Humidity, Clouds & Rainfall – NDA Geography Notes

Introduction

Water is constantly moving between the Earth’s surface and the atmosphere — evaporating from oceans, lakes, and rivers, rising into the atmosphere, condensing into clouds, and falling back as precipitation. This movement is the water cycle or hydrological cycle — one of the most important processes shaping the Earth’s climate and supporting all life.

This chapter explains how water enters the atmosphere through evaporation, how humidity is measured, how clouds form, how different cloud types are classified, and how different types of rainfall form. For NDA, the most heavily tested topics are cloud types and their altitude, types of rainfall and the conditions that produce them, and specific humidity-related terms.

Understand the Topic

Evaporation and the Water Cycle

Evaporation is the process by which liquid water at the surface converts to water vapour and enters the atmosphere. It is driven by solar energy — the Sun heats the water surface, giving water molecules enough energy to escape into the air above.

Factors that increase evaporation:

• High temperature
• Low humidity (dry air can absorb more moisture)
• Strong wind (carries away humid air and replaces it with dry air)
• Large surface area of water body

Transpiration is the loss of water vapour from plants through small pores in their leaves called stomata.

Evapotranspiration = evaporation + transpiration combined. It is the total water loss from land surfaces through both physical and biological processes.

The Hydrological Cycle:

The hydrological cycle describes the continuous movement of water through the Earth system:
Evaporation from surface → Water vapour rises → Condensation into clouds → Precipitation (rain, snow) → Runoff back to rivers and oceans → Evaporation again.

The hydrological cycle is driven entirely by solar energy — the Sun powers evaporation, which powers the entire cycle. [NDA tested via CDS]

Humidity

Humidity is the amount of water vapour present in the air.

Absolute Humidity:
The actual amount of water vapour present in a given volume of air — expressed in grams per cubic metre (g/m³).

Absolute humidity decreases with altitude — because the total amount of air and water vapour both decrease with altitude. [NDA tested via CDS]

Specific Humidity:
The mass of water vapour per unit mass of air (including water vapour) — expressed in grams per kilogram (g/kg). Unlike absolute humidity, specific humidity does not change when pressure or temperature changes alone — it only changes when water vapour is added or removed. It is more useful in meteorology than absolute humidity. [NDA tested via CDS]

Relative Humidity:
The ratio of the actual amount of water vapour in the air to the maximum amount it could hold at that temperature — expressed as a percentage.

Relative Humidity (%) = (Actual water vapour / Maximum possible water vapour at that temperature) × 100

If relative humidity is 70%, the air is holding 70% of the maximum water vapour it can hold at that temperature.

When relative humidity reaches 100%, the air is fully saturated — it cannot hold any more water vapour. Any further addition of moisture or drop in temperature will cause condensation.

Relative humidity is always expressed as a percentage. [NDA 2014-II]

Relative humidity changes with temperature — as temperature rises, air can hold more water vapour, so relative humidity falls (even if no moisture is added or removed). As temperature falls, air can hold less, so relative humidity rises. [NDA tested via CDS]

Dew Point:
The temperature to which air must be cooled (at constant pressure and humidity) for it to become saturated — relative humidity reaches 100%. Below the dew point, condensation begins.

The dew point is the temperature at which relative humidity is 100%. [NDA tested via CDS]

When the dew point is close to the current air temperature, the air is nearly saturated — rain or fog is likely. When the dew point is far below the air temperature, the air is dry.

Condensation

When air cools below the dew point, water vapour condenses into tiny liquid droplets. This is condensation — the reverse of evaporation.

For condensation to occur in the atmosphere, two conditions are needed:

1. Air must cool below its dew point
2. There must be tiny particles (dust, pollen, salt) in the air for the droplets to form around — these are called condensation nuclei

Forms of condensation near the surface:

Dew: Water droplets that form on cold surfaces at night when the surface temperature falls below the dew point. Forms when the sky is clear and the air is calm — because radiation escapes freely and cools the surface rapidly. [NDA tested via CDS]

Frost: When the dew point is below 0°C, water vapour deposits directly as ice crystals on cold surfaces — this is frost. [NDA tested via CDS]

Fog: A cloud at ground level. Forms when a large mass of air near the surface cools below its dew point — creating tiny suspended water droplets that reduce visibility. [NDA 2016-I | NDA tested via CDS]

Mist: Similar to fog but thinner — visibility is reduced but not as severely as in fog. The distinction between fog and mist is usually based on visibility (fog reduces visibility below 1 km; mist below 2 km). [NDA tested via CDS]

Smog: A mixture of smoke, dust, and fog. The word is a combination of smoke and fog. Common in industrial cities when temperature inversion traps pollutants near the surface. [NDA tested via CDS]

Types of Fog:

Radiation Fog: Forms on clear, calm nights when the ground radiates heat and cools rapidly. The air near the ground also cools — eventually reaching the dew point. Most common in valleys where cold air drains and accumulates. [NDA tested via CDS]

Advection Fog: Forms when warm, moist air moves horizontally over a cold surface. The air near the surface cools below its dew point. Famous example — the fog along the California coast when warm Pacific air moves over the cold California Current. [NDA tested via CDS]

Steam Fog (Arctic Sea Smoke): Forms when very cold air moves over relatively warm water — water evaporates and immediately condenses in the cold air above, creating wisps of fog. [NDA tested via CDS]

Clouds

A cloud is a visible mass of tiny water droplets or ice crystals suspended in the atmosphere. Clouds form when air rises, expands, cools below its dew point, and condensation occurs on condensation nuclei.

How clouds form — the mechanism:

Air can be forced upward by:

• Convection (surface heating)
• Orographic lifting (air forced up by a mountain)
• Frontal lifting (warm air forced up over cold air at a front)
• Convergence (air flowing together and being forced up)

As air rises, it expands and cools. When it reaches the dew point, condensation begins and a cloud forms. The altitude at which this happens is called the cloud base.

Cloud Classification

Clouds are classified by two characteristics:

1. Altitude — how high they form (high, middle, or low)
2. Form — their shape (layered/flat = stratus type; heaped/puffy = cumulus type; wispy = cirrus type)

The classification system was devised by Luke Howard in 1803. [NDA tested via CDS]

High Clouds (above 6,000 m):

High clouds are made entirely of ice crystals — because temperatures at these altitudes are well below freezing.

Cirrus: Thin, wispy, feathery clouds made of ice crystals. They look like white wisps or curls against a blue sky. They do not produce rain. They often signal approaching weather — particularly a warm front. [NDA tested via CDS]

Cirrostratus: A thin, sheet-like layer of ice crystals covering much of the sky. Creates a halo effect around the Sun or Moon. Does not produce rain directly but indicates approaching precipitation. [NDA tested via CDS]

Cirrocumulus: Small, white puffs of cloud arranged in rows at high altitude. Sometimes called a “mackerel sky.” [NDA tested via CDS]

Middle Clouds (2,000–6,000 m):

Altostratus: A grey or blue-grey sheet of cloud covering the whole sky. Often thick enough to block the Sun. Produces widespread, steady rain or snow. [NDA tested via CDS]

Altocumulus: White or grey patches, sheets, or layers arranged in rows. Sometimes produce light rain. [NDA tested via CDS]

Low Clouds (below 2,000 m):

Stratus: A flat, grey, featureless sheet of cloud close to the ground. Like fog that does not quite reach the surface. Produces drizzle. Common on overcast days. [NDA tested via CDS]

Stratocumulus: Low, lumpy, grey clouds in patches or rolls. The most common cloud type globally. May produce light rain. [NDA tested via CDS]

Nimbostratus: A dark grey, thick layer of cloud that produces continuous, moderate to heavy rain or snow. No distinct base — the rain often merges with the cloud itself. The “nimbo” prefix means rain-producing. [NDA tested via CDS]

Vertically Developed Clouds (span all altitudes):

Cumulus: Puffy, heaped, white clouds with flat bases and rounded tops. The classic “fair weather” cloud. Formed by convection. Do not usually produce rain when small — but can grow into cumulonimbus. [NDA tested via CDS]

Cumulonimbus: The largest and most powerful cloud type. Extends from low altitude to the top of the troposphere (tropopause) — sometimes 15 km tall. Dark, anvil-shaped top (called an anvil cloud). Produces heavy rain, hail, lightning, thunder, and tornadoes. The cloud associated with thunderstorms. [NDA 2022-I | NDA tested via CDS]

Cumulonimbus is the cloud type associated with thunderstorms. [NDA 2022-I]

The correct sequence of clouds in terms of increasing height: Nimbostratus → Altostratus → Cirrus. [NDA tested via CDS]

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Precipitation

Precipitation is any form of water — liquid or solid — that falls from clouds to the Earth’s surface.

Types of precipitation: rain, drizzle, snow, sleet, hail, freezing rain.

Rainfall specifically refers to liquid precipitation.

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Types of Rainfall

There are three main mechanisms by which air is forced upward to produce rainfall. Each produces a distinctive type of rainfall with characteristic patterns.

1. Convectional Rainfall

Mechanism:
The surface heats up intensely — particularly in the tropics or during summer afternoons. Air near the surface warms rapidly, becomes less dense, and rises vigorously in convection currents. As it rises, it cools and condensation occurs, forming large cumulonimbus clouds. Heavy rainfall — often with thunder and lightning — results.

Characteristics:

• Intense and heavy
• Short duration — usually a few hours
• Accompanied by thunder and lightning
• Very localised — may rain heavily on one side of a street while the other side is dry

Where it occurs:

• Equatorial regions — where intense heat drives convection almost every afternoon
• Continental interiors in summer — particularly the Gangetic Plain in India

Equatorial regions receive convectional rainfall almost every afternoon because of intense surface heating. [NDA tested via CDS]

2. Orographic Rainfall (Relief Rainfall)

Mechanism:
When moisture-laden winds blow against a mountain range, they are forced upward. As the air rises, it cools, condenses, and produces rainfall on the windward side (the side facing the wind). After crossing the mountains, the air descends on the leeward side. As it descends, it warms and becomes drier — so the leeward side receives little or no rain.

The Rain Shadow:
The dry area on the leeward side of a mountain range where little rain falls is called a rain shadow. [NDA tested via CDS]

Characteristics:

• Heavy rainfall on the windward side
• Dry conditions on the leeward side (rain shadow)
• Persistent — occurs whenever moisture-laden winds blow against the mountains

Examples:

• The Western Ghats receive heavy rainfall on their western windward side (up to 6,000 mm per year) while the Deccan Plateau on the eastern leeward side receives very little — the classic rain shadow of India.
• The Himalayas force the monsoon winds upward — producing enormous rainfall on the southern slopes.

[NDA tested via CDS]

3. Cyclonic (Frontal) Rainfall

Mechanism:
When a warm air mass meets a cold air mass at a front, the warm air is forced upward — either steeply (at a cold front) or gradually (at a warm front). As the warm air rises, it cools and produces precipitation.

Characteristics:

• At a cold front: heavy, intense rain over a narrow belt
• At a warm front: steady, widespread, moderate rain over a large area
• Associated with temperate cyclones
• Most common in mid-latitudes (30°–60°)

This type of rainfall is also called frontal rainfall. It is most common in temperate regions — particularly in Western Europe and North America. [NDA tested via CDS]

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Snow

Snow forms when temperatures in the cloud are below freezing and ice crystals grow large enough to fall. Each snowflake has a unique hexagonal (six-sided) crystal structure.

Snow accumulates in polar regions and high mountains — contributing to glaciers.

Precipitation at the poles is mostly snow because temperatures are too cold for liquid water. [NDA tested via CDS]

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Hail

Hail forms in large cumulonimbus clouds where strong updrafts carry raindrops upward into freezing temperatures. The droplet freezes, falls, gets swept back up, acquires another layer of ice, falls again — growing in concentric layers until it is heavy enough to fall to the ground as a hailstone.

Hail is always associated with cumulonimbus clouds and thunderstorms. [NDA tested via CDS]

Global Rainfall Distribution

Rainfall is not evenly distributed around the world. The pattern largely follows the pressure and wind belts:

High rainfall areas:

• Equatorial zone — convectional rainfall almost daily
• Windward sides of mountains in the path of moisture-bearing winds
• Tropical monsoon coasts
• Mid-latitude west coasts (frontal rainfall from westerlies)

Low rainfall areas:

• Subtropical high pressure belts — descending air suppresses rain (location of world’s hot deserts)
• Leeward sides of mountains (rain shadow)
• Continental interiors far from ocean moisture
• Polar regions — cold air holds very little moisture

Why deserts exist in subtropical zones:
The subtropical high pressure belt (25°–35° latitude) is characterised by sinking, diverging air. Sinking air warms — which increases its capacity to hold moisture rather than releasing it. So precipitation is suppressed. This is why the world’s major hot deserts — Sahara, Arabian, Thar, Australian — are all in this zone. [NDA tested via CDS]

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Measurement of Rainfall

Rainfall is measured by a rain gauge — a cylindrical container that collects rain. The depth of collected water is measured in millimetres or inches. [NDA tested via CDS]

Isohyets are lines on a map connecting places of equal annual rainfall — similar to how isotherms connect equal temperature and isobars connect equal pressure. [NDA tested via CDS]