —
Blocking Anticyclone Associated with NW Europe in both winter and summer. It causes prolonged cold dry seasons in Western Europe and exceptionally fine weather in summer. NW Europe

Atmospheric Pressure, General Circulation and Planetary Winds

Warning📘 Syllabus Coverage
Syllabus Topic Details
NEP-2020 Climatology section — Pressure and winds
UGC NET Unit II — Atmospheric pressure and general circulation

Distribution of Pressure

  • Lucian Vidie (1843) — invented the Aneroid Barometer
  • Isobar — imaginary line joining equal atmospheric pressure
  • Sea level pressure = 1013.25 mb

Factors Affecting Pressure

  • Temperature, Water Vapour, Rotation of Earth, Altitude, Gravitational Pull

Vertical Distribution

  • 34 millibars per 300 meters of ascent
  • 500 mb at 5.5 km height; 100 mb at 17 km height

Horizontal Distribution (Pressure Belts)

Belt Latitude Type
Equatorial Low 5°–5° North and South Low Pressure (Doldrums)
Sub-Tropical High 25°–35° North and South High Pressure (Horse Latitudes)
Sub-Polar Low 60°–65° North and South Low Pressure
Polar High 85°–90° North and South High Pressure
  • Doldrums — extremely calm air movement (Equatorial region)
  • Horse Latitude — Sub-tropical high pressure zone
  • Aleutian Low — Sub-polar low in the Pacific

General Circulation of Winds

  • Hadley Cell — Edmund Hadley (1686), modified by George Hadley (1735)
  • Pressure Gradient — change of pressure per unit distance
  • Coriolis Force — G.G. de Coriolis (1835)
    • Right deflection in Northern Hemisphere; Left in Southern Hemisphere
  • Geostrophic Balance: Maintained with the exact balance of the Coriolis force and the horizontal pressure gradient force. Geostrophic Wind is originated when the pressure gradient force is balanced by the Coriolis force acting in the opposite direction.
  • Geostrophic Thermal Wind: The speed of geostrophic thermal wind increases with height because each isobaric surface slopes more steeply than the one below it.
  • 1855 — W. Ferrel → Ferrel’s Law / **Buys Ballot’s Law*
  • Buys Ballot (1857): ‘0’ at Equator → Maximum at Poles; 30° lat = 50% deflection; 60° lat = 86.7% deflection

Tricellular Theory

Cell Latitude Named After
Hadley Cell 0°–30° N/S Edmund Haley (1686) → George Hadley (1735)
Ferrel Cell 30°–60° N/S William Ferrel
Polar Cell 60°–90° N/S —
  • Eddy Theory — circulation as a result of eddies

Planetary Winds

Trade Winds (30° N/S)

  • Speed: 15–25 km/h
  • Flow from sub-tropical high → equatorial low
  • Easterlies: NH → NE Trade; SH → SE Trade

ITCZ (Inter-Tropical Convergence Zone)

  • Also called **Equatorial Trough*

Westerlies

  • From sub-tropical high → sub-polar low
  • Cause rainfall on western margins of continents
  • NH → SW to NE; SH → NW to SE
  • Roaring Forties = 40°S; Furious Fifties = 50°S; Shrieking Sixties = 60°S
  • Dish-pan experiment relates to the formation of Rossby Waves embedded in the Westerlies.
  • Global Rainfall Changes: Rainfall distribution at a global-scale in recent decades is altered due to the alteration of the Hadley cell of general wind circulation.

Polar Winds

  • From Polar High → Sub-Polar Low

Monsoons and Jet Streams

Warning📘 Syllabus Coverage
Syllabus Topic Details
NEP-2020 Climatology section — Monsoons and jet streams
UGC NET Unit II — Monsoons and jet streams

Monsoon

  • ‘Mausin’ (Arabic) / ‘Mansin’ (Malayan) = **Season*

Theories of Monsoon Origin

Theory Scholar Year
Classical Theory (thermal contrast between continents & ocean) Edmund Halley 1686
Air Mass Theory — —
Seasonal Shift of ITCZ H. Flohn —
Jet Stream Theory — —
Upper Air Circulation Theory M.T. Yin 1949
The Monsoon (book) Pierre Pedelaborde 1963
Tibetan Plateau influence P. Koteswaram 1952

Historical References

  • Rig Veda — first mention of monsoon
  • Al-Masudi — documented monsoon
  • Sidi Ali — 1564 AD
  • Indian Ocean Dipole (IOD): Refers to the Sea surface temperature anomaly and affects the Rainfall of countries that surround the Indian Ocean Basin.
  • Monsoon Delay Factors: Delay in monsoon onset in India is often caused by (1) El Niño, (2) Weak Tibetan anticyclone, (3) Presence of Westerly Jet Stream south of the Himalaya during summer, and (4) Late shifting of ITCZ.
  • Major Monsoon Factors: The major factors responsible for the monsoon type of climate in India include Location, Thermal contrast, Upper air circulation, and the Inter-tropical convergence zone.
  • Rainiest Month: According to the IMD, July is the rainiest month in India. The South-West Monsoon usually covers the whole country by the middle of July.

Monsoon Research Programmes

Programme Full Form Year
ISMEX Indo-Soviet Monsoon Experiment 1973–74
Monsoon 77 — 1977
MONEX Monsoonal Experiment 1979 (104 aircraft missions)
GARP Global Atmospheric Research Programme —
ICSU International Council of Scientific Unions —
IMMC International Monex Management Centres, Kuala Lumpur —

Jet Stream

  • Narrow band of strong wind blowing west to east in the upper troposphere
  • Speed: 150–430 km/h
  • Depth: 900–2150 meters
  • Seasonal shift in position

Types of Jet Streams

Type Latitude / Level Discovery
Circumpolar / Polar Front Jet Stream 40°–60° N/S; 300 mb pressure Bjerknes (1933); World War II; strongest in winter
Sub-Tropical Westerly Jet Stream 30°–35° N/S; 9100 m to 31,700 m —
Tropical Easterly Jet Stream Over India and Africa —
Subtropical-Subpolar Jet Stream Above 30 km —
Polar Night Jet Stream — Due to steep pressure & temperature gradient
Local / Regional Jet — Due to thermal and dynamic changes

Rossby Waves

  • Discovered by **C.J. Rossby in 1937*
  • **Index Cycle:*
    1. High Zonal Index
    2. Jet stream transforms into wavy path
    3. Shifts towards equator
    4. Low Zonal Index

Air Masses and Fronts

Warning📘 Syllabus Coverage
Syllabus Topic Details
NEP-2020 Climatology section — Air masses and fronts
UGC NET Unit II — Air masses and fronts

Air Mass

  • A massive and thick, horizontally homogeneous air body with regard to temperature and humidity

Source Regions & Types

Code Source Region Type
cP Continental Polar Cold, Dry
mP Maritime Polar Cold, Moist
cT Continental Tropical Hot, Dry
mT Maritime Tropical Warm, Moist
mE Maritime Equatorial Warm, Very Moist
— Arctic and Antarctic Region Cold Air Mass

Thermodynamic Modification

  • Air mass heated from below → K (Kalt — cold air mass)
  • Air mass cooled from below → W (Warm air mass)

Fronts

Concept given by **Vilhelm Bjerknes & Jakob Bjerknes*

  • Frontogenesis — process of front formation
  • Inclined at surface: angle 1/30 to 1/200
  • Depth: 3000–5000 m; Width: 50–80 km; Movement: 50–80 km/h

Warm Front

  • Light warm air becomes aggressive and **rises slowly over cold dense air*
  • Slope: 1:100 to 1:400
  • Clouds: Cirrus (Ci) → Cirrostratus (Cs) → Altocumulus (Ac) → Altostratus (As)

Katafront

  • In Katafronts, the warm airmass sinks relative to the cold airmass. They are less active, and warm air is overrun by dry descending air.

Cold Front

  • Cold air becomes aggressive and invades the warm air; forcibly lifts warm air
  • Slope: 1:50 to 1:100
  • Dry climate conditions

Occluded Front

  • When a cold front overtakes a warm front and lifts the warm air mass completely off the ground.
  • Warm Occluded Front: The air behind the cold front is warmer than the cool air it is overtaking.

Quasi-Stationary Front

  • Front becomes almost stationary

Stationary Front

  • No forward motion along the line of transition between two air masses

Cyclones — Temperate and Tropical

Warning📘 Syllabus Coverage
Syllabus Topic Details
NEP-2020 Climatology section — Cyclones
UGC NET Unit II — Temperate and tropical cyclones

Temperate Cyclone

  • Also called **Extratropical Cyclones / Wave Cyclone*
  • Latitude: 35°–65° N/S
  • Shape and Size: Radius — 400 to 800 km
  • Wind velocity: 40–60 km/h
  • Moves **west to east*

Theories of Origin

Theory Scholar Year
Due to convergence of two opposing air masses Fitzroy 1863
Dynamic Theory Shaw and Lempfert 1911
Eddy Theory — —
Bergen Theory / Wave Theory / Frontal Theory V. Bjerknes & J. Bjerknes (Norway) 1918

Stages of Temperate Cyclone (Bergen Theory)

  1. Incipient Stage
  2. Juvenile Stage
  3. Early Maturity
  4. Full Maturity
  5. Old Stage
  6. End Stage

Weather Regions

  • North Atlantic Ocean
  • Mediterranean Sea
  • North Pacific Region
  • China Sea

Tropical Cyclone

  • Diameter: 80–300 km (some 50 km or less)
  • High wind speed; sea temperature must be **27°C or more*
  • They mostly occur in late summer and autumn. They do not occur beneath the jet stream.
  • No cyclones occur along the equator because the Coriolis force is zero.
  • Eye of the cyclone — centre of extreme low pressure in spite of descending wind. At the eye, adiabatic warming of descending airmass accentuates high temperature. Note that some cyclones may not develop an eye.

Classification (Christopherson, 1995)

Type Wind Speed
Tropical Disturbance Low wind speed; 5°N–20°N; >34 knots
Tropical Depression Up to 60 km/h (34 knots)
Tropical Storm 63–118 km/h

Hurricanes / Typhoons

  • Region: Caribbean Sea & Gulf of Mexico; 8°–15° N/S
  • Diameter: 150–500 km
  • Pressure: 950 mb
  • Wind speed: 120–200 km/h
  • Eye = 8–50 km
  • **Saffir-Simpson Hurricane Damage Scale (1–5)*

Tornadoes (Twisters)

  • In USA; associated with thunderstorms
  • Diameter: 100 metres
  • Pressure at centre: 10 mb
  • Wind speed: 400 km/h
  • Dry Line — in Mexico, zone of great turbulence
  • Fujita Scale (F-scale) — T. Theodore Fujita, University of Chicago
  • Mamma clouds — formed at the base of severe thunderstorms
  • News clouds — appear like a long rolling cylinder / wedge
  • Doppler Radar: Used for Nowcasting (very short-range weather forecasting).
  • Water spout: An intense low pressure system similar to a tornado but develops over a sea or large water-body; typically associated with a cumulonimbus cloud-base.

Structure of Tropical Cyclone

Layer Description
The Eye Central part — calm, extreme low pressure
Eye Wall Ring of intense convection
Spiral Bands Bands of rain
Annular Zone —
Outer Convective Band Trade wind cumulus
Lower Layer Inflow layer
Middle Layer —
Upper Layer Outflow layer

Precipitation: Types and Distribution

Warning📘 Syllabus Coverage
Syllabus Topic Details
NEP-2020 Climatology section — Precipitation
UGC NET Unit II — Precipitation types and distribution

Types of Precipitation

Type Description
Snowfall Temperature below freezing point
Sleet Frozen raindrops and re-frozen melted snow water
Hail Hard rounded pellets; diameter 5 mm to 5 cm; formed by supercooled water or ascending air currents
Drizzle Spray-like rainfall; small water drops; fine drizzle = ‘mist’ (North America)
Rainfall Cloud particles; humid air rises, cools, condensation → precipitation
  • Fog: Geographically, the most extensive fogs in India during winter are mainly Radiation fogs.

Measurement of Humidity

  • Specific Humidity: The ratio of the weight of water vapour in a parcel of the atmosphere to the total weight of moist air.
  • Relative Humidity (RH): \(\frac{\text{Actual water vapour content}}{\text{Water vapour capacity (saturation)}} \times 100\). (e.g., at 25°C, if capacity is 20g and content is 15g, \(RH = (15/20) \times 100 = 75\%\)).
  • Lithium chloride-based hygrometer: A technique for measuring atmospheric moisture based on electrical resistance.

Dew Formation

Favourable conditions for the formation of dew include: 1. **Clear sky.* 2. Calm evening without turbulence in the atmosphere. 3. Nocturnal radiative cooling below the dew point temperature of the air resting near the earth’s surface. 4. Warm previous day to raise the moisture content of the air.

Rainfall Formation Processes

Process Scholars Year
Bergeron–Findeisen Process (Ice Crystal Process) Tor Bergeron and von Findeisen. It involves mixed clouds with co-existence of ice and water, differences in saturation vapour pressure, and migration of water vapour towards sublimation nuclei. 1933
Collision–Coalescence Process George Simpson and Mason; modified by Langmuir —

Key Definitions

  • Cloudburst: Defined by the IMD as rainfall of \(\ge\) 10 cm / hour.

Types of Rainfall

  • **Convectional Rainfall*
  • Orographic Rainfall (Rain Shadow on lee side)
  • **Cyclonic / Frontal Rainfall*

Rainfall Distribution Classes

Class Amount
Heavy 250 cm+
Moderate 1000–2000 mm
Inadequate 500–1000 mm
Low 250–500 mm
Extremely Low Below 250 mm

Classification of World Climates

Warning📘 Syllabus Coverage
Syllabus Topic Details
NEP-2020 Climatology section — Köppen, Thornthwaite
UGC NET Unit II — Köppen’s scheme, Thornthwaite’s scheme

Koeppen’s Climate Classification

  • Dr. Wladimir Koeppen — University of Graz
  • Empirical Classification based on:
    • Annual and monthly means of temperature
    • Annual and monthly means of precipitation
    • Based on **five vegetation zones*

Timeline of Revisions

Year Work
1931 Grundriss und Klimakunde — world map of climatic classification
1936 Koeppen–Geiger: Handbuch der Klimatologie (Vol. I–V)
1953 Koeppen–Geiger–Pohl’s model

Major Climate Groups

Symbol Climate Type Definition
A Tropical Rainy (Megathermal) Average temperature above 18°C
B Dry Climates Defined by precipitation-to-evaporation ratios
C Mid-Humid (Mesothermal) Coldest month: −3°C to 18°C; one month avg. >10°C
D Snowy Forest (Microthermal) / Boreal Coldest month below −3°C; warmest month >10°C
E Polar Avg. temperature of warmest month below 10°C

A, C, D, E → defined by temperature; B → defined by precipitation-to-evaporation ratios

Sub-Types (Second Letter)

Letter Meaning
f No dry season; minimum precipitation 6 cm every month
w Dry season in winter
s Well-defined summer dry season
m Monsoon — rainforest despite short dry season

Third Letter (Temperature)

Letter Meaning
a Hot summer (+22°C)
b Cool summer (max 22°C)
c Short cool summer (10°C)
x Rainfall in late spring

B (Dry) Subdivisions

Symbol Meaning
h (heiss) Hot
k (kalt) Cold
n (Nebel) Frequent fog
w Winter drought
s Summer drought
BW ‘Wüste’ — desert arid climate (<40 cm)
BS ‘Steppe’ — dry grassland semi-arid
BWh Tropical desert (+18°C)
BWk Middle latitude cold desert
BSh Tropical steppe
BSk Mid-latitude cold steppe
BWn / BSn Along littorals with cool ocean currents

Detailed Climate Codes

Code Climate
Af Rainforest / Tropical Wet; driest month ≥ 6 cm
Aw Savanna — Tropical Wet and Dry
Am Monsoon Climate
Cf Precipitation throughout the year
Cw Dry winter
Cs Dry summer
Df Cold climate, humid winter, no dry season
Dfc Long warm summer
Dfb Long cool summer
Dfc Subarctic, short cool summer
ET Tundra Climate
EF Perpetual Frost (Ice Cap)

Thornthwaite’s Climate Classification

  • **C.W. Thornthwaite*
  • 1931 — Applied to North America; 1933 — Whole world classification
  • 1948 — Modified scheme
  • Complex and empirical in nature

1931 Scheme

Precipitation Effectiveness (P/E Index)
Symbol Climate P/E Index
A Wet — Rainforest 128 and above
B Humid — Forest 64–127
C Sub-Humid — Grassland 32–63
D Semi-Arid — Steppe 16–31
E Arid — Desert Below 16
Small Letter Suffixes
Letter Meaning
r Adequate rainfall in all seasons
s Rainfall deficient in summer
w Rainfall deficient in winter
d Rainfall deficient in all seasons
Thermal Efficiency (T/E Index)
Symbol Climate T/E Index
A’ Tropical Above 128
B’ Mesothermal 64–127
C’ Microthermal 32–63
D’ Taiga 16–31
E’ Tundra 1–15
F’ Frost 0

Thornthwaite gave **32 types of climates*

1948 Revised Scheme

  • Precipitation Effectiveness
  • Seasonal Distribution of Rainfall
  • Thermal Efficiency
  • Moisture Index (Im) — key addition

Criticisms of Thornthwaite’s Classification

  • Limited climatic variables: It largely ignores the role of prevailing winds, air pressure, and air masses.
  • Complexity and Replicability: It uses complex indices, making it difficult to replicate for other areas.
  • Overemphasis on Water Balance: This rigid mathematical approach may not accurately capture true, holistic climate variability.

Köppen Symbol Matching

  • Tropical Monsoon: Am
  • Mediterranean: Csa / Csb
  • Tropical Hot Desert: Bwh
  • Arctic: ET / EF

Major Classification Systems Summary

Classifier Basis Approach
Koeppen (1931) Temperature + precipitation + vegetation Empirical
Thornthwaite (1931, 1948) P/E index, T/E index, Moisture Index Empirical
Trewartha Modified Koeppen Empirical
Flohn Atmospheric circulation Generic / Dynamic
Strahler Solar radiation + wind belts Dynamic

Hydrological Cycle

Warning📘 Syllabus Coverage
Syllabus Topic Details
NEP-2020 Climatology section — Hydrological cycle
UGC NET Unit II — Hydrological cycle

**Evaporation → Transpiration → Condensation → Precipitation → Infiltration & Run-off*

  • Stomata — process of water absorption through roots (transpiration)

Water Distribution on Earth

Type Percentage
Saline Ocean Water 97.25%
Fresh Water (total) 2.75%
Fresh Water in Glaciers/Ice 68.7% of fresh water
Groundwater 29.9% of fresh water

Global Warming and Ozone Depletion

Warning📘 Syllabus Coverage
Syllabus Topic Details
NEP-2020 Climatology section — Global warming
UGC NET Unit II — Global warming

Global Warming

  • Little Climate Optimum — last 1000 years
  • 1250–1450 — Climate Optimum period
  • 1450–1880 — Little Ice Age
  • 1816 — The Year Without a Summer
  • Sunspots — dark circular areas on the outer surface (photosphere) of the sun
    • 11-year cycle
  • 90,000–100,000 years — Earth’s orbit changes its shape (Milankovitch Cycles)

CO₂ Concentration Over Time

Year CO₂ Level
1825 210 ppm
1885 280 ppm
1985 350 ppm
2000 360–400 ppm

Greenhouse Gases & Radiative Forcing

  • CO₂ (50% share), CFC, CH₄, N₂O, O₃, NO
  • Global Warming Potential (GWP) Order: CO₂ < CH₄ < N₂O < CFC-11
  • Anthropogenic Alteration: Non-radiative, external radiative, and internal radiative forcing agents influence climate. Among these, radiative forcing agents (especially internal ones like GHG concentrations) are highly susceptible to anthropogenic alteration.

CO₂ Emissions by Region

Region Percentage
USA 30.3%
Europe 27.7%
Asia (developing countries) 12.2%
  • 18,000 years ago — sea level was 82 metres below present level
  • Spread of tropical diseases
  • R.K. Pachauri — 0.5°C rise in winter temperature = 0.45 tonne/hectare fall in crop yield

Kyoto Protocol

  • 1997 — 141 countries signed
  • Goal: reduce greenhouse gas emissions
  • Net Zero Target: In COP-26, India pledged to achieve Net Zero by 2070.

Ozone Depletion

  • Troposphere Ozone — absorbs temperature → **Bad Ozone*
  • Stratosphere Ozone — acts as a filter against UV-B → **Good Ozone*
  • Chapman Mechanism: Cl + O₃ → ClO + O₂

Causes of Ozone Depletion

  • CFC (Chlorofluorocarbons)
  • Nitrogen Oxide
  • Hydrocarbons

Key Facts

  • Ozone hole found **above Antarctica*
  • 1% ozone lost → 2% more UV rays reach Earth
  • Ice Climbing effects on polar ozone

International Conferences on Ozone

Conference Year Key Decision
Vienna Conference 1985 First conference to discuss ozone depletion
Montreal Protocol 1987 CFC reduction step by step
Saving the Ozone Layer Conference 1989 British Govt + UNEP; final withdrawal of all ozone-depleting gases
Helsinki Conference 1989 Devised the Montreal Protocol

Part B: NEP-2020 Specific Topics

These topics are part of the NEP-2020 undergraduate programme only.

Stability and Instability of the Atmosphere

Warning📘 Syllabus Coverage
Syllabus Topic Details
NEP-2020 Climatology section — Stability and instability
UGC NET Unit II — Stability and instability of the atmosphere
  • Stable Atmosphere — air resists vertical displacement
  • Unstable Atmosphere — air parcel, once displaced, continues to move away from original position
  • Neutral Stability — displaced air has same temperature as surroundings

Key Processes

  • DALR (Dry Adiabatic Lapse Rate): 1°C per 100 m (for unsaturated air). It is constant.
  • SALR (Saturated / Wet Adiabatic Lapse Rate): 0.5°C per 100 m (for saturated air). It is high under high temperatures.
  • ELR (Environmental Lapse Rate): 6.4°C per km (normal).

Stability Conditions

  • Absolutely Stable: ELR < WALR (or SALR)
  • Conditionally Stable: WALR < ELR < DALR
  • Absolutely Unstable: ELR > DALR
  • Neutral Stability: ELR = DALR
  • Vertical Change of Temperature (Descending order of lapse rate): Superadiabatic > Adiabatic > Subadiabatic > Inversion

Humidity

Absolute Humidity

  • Actual amount of water vapour present in the air
  • Higher water vapour = higher absolute humidity

Relative Humidity (RH)

  • Measure of water vapour relative to the temperature of air
  • Measure of actual amount vs. maximum possible at current temperature
  • Formula:

**RH = (Actual water vapour content / Max. water vapour capacity) × 100*

  • Temperature increase → Relative Humidity **decreases*

Specific Humidity

  • Weight of water vapour per unit of air
  • Proportion of mass of water vapour to total mass of air

Part C: UGC NET Specific Topics

These topics are part of the UGC NET syllabus only.

El Niño, La Niña & Southern Oscillation (ENSO)

Warning📘 Syllabus Coverage
Syllabus Topic Details
UGC NET Unit II — ENSO events

El Niño

  • A warm surface current flowing south to north between 36°S and 3°S latitude; 180 km from Peruvian Coast
  • Results from southward shifting of the counter equatorial warm current
  • First reported 1541 → named “El Niño” (The Child Christ)

Effects of El Niño

  • Rainfall in Atacama Desert
  • Winds, Tornadoes and Typhoons
  • Damage to fish and birds (upwelling of cold water disappears → plankton loss)
  • Destruction of Coral Reefs (rises temperature)
  • Spread of diseases
  • Disrupts weather
  • Associated with occasional weak monsoon rains, less rainfall and drought in the Indian sub-continent.

Southern Oscillation

  • Discovered by Gilbert Walker (1920) — curious see-saw pattern of meteorological changes between Pacific and Indian Oceans
  • The term oscillation is associated with El Niño and La Niña because their fluctuations are not perfectly periodic.
  • Walker Circulation — Walker’s model of atmospheric circulation. In an El Niño year, the Western Pacific warm pool moves towards the east, which modifies the Walker Circulation.
  • SOI = Southern Oscillation Index (negative SOI = favourable for El Niño)

ENSO

  • ENSO = El Niño + Southern Oscillation; a periodic fluctuation with a cycle of 2 – 7 years. ENSO events are specifically associated with the Walker cell.

La Niña

  • Abnormal accumulation of cold water in the central and eastern Pacific region (‘Little Girl’ — Spanish)
  • Thermocline — layer of water where temperature changes rapidly in vertical direction

Meteorological Hazards and Disasters

Warning📘 Syllabus Coverage
Syllabus Topic Details
UGC NET Meteorological Hazards and Disasters (Cyclones, Thunderstorms, Tornadoes, Hailstorms, Heat/Cold Waves, Drought, Cloudburst, GLOF)

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NoteKey Concepts
  • Cyclones: Tropical and extra-tropical — wind damage, storm surge, flooding. Warning systems and preparedness.
  • Thunderstorms: Convective storms — lightning, heavy rain, hail, gusty winds. Nor’westers in eastern India.
  • Tornadoes: Violently rotating columns of air — Fujita scale, damage patterns. Rare in India but occur.
  • Hailstorms: Large ice pellets — damage to crops, property, livestock.
  • Heat and Cold Waves: Extreme temperature events — health impacts, agricultural losses, urban vulnerability.
  • Drought: Meteorological, hydrological, agricultural — causes, monitoring (SPI, PDSI), management.
  • Cloudburst: Sudden, extremely heavy rainfall — flash floods, landslides. Common in Himalayan regions.
  • GLOF (Glacial Lake Outburst Flood): Catastrophic release of water from glacial lakes — Himalayan risk.

Applied Climatology

Warning📘 Syllabus Coverage
Syllabus Topic Details
UGC NET Climate applications in various sectors

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NoteKey Concepts
  • Agricultural Climatology: Crop-weather relationships, agroclimatic zones, growing season, frost-free period.
  • Urban Climatology: Urban heat island, air quality, urban wind patterns, building energy requirements.
  • Aviation Climatology: Weather impacts on flight operations — visibility, turbulence, icing, crosswinds.
  • Bioclimatology: Climate-health relationships — thermal comfort indices, disease ecology, seasonal patterns.
  • Renewable Energy: Solar and wind resource assessment — site selection for solar/wind farms.
  • Climate Data and Monitoring: Weather stations, satellites, radar, automated weather stations, climate databases.

Clouds

  • Luke Howard (1803) — 1st cloud classification; biologist from England; used Latin names
  • 1895 — International Meteorological Committee published cloud classification
  • WMO (1956) — The International Cloud Atlas

Cloud Types by Altitude

High Clouds (6–14 km)

Cloud Symbol Features
Cirrus Ci ‘Hair’ — fibrous, feathery appearance
Cirrostratus Cs —
Cirrocumulus Cc —

Middle Clouds (2–6 km)

Cloud Symbol Features
Altocumulus Ac —
Altostratus As —

Low Clouds (0–2 km)

Cloud Symbol Features
Stratocumulus Sc —
Stratus St ‘Layer’ — laying in a level sheet
Nimbostratus Ns Rain-producing layer cloud

Vertical Clouds

Cloud Features
Cumulus ‘Pile’ — flat base, rounded tops
Cumulonimbus Storm cloud; reaches from low to high level

Cloud Sequence by Height

The correct sequence of cloud types with increasing height from the ground level is: Cumulus → Stratocumulus → Altocumulus → Cirrocumulus.

Local Winds

Warm Local Winds

Wind Region Features
Chinook Rocky Mountains, USA Warm, dry; speed 160 km/h; ‘Snow-eater’
Foehn / Föhn Alps, Switzerland; oasis valleys Warm, dry (German)
Harmattan West Africa Dry, hot; called ‘doctor’
Sirocco Sahara Desert, Mediterranean Sea Warm, dusty, dry; ‘blood rain’ = dry sand with rain

Cold Local Winds

Wind Region Features
Mistral Alps, Mediterranean Sea Cold and dry

Equivalent Wind Names

Group Winds
Warm Dry Winds Beng (S. Africa) = Chinook (Rockies) = Norwester (New Zealand) = Samun/Samoon (Iran) = Santa Ana (California) = Zonda (Argentina)
Warm Dusty & Dry Sirocco (Sahara) = Khamsin (Egypt) = Gibli (Libya) = Chili (Tunisia) = Leveche (Spain) = Simoom = (Arabian Desert)

Mountain Winds

  • Catabatic winds: Move downslope when the mountain surface is colder than the surrounding air.
  • Anabatic winds: Move upslope driven by warmer surface temperature on the mountain slope.

Quick Reference

Key Scholars — Climatology

Contributors and Their Contributions (NET Notes — Pulakesh Pradhan)

Scholar Contribution
Edmund Halley (1686) Monsoon thermal theory
G.G. de Coriolis (1835) Coriolis Force
Buys Ballot (1857) Buys Ballot’s Law of pressure gradient
W. Ferrel (1855) Ferrel’s Law / Ferrel Cell
Lord Rayleigh Rayleigh Scattering
Gilbert Walker (1920) Southern Oscillation discovery
Bjerknes (1918) Bergen Theory of cyclones; Fronts concept
C.J. Rossby (1937) Rossby Waves
Wladimir Koeppen (1931) Empirical climate classification
C.W. Thornthwaite (1931, 1948) P/E and T/E climate classification
P. Koteswaram (1952) Tibetan Plateau role in monsoon
Tor Bergeron & Findeisen (1933) Ice Crystal Process of precipitation
Luke Howard (1803) First cloud classification
Lucian Vidie (1843) Invented Aneroid Barometer
Admiral Beaufort Beaufort scale: relates to the Velocity of Winds.
R.K. Pachauri Crop yield impact of global warming

Notes compiled by Pulakesh Pradhan — Climatology (NET)

Climatology Quick Reference

Key Books and Authors

Book Author
General Climatology Howard J. Critchfield
Introduction to Climate G.T. Trewartha
Climatology D.S. Lal
The Climates of the Continents W.G. Kendrew
Atmosphere, Weather and Climate Roger G. Barry & Richard J. Chorley

Classifications and Theories

Theory / Concept Propounder Details
Empirical Climate Classification Wladimir Köppen (1918) Based on temperature and precipitation (A, B, C, D, E, H)
Genetic Climate Classification Hermann Flohn (1950) Based on global wind belts and precipitation
Moisture Index & Evapotranspiration C.W. Thornthwaite (1931, 1948) Thermal Efficiency, Moisture Index (P/E ratio)
Polar Front Theory V. Bjerknes & J. Bjerknes Genesis of temperate cyclones
Air Mass Concept Tor Bergeron Large body of air with uniform T and humidity

Important Concepts

  • Albedo: Reflectivity of a surface (Snow has highest albedo).
  • Lapse Rate: Normal lapse rate is 6.5°C per 1000m. Dry Adiabatic is 10°C/km, Wet Adiabatic is ~5°C/km.
  • Inversion of Temperature: Temperature increases with altitude.
  • Coriolis Force: Deflective force of Earth’s rotation (Ferrel’s Law).
  • El Niño & La Niña: ENSO cycle affecting global weather patterns.
  • Rossby Waves: Large-scale meanders in high-altitude winds (jet streams).

Notes compiled by Geography Team