These climatology notes for OPSC OAS Part 2 cover humidity and its measures, the forms of condensation, cloud classification, the three types of rainfall, air masses and fronts, tropical and temperate cyclones, El Nino, La Nina and the Southern Oscillation, and Koppen's climate classification. Atmospheric composition, insolation, pressure belts and winds are covered in Part 1.
Humidity and Its Measures
Humidity is the water vapour content of the atmosphere. Meteorology uses four distinct measures, and OPSC and UPSC test the difference between them.
| Measure | Definition | Unit |
|---|---|---|
| Absolute humidity | Mass of water vapour per unit volume of air | g/m³ |
| Specific humidity | Mass of water vapour per unit mass of moist air | g/kg |
| Relative humidity | Ratio of actual vapour to the vapour the air can hold at that temperature, expressed as a percentage | % |
| Dew point | Temperature at which the air becomes saturated on cooling | °C |
NDA 2009 asked for the mass of water vapour per unit volume — absolute humidity. NDA 2009 also asked which instrument measures humidity — the wet and dry bulb thermometer, which works on the hygrometer principle.
The dew point is the temperature to which air must be cooled at constant pressure for it to become saturated. The saturation deficit is the difference between the vapour the air could hold at that temperature and the vapour it actually holds. A small saturation deficit means the air is close to condensation and rain is likely.
Relative humidity is the measure that decides whether precipitation is likely, because it expresses how near the air is to saturation. It changes with temperature even when the actual vapour content stays the same: warming lowers relative humidity and cooling raises it. This is why relative humidity typically peaks near dawn and falls in the afternoon.
- Relative humidity is a ratio, not the weight of moisture per volume — that is absolute humidity.
- Specific humidity, expressed in g/kg, does not change with temperature or pressure as long as no vapour is added or removed, so it is the most conservative measure.
Forms of Condensation
Condensation occurs when air cools below its dew point. The form it takes depends on the surface and the temperature.
| Form | How it forms | Where |
|---|---|---|
| Dew | Vapour condenses as droplets on cool surfaces such as leaves and grass | Clear, calm nights |
| Frost | Vapour changes directly to ice when the dew point is below 0°C | Cold clear nights |
| Mist | Fine droplets suspended near the surface; visibility reduced only slightly | Humid valleys, mornings |
| Fog | A cloud at ground level; visibility reduced below 1 km | Coasts, valleys, industrial areas |
| Cloud | Droplets or ice crystals suspended aloft | Free atmosphere |
IAS 2019 asked why dew does not form on a cloudy night — clouds absorb and re-radiate long-wave terrestrial radiation back to the surface, preventing enough cooling to reach the dew point.
Dew and frost need clear, calm nights for rapid radiative cooling. Cloud cover blocks that cooling, which is why cloudy nights are warmer and dew-free.
Cloud Classification
Clouds are classified by height and form. The Latin roots are cirrus (curl), stratus (layer), cumulus (heap) and nimbus (rain).
| Family | Height | Types | Character |
|---|---|---|---|
| High | Above 6 km | Cirrus, cirrostratus, cirrocumulus | Thin, wispy, ice crystals; little rain |
| Middle | 2–6 km | Altostratus, altocumulus | Layered or patchy; light rain possible |
| Low | Below 2 km | Stratus, stratocumulus, nimbostratus, cumulus | Nimbostratus gives steady continuous rain |
| Vertical development | Base low, top high | Cumulonimbus | Greatest vertical extent; thunderstorms, hail, showery rain |
IAS 2004 asked for the low clouds — stratus, nimbus and cumulus. IAS 2003 asked which cloud has the greatest vertical extent and brings convectional rain with lightning and thunder — cumulonimbus.
Rain-bearing clouds look dark because their large number of water droplets and their great thickness absorb and scatter most of the sunlight instead of transmitting it.
IMAGE (Diagram): Vertical arrangement of cloud families with typical heights.
Labels: high clouds (cirrus, cirrostratus, cirrocumulus), middle clouds (altostratus, altocumulus), low clouds (stratus, nimbostratus, cumulus), cloud with vertical development (cumulonimbus); heights 2 km, 6 km; tropopause at 12 km.
Caption: Cloud families by height and form.
Find it: NCERT Class 11 Fundamentals of Physical Geography cloud classification figure, or Wikimedia Commons ‘Cloud types’.
Precipitation and Rainfall Types
Precipitation is the fall of water in solid or liquid form — rain, snow, sleet, hail. It requires abundant moisture, cooling of air to the dew point by ascent, and condensation nuclei. Two mechanisms produce raindrops: the Bergeron process in cold clouds that extend above the freezing level, and the collision-coalescence process in warm clouds that stay below it.
| Type | Mechanism | Character | Example |
|---|---|---|---|
| Convectional | Surface heating makes moist air rise, cool and condense | Afternoon, short, heavy, cumulonimbus, thunder | Amazon basin, equatorial belt |
| Orographic | Moist air forced up a mountain barrier | Heavy on windward slope, rain shadow on leeward | Western Ghats, Himalaya foothills |
| Frontal or cyclonic | Warm air forced to rise over cold air along a front | Widespread, steady, temperate latitudes | North-west India in winter |
In orographic rainfall, precipitation increases up the windward slope and then decreases above a certain height once the moisture is exhausted. This is the inversion of rainfall. The leeward side lies in the rain shadow, receiving dry descending air.
IAS 2007 asked which basin gets convectional rainfall — the Amazon. NDA 2019 asked about inversion of rainfall — it belongs to orographic rainfall. NDA 2011 asked about the rain shadow — it is associated with orographic rainfall.
A cloudburst is an extreme, localised downpour of about 100 mm or more in an hour, often 250–300 mm in a couple of hours, common in hilly terrain. It is not restricted to daytime.
Other precipitation measures: an isohyet joins places of equal rainfall, an isoneph joins places of equal cloud cover, an isobar joins equal pressure and an isotach joins equal wind speed.
IMAGE (Diagram): Three panels showing convectional, orographic and frontal rainfall.
Labels: convectional — heated surface, rising moist air, cumulonimbus, afternoon shower; orographic — windward slope, forced ascent, rain, leeward rain shadow; frontal — warm air over cold air, front line, widespread rain.
Caption: The three mechanisms of rainfall.
Find it: NCERT Class 11 Fundamentals of Physical Geography chapter on Water in the Atmosphere, or Wikimedia Commons ‘Orographic precipitation’.
Air Masses and Fronts
An air mass is a large body of air with uniform temperature and humidity acquired from its source region. Air masses form over broad, uniform surfaces under stable, anticyclonic conditions, not in cyclonic conditions.
| Air mass | Source region | Character |
|---|---|---|
| Continental polar (cP) | High-latitude land, e.g. Siberia, Canada | Cold, dry |
| Maritime polar (mP) | Cold oceans at high latitude | Cold, moist |
| Continental tropical (cT) | Subtropical deserts | Hot, dry |
| Maritime tropical (mT) | Warm tropical oceans | Warm, moist |
| Maritime equatorial (mE) | Warm equatorial oceans | Hot, very humid, unstable |
A front is the boundary between two contrasting air masses. Frontogenesis is the creation or regeneration of a front; frontolysis is its destruction. Cold fronts move faster than warm fronts and eventually overtake them, producing an occlusion.
NDA 2019 asked which statement about air masses is not true — that they develop in cyclonic conditions. NDA 2015 asked the correct air mass and source region pair — maritime equatorial over warm equatorial oceans.
Tropical Cyclones
A tropical cyclone is an intense low-pressure system that forms over warm tropical oceans. Conditions for formation: sea surface temperature above about 26.5°C, abundant moisture, low vertical wind shear, the Coriolis force (so not within about 5° of the equator), upper-level divergence and a pre-existing disturbance usually linked to the ITCZ.
| Feature | Description |
|---|---|
| Eye | Calm, clear centre with descending air, lowest pressure and slightly higher temperature than the surroundings |
| Eyewall | Ring of strongest winds and heaviest rain around the eye |
| Diameter | Typically 100–1,500 km |
| Circulation (NH) | Counter-clockwise and inward |
| Circulation (SH) | Clockwise and inward |
| Barometer | Falls sharply on approach |
Regional names: Typhoon in the western Pacific and China Sea, Hurricane in the Atlantic and eastern Pacific, Willy-Willy around Australia, Baguio in the Philippines, Cyclone in the Indian Ocean.
CDS 2016 asked what the eye of a cyclone has — abnormally high temperature and the lowest pressure. APSC 2017 asked the Philippine name — Bagyo. UPPSC 2020 matched Baguios, hurricanes, typhoons and Willy-Willies with their countries.
Temperate Cyclones and the Polar Front
Temperate cyclones form along the polar front, where warm tropical air meets cold polar air. The Polar Front Theory was proposed by V. Bjerknes and his school. These cyclones are larger, less violent and longer-lived than tropical cyclones, and they travel from west to east steered by the westerlies.
| Feature | Tropical cyclone | Temperate cyclone |
|---|---|---|
| Origin | Warm tropical oceans | Polar front, mid-latitudes |
| Diameter | Smaller, 100–1,500 km | Larger, often over 2,000 km |
| Fronts | No fronts | Warm and cold fronts present |
| Eye | Present | Absent |
| Movement | Steered by trade winds, generally westward then poleward | West to east with the westerlies |
| Season | Late summer and autumn | Mainly winter |
The Arctic front, between Arctic and polar air, is relatively inactive because the temperature contrast between the two air masses is small.
NDA 2019 asked which phenomenon the Polar Front Theory is essentially related to — the temperate cyclone. CDS 2015 asked which statement about fronts is correct — cold fronts move faster and frequently overtake warm fronts.
- An anticyclone has sinking air at the centre; a cyclone has rising air. The eye is the small exception where air sinks.
- Jet streams exist in both hemispheres, not only the Northern Hemisphere.
- Tropical cyclones do not form over the South Atlantic or south-eastern Pacific mainly because the ITCZ disturbance is rarely present there.
El Nino, La Nina and the Southern Oscillation
El Nino is the irregular warming of sea surface water in the central and eastern equatorial Pacific, recurring every two to seven years. La Nina is the opposite cooling phase. The atmospheric counterpart of El Nino is the Southern Oscillation, a seesaw of pressure between the eastern and western Pacific. Together they form ENSO.
| Effect | El Nino | La Nina |
|---|---|---|
| Pacific sea surface | Warm anomaly, eastern and central Pacific | Cold anomaly, eastern Pacific |
| Indian monsoon | Often weak or deficient | Often normal or strong |
| Australia and Indonesia | Drought | Wet, floods |
| Peru and Ecuador | Heavy rain and floods | Dry |
| Atlantic hurricanes | Suppressed by increased wind shear | Favoured |
| Marine life | Disrupted by reduced upwelling | Enhanced upwelling |
El Nino Modoki is a variant in which warming is concentrated in the central Pacific rather than the eastern Pacific. Unlike normal El Nino, Modoki weakens the wind shear over the Atlantic and can favour more Atlantic hurricanes.
CDS 2011 asked what El Nino is — an abnormally warm ocean current. IAS 2010 asked about El Nino Modoki and the Atlantic hurricanes. IAS 2011 asked about La Nina — it cools the equatorial Pacific, not the Indian Ocean, and it does affect the Indian monsoon.
Koppen Climate Classification
Wladimir Koppen classified world climates using monthly temperature and precipitation, with natural vegetation as the visible index of climate. The five main groups are A, B, C, D and E.
| Group | Meaning | Key criterion | Example code |
|---|---|---|---|
| A | Tropical | Every month above 18°C | Af tropical wet; Am tropical monsoon; Aw tropical wet and dry |
| B | Dry | Potential evaporation exceeds precipitation | BWh hot desert; BSh semi-arid steppe |
| C | Warm temperate | Coldest month between −3°C and 18°C | Cfa humid subtropical; Csa Mediterranean |
| D | Cold snow forest | Coldest month below −3°C, warmest above 10°C | Dfb humid continental; Dfc subarctic |
| E | Polar | Warmest month below 10°C | ET tundra; EF ice cap |
India falls mainly into Am (tropical monsoon) over the peninsula, Aw in parts of the interior, BSh in the semi-arid north-west, BWh in the Thar desert, Cwa in the Gangetic plain and ET in the high Himalaya.
NDA 2019 asked what Cfa denotes — humid subtropical climate. UPPSC 2018 asked who linked vegetation as the true index of climate — Koppen.
IMAGE (World map): Koppen climate classification map.
Labels: Af, Am, Aw, BWh, BSh, Cfa, Csa, Cfb, Dfb, Dfc, ET, EF; mark India with Am, BSh, BWh, Cwa and ET.
Caption: World climatic types after Koppen.
Find it: NCERT Class 11 Fundamentals of Physical Geography climate map, or Wikimedia Commons ‘Koppen-Geiger climate map’.
Odisha Angle: Cyclones, Kalabaisakhi and Coastal Vulnerability
Odisha is one of India's most cyclone-exposed states. Its concave coastline, shallow continental shelf and location on the head of the Bay of Bengal make landfalls frequent and destructive. The 1999 super cyclone, which made landfall near Paradip with wind speeds of about 260 km/h and killed thousands, remains the benchmark disaster. More recent events include Cyclone Fani (May 2019), which struck Puri, Amphan (2020) and Yaas (2021).
The pre-monsoon Kalabaisakhi thunderstorms of April and May, the Nor'westers, bring sharp showers, lightning and gusty squalls to the coastal districts. The India Meteorological Department's Regional Meteorological Centre at Bhubaneswar issues cyclone, thunderstorm and heat wave warnings, and the Odisha State Disaster Management Authority coordinates evacuation and shelter operations, which have become a widely cited model for cyclone preparedness.
Odisha cyclones: Bay of Bengal origin, concave coast, shallow shelf. Benchmark events: 1999 super cyclone, Fani 2019, Amphan 2020, Yaas 2021. Pre-monsoon thunderstorms = Kalabaisakhi.
Practice Questions
The mass of water vapour per unit volume of air is known as:
- Relative humidity
- Specific humidity
- Absolute humidity
- Variable humidity
Show answer
(c) – Absolute humidity is mass of vapour per unit volume, in g/m³; specific humidity is per unit mass of air. NDA 2009
Which one of the following instruments is used for measuring the humidity of the air?
- Wind vane
- Aneroid barometer
- Wet and dry bulb thermometer
- Maximum and minimum thermometer
Show answer
(c) – The difference between wet-bulb and dry-bulb readings gives humidity through a hygrometric table. NDA 2009
Why are dewdrops not formed on a cloudy night?
- Clouds absorb the radiation released from the Earth's surface
- Clouds reflect back the Earth's radiation
- The Earth's surface would have low temperature on a cloudy night
- Clouds deflect the blowing wind to ground level
Show answer
(b) – Clouds absorb and re-radiate long-wave terrestrial radiation back to the surface, so the ground does not cool to the dew point. IAS 2019
Which one of the following clouds has the greatest vertical extent and brings convectional rain accompanied by lightning and thunder?
- Altocumulus
- Cirrocumulus
- Cirrostratus
- Cumulonimbus
Show answer
(d) – Cumulonimbus towers through the troposphere and produces thunderstorms, hail and heavy showers. IAS 2003
The line joining points with equal cloud cover is called:
- Isobar
- Isoneph
- Isohyet
- Isotach
Show answer
(b) – Isoneph joins equal cloud cover; isohyet joins equal rainfall and isotach equal wind speed. OPSC 2018
Consider the following statements about tropical cyclones:
- They require a sea surface temperature above about 26.5°C.
- They require low vertical wind shear.
- They form within about 5° of the equator because the Coriolis force is strongest there.
How many of the statements given above are correct?
- Only one
- Only two
- All three
- None
Show answer
(b) – Warm sea surface and low wind shear are needed. Tropical cyclones do not form within about 5° of the equator because the Coriolis force is too weak there, not strongest.
Match List I with List II:
| List I (Tropical cyclone) | List II (Region) |
|---|---|
| A. Baguio | 1. Australia |
| B. Hurricane | 2. Philippines |
| C. Typhoon | 3. United States of America |
| D. Willy-Willy | 4. China and western Pacific |
Select the correct answer using the code given below:
- A-2, B-3, C-4, D-1
- A-1, B-3, C-4, D-2
- A-2, B-4, C-3, D-1
- A-3, B-2, C-1, D-4
Show answer
(a) – Baguio is the Philippine name, hurricane the Atlantic and US name, typhoon the western Pacific and China Sea name, and Willy-Willy the Australian name. UPPSC 2020
El Nino is associated with the formation of the south-west monsoon of India. It is:
- An abnormally warm ocean current
- A periodic warm air mass
- A periodic warm wind
- A periodic low-pressure centre
Show answer
(a) – El Nino is the periodic warming of sea surface water in the central and eastern equatorial Pacific. CDS 2011
According to Koppen's climatic classification, the letter code Cfa denotes:
- Tropical wet climate
- Humid subtropical climate
- Tundra climate
- Tropical wet and dry climate
Show answer
(b) – C is warm temperate, f means no dry season and a means hot summer, giving the humid subtropical climate. NDA 2019
Consider the following statements about Odisha's cyclone vulnerability:
- The concave coastline and shallow continental shelf increase the state's exposure to storm surges.
- The 1999 super cyclone made landfall near Paradip.
- Cyclone Fani struck Puri district in May 2019.
How many of the statements given above are correct?
- Only one
- Only two
- All three
- None
Show answer
(c) – The concave coast and shallow shelf amplify storm surges, the 1999 super cyclone struck near Paradip, and Fani made landfall near Puri in May 2019.