Moisture, Air Masses, Fronts and World Climates

From water vapour to world climates

Weather depends on how much moisture the air holds, how air masses with different properties meet, and how these patterns repeat over the year to form climate. This lesson covers humidity, condensation and precipitation, air masses and fronts, temperate cyclones, the Köppen climate classification, and the ocean–atmosphere oscillations – El Niño, La Niña and the Indian Ocean Dipole – that strongly influence India's monsoon. Tropical cyclones are covered in the module on geophysical phenomena.

Humidity and condensation

• Absolute humidity – the actual amount of water vapour in a given volume of air (grams per cubic metre).
• Relative humidity – the percentage of moisture present compared with the maximum the air can hold at that temperature. Warm air can hold more vapour, so relative humidity falls as air warms and rises as it cools.
• Dew point – the temperature at which air becomes saturated (100 per cent relative humidity) and condensation begins.

Forms of condensation: dew (on cool surfaces above freezing), frost (below freezing), fog and mist (condensation near the ground, common in the Indo-Gangetic plain in winter), and clouds.

Clouds are classified by height and form:

• High: cirrus (feathery, ice crystals), cirrocumulus, cirrostratus (halo around the Sun).
• Middle: altocumulus, altostratus.
• Low: stratus (grey sheet), stratocumulus, nimbostratus (steady rain).
• Vertical development: cumulus (fair-weather "cotton wool") and cumulonimbus (towering thunderclouds with an anvil top, bringing thunder, lightning, hail and cloudbursts).

Types of rainfall

1. Convectional – intense heating causes air to rise, cool and form cumulonimbus clouds; afternoon thunderstorms in equatorial regions and pre-monsoon storms in India.
2. Orographic (relief) – moist air forced up a mountain cools and rains on the windward side; the leeward side lies in a rain shadow. Example: heavy rain on the western slopes of the Western Ghats (Mahabaleshwar, Agumbe) and dry conditions on the Deccan plateau; Mawsynram and Cherrapunji in Meghalaya receive some of the world's highest rainfall because of the funnel-shaped Khasi hills.
3. Cyclonic (frontal) – warm air rises over cold air along fronts, bringing widespread rain in temperate regions and winter rain in north-west India from western disturbances.

Air masses and fronts

An air mass is a large body of air with uniform temperature and humidity, taking on the character of its source region. They are classified as maritime (m) or continental (c), and tropical (T), polar (P), arctic (A) or equatorial (E) – for example, mT (warm and moist, from tropical oceans) and cP (cold and dry, from Siberia or Canada).

When two air masses meet, the boundary is a front:

• Cold front – cold air pushes under warm air, lifting it sharply; brings short, heavy showers and thunderstorms and a drop in temperature.
• Warm front – warm air slides gently over cold air; brings long periods of light rain and drizzle, with a sequence of clouds from cirrus to nimbostratus.
• Occluded front – a fast cold front overtakes a warm front, lifting the warm air off the ground.
• Stationary front – neither air mass advances.

Frontogenesis is the formation of a front; fronts are common in middle latitudes and rare in the tropics, where air masses are similar.

Temperate (extra-tropical) cyclones

Temperate cyclones form along the polar front in middle latitudes (35°–65°) where warm tropical and cold polar air meet (the Bjerknes polar front theory). They are large (up to 2,000 km across), move from west to east with the westerlies, and bring changeable weather over several days – a sequence of warm-front rain, a warm sector, then cold-front showers. They form over land and sea, unlike tropical cyclones, which need warm seas. They influence the weather of Europe, North America and East Asia, and the western disturbances that reach India are their weakened eastern extensions.

Anticyclones are high-pressure systems with descending air, clockwise winds in the Northern Hemisphere, and clear, stable weather – causing heat waves in summer and fog or cold waves in winter.

Köppen's classification of climates

Wladimir Köppen (1918, later revised) classified climates using temperature and precipitation and the vegetation they support. He used capital letters for main groups:

Small letters add detail: f (no dry season), m (monsoon), w (dry winter), s (dry summer).

Major world climates in brief

• Equatorial – hot and wet throughout the year, daily convectional rain, dense rainforest.
• Tropical monsoon – seasonal reversal of winds, wet summers and dry winters; South and South-East Asia.
• Savanna (Sudan type) – wet summer, long dry season, grasslands with scattered trees; Africa, Brazil (campos), Venezuela (llanos).
• Hot deserts – on the western margins of continents between about 15° and 30°, where subtropical highs and cold ocean currents (Benguela – Namib; Peru/Humboldt – Atacama; Canaries – Sahara; California – Sonoran) keep rainfall very low.
• Mediterranean – dry, hot summers and mild, wet winters on the western sides of continents around 30°–40° (Mediterranean basin, California, central Chile, Cape Town, south-west Australia); famous for citrus, grapes and olives.
• Temperate grasslands – prairies (North America), steppes (Eurasia), pampas (Argentina), veld (South Africa), downs (Australia).
• Taiga – long, cold winters; coniferous forests; and tundra – permafrost, mosses and lichens.

El Niño, La Niña and the Indian Ocean Dipole

El Niño is the periodic warming of sea-surface temperatures in the central and eastern equatorial Pacific, off Peru and Ecuador, every 2–7 years. Normally strong trade winds push warm water westward towards Indonesia and Australia, and cold water wells up along South America (the Humboldt current). During El Niño the trade winds weaken, warm water spreads east, the Walker circulation weakens or shifts, and rainfall moves from the western to the central Pacific. The atmospheric part is the Southern Oscillation, and together they are called ENSO.

• Effects: drought in Australia and Indonesia, heavy rain and floods in Peru, collapse of Peruvian anchovy fisheries, and often a weak Indian monsoon – major Indian droughts in 1987, 2002, 2009 and 2015 coincided with El Niño, though not every El Niño causes drought.
• La Niña is the opposite – stronger trade winds and cooler eastern Pacific – usually associated with good monsoon rains in India.

The Indian Ocean Dipole (IOD) is the difference in sea-surface temperature between the western Indian Ocean (near Africa) and the eastern Indian Ocean (near Indonesia). A positive IOD (warmer west) usually helps the Indian monsoon and can offset El Niño, as in 1997 and 2019; a negative IOD weakens it. The Madden–Julian Oscillation (MJO), an eastward-moving band of clouds and rain, influences monsoon "breaks" and cyclone formation on timescales of 30–60 days.

Mains practice question

What is El Niño? Explain how El Niño and the Indian Ocean Dipole influence the Indian summer monsoon. (150 words)

Answer framework

• Define El Niño and ENSO; normal vs El Niño Walker circulation.
• Impact on India: weaker monsoon, examples of drought years.
• IOD: positive IOD strengthens monsoon; can offset El Niño (1997, 2019).
• Conclusion: IMD forecasts use both; need for climate-resilient agriculture.