Water in the Atmosphere
1. Learning Objectives
After reading these notes, you will be able to:
2. Humidity — Types and Concepts
💧 Absolute Humidity
The actual amount of water vapour present in the atmosphere. It is the weight of water vapour per unit volume of air, expressed in grams per cubic metre. It differs from place to place on Earth’s surface.
💦 Relative Humidity
The percentage of moisture present in the atmosphere as compared to its full capacity at a given temperature. With change in temperature, capacity to retain moisture changes → relative humidity is also affected. It is greater over oceans and least over continents.
🌫️ Saturated Air
Air containing moisture to its full capacity at a given temperature is said to be saturated. It is incapable of holding any additional moisture at that stage.
🌡️ Dew Point
The temperature at which saturation occurs in a given sample of air is known as the dew point. When air cools to dew point, it can no longer hold all its water vapour → condensation begins.
3. Evaporation and Condensation
☀️ Evaporation
Process by which water is transformed from liquid to gaseous state. Heat is the main cause for evaporation. Latent Heat of Vaporisation = amount of heat energy required to convert a unit mass of liquid into vapour without a change in temperature. Higher temperature → more absorption and retention. Greater movement of air → greater evaporation.
🌧️ Condensation
Transformation of water vapour into water. Caused by loss of heat. When moist air is cooled, its capacity to hold water vapour ceases → excess water vapour condenses into liquid form. In free air, condensation results from cooling around very small particles called hygroscopic condensation nuclei — dust, smoke, salt from ocean.
📋 Conditions for Condensation
- When the temperature of air is reduced to dew point with volume remaining constant.
- When both volume and temperature are reduced.
- When moisture is added to the air through evaporation.
- Most favourable condition = decrease in air temperature.
4. Forms of Condensation
On surfaces
Dew point above
freezing point
Ice crystals
Dew point at/below
0°C
Near ground
Fog <1 km visibility
Mist 1–2 km
High in atmosphere
Water droplets
or ice crystals
🌱 Dew
Moisture deposited as water droplets on cooler surfaces of solid objects — stones, grass blades, plant leaves. Ideal conditions: clear sky, calm air, high relative humidity, cold and long nights. Dew point must be above freezing point (0°C) for dew formation.
🧊 Frost
Forms on cold surfaces when condensation takes place below freezing point (0°C) — i.e., dew point is at or below freezing point. Excess moisture is deposited in the form of minute ice crystals instead of water droplets. Ideal conditions same as dew, except air temperature must be at or below 0°C.
🌫️ Fog and Mist
When temperature of an air mass falls suddenly, condensation takes place on fine dust particles near the ground. Fog = cloud with base at or near ground; reduces visibility to less than 1 km. Mist limits visibility to 1–2 km. Mist contains more moisture than fog. Smog = fog mixed with smoke.
☁️ Clouds
A mass of minute water droplets or tiny ice crystals formed by condensation of water vapour in free air at considerable elevations. They take various shapes. Classified based on height, expanse, density and transparency/opaqueness into four types: Cirrus, Cumulus, Stratus, Nimbus.
5. Types of Clouds
| Cloud Type | Altitude | Appearance | Key Feature |
|---|---|---|---|
| Cirrus | 8,000–12,000 m (High) | Thin, detached, feathery | Always white in colour |
| Cumulus | 4,000–7,000 m (Middle) | Look like cotton wool; patchy | Flat base, exist in patches |
| Stratus | Low levels | Layered, cover large portions of sky | Formed due to loss of heat or mixing of air masses |
| Nimbus | Middle level or near surface | Black or dark grey; shapeless, thick | Rain-bearing clouds; extremely dense and opaque |
Middle clouds: Altostratus, Altocumulus
Low clouds: Stratocumulus, Nimbostratus
Clouds with extensive vertical development: Cumulus, Cumulonimbus
6. Precipitation — Types and Forms
🌧️ Rainfall
Precipitation in the form of water. Most common form. Occurs when temperature is above 0°C.
❄️ Snowfall
When temperature is lower than 0°C, precipitation takes place as fine flakes of snow. Moisture is released in the form of hexagonal crystals that form snow flakes.
🧊 Sleet
Frozen raindrops and refrozen melted snow-water. When a layer of air above freezing overlies a sub-freezing layer near the ground → raindrops encounter colder air → solidify → reach ground as small pellets of ice (not bigger than raindrops).
⚡ Hailstones
Drops of rain after being released by clouds become solidified into small rounded solid pieces of ice. Formed by rainwater passing through colder layers. Hailstones have several concentric layers of ice one over the other.
7. Types of Rainfall
🌀 1. Convectional Rain
Air on being heated becomes light and rises up in convection currents. As it rises, it expands → loses heat → condensation → cumulous clouds → heavy rainfall with thunder and lightning. Does not last long. Common in the hotter part of the day. Very common in equatorial regions and interior parts of continents (especially Northern Hemisphere).
⛰️ 2. Orographic (Relief) Rain
When saturated air mass comes across a mountain, it is forced to ascend → expands → temperature falls → moisture condenses → rain on windward slope. Winds reach leeward side → descend → temperature rises → capacity to absorb moisture increases → leeward slope remains dry (rain-shadow area). Also called Relief Rain.
🌪️ 3. Cyclonic (Frontal) Rain
Caused by cyclonic activity and fronts. When two air masses of different temperatures meet at a front, the warm air is forced to rise over the cold air → cooling → condensation → precipitation. Refer to Chapter 9 (Extra Tropical Cyclones) for detailed understanding.
8. World Distribution of Rainfall
📊 General Pattern
- As we proceed from the equator towards the poles, rainfall goes on decreasing steadily.
- Coastal areas receive greater amounts of rainfall than interior of continents.
- Rainfall is more over oceans than on landmasses — oceans are great sources of water.
- Between latitudes 35°–40° N and S of equator — rain is heavier on eastern coasts and decreases towards west.
- Between latitudes 45°–65° N and S of equator — due to westerlies, rainfall is first received on western margins of continents and decreases eastwards.
- Where mountains run parallel to the coast — rain is greater on coastal plain (windward side) and decreases towards leeward side.
📋 Precipitation Regimes — Annual Amounts
| Region | Annual Rainfall |
|---|---|
| Equatorial belt, windward slopes of mountains (west coasts — cool temperate zone), coastal monsoon areas | Over 200 cm |
| Interior continental areas; coastal areas of continents | 100–200 cm |
| Central parts of tropical land; eastern and interior parts of temperate lands | 50–100 cm |
| Rain shadow zones, interior of continents, high latitudes | Less than 50 cm |
Summary — Quick Revision
Humidity: Water vapour in air = 0–4% by volume. Absolute humidity = actual amount (g/m³). Relative humidity = % of moisture vs full capacity at that temperature. Greater over oceans, least over continents.
Saturation & Dew Point: Air at full moisture capacity = saturated. Temperature at which saturation occurs = dew point. Most favourable condition for condensation = decrease in air temperature.
Evaporation: Liquid → gas. Heat is main cause. Latent heat of vaporisation converts liquid to vapour. Greater air movement = greater evaporation. Higher temperature = more moisture retention capacity.
Condensation: Vapour → water. Caused by loss of heat. Results from cooling around hygroscopic condensation nuclei — dust, smoke, ocean salt. Influenced by volume, temperature, pressure, humidity.
4 Forms of Condensation: Dew (on surfaces, dew point above 0°C), Frost (ice crystals, dew point ≤ 0°C), Fog (visibility <1 km), Mist (visibility 1–2 km). Smog = fog + smoke.
4 Cloud Types: Cirrus (8,000–12,000 m, feathery, white), Cumulus (4,000–7,000 m, cotton wool, flat base), Stratus (layered, low), Nimbus (rain-bearing, black/dark grey, shapeless).
4 Forms of Precipitation: Rain (liquid, temp > 0°C), Snow (hexagonal crystals, temp < 0°C), Sleet (frozen raindrops — warmer air over subfreezing layer), Hailstones (concentric ice layers, formed in colder layers).
3 Types of Rainfall: Convectional (hot air rises, equatorial regions, afternoon thunder), Orographic/Relief (forced uplift by mountain — windward more, leeward = rain shadow), Cyclonic/Frontal (front-based, Chapter 9).
World Distribution: Rainfall decreases equator → poles. Coasts > interiors. Oceans > landmasses. Rainfall >200 cm: equatorial belt + monsoon coasts + windward mountain slopes. <50 cm: rain shadow + interior + high latitudes.
Important Terms to Remember
- Humidity: Water vapour present in the air. Varies from zero to four per cent by volume of the atmosphere.
- Absolute Humidity: The actual amount of water vapour present in the atmosphere — weight of water vapour per unit volume of air, expressed in grams per cubic metre.
- Relative Humidity: The percentage of moisture present in the atmosphere as compared to its full capacity at a given temperature. Greater over oceans, least over continents.
- Saturated Air: Air containing moisture to its full capacity at a given temperature. Incapable of holding any additional moisture at that stage.
- Dew Point: The temperature at which saturation occurs in a given sample of air. When temperature falls to dew point, condensation begins.
- Evaporation: Process by which water is transformed from liquid to gaseous state. Heat is the main cause. Greater temperature and greater air movement = greater evaporation.
- Latent Heat of Vaporisation: The amount of heat energy required to convert a unit mass of a liquid into vapour without a change in temperature.
- Condensation: Transformation of water vapour into water — caused by loss of heat. Results from cooling around hygroscopic condensation nuclei.
- Hygroscopic Condensation Nuclei: Very small particles of dust, smoke and salt from the ocean around which condensation takes place in free air.
- Dew: Moisture deposited as water droplets on cooler surfaces of solid objects like stones, grass, leaves. Requires dew point above freezing point. Ideal conditions: clear sky, calm air, high relative humidity, cold long nights.
- Frost: Forms when condensation takes place below 0°C. Excess moisture deposited as minute ice crystals. Dew point at or below freezing point.
- Fog: Cloud with its base at or near the ground. Reduces horizontal visibility to less than 1 km. Occurs where warm air currents contact cold currents.
- Mist: Similar to fog but limits visibility to 1–2 km. Contains more moisture than fog — each nucleus has a thicker layer of moisture. Frequent over mountains.
- Smog: A condition when fog is mixed with smoke. Common in urban and industrial centres.
- Cloud: A mass of minute water droplets or tiny ice crystals formed by condensation of water vapour in free air at considerable elevations.
- Cirrus: Highest clouds, formed at 8,000–12,000 m. Thin, detached, feathery appearance. Always white in colour.
- Cumulus: Clouds formed at 4,000–7,000 m. Look like cotton wool. Exist in patches with flat base.
- Stratus: Layered clouds covering large portions of the sky. Formed due to loss of heat or mixing of air masses of different temperatures.
- Nimbus: Black or dark grey clouds formed at middle levels or near the surface. Extremely dense, shapeless — rain-bearing clouds.
- Cumulonimbus: Clouds with extensive vertical development — associated with thunderstorms and heavy rainfall.
- Precipitation: The release of moisture from the atmosphere after condensation — when condensed particles grow too heavy to be held by air resistance against gravity.
- Rainfall: Precipitation in the form of water. Temperature above 0°C.
- Snowfall: Precipitation in the form of fine flakes of snow when temperature is below 0°C. Moisture released as hexagonal crystals.
- Sleet: Frozen raindrops and refrozen melted snow-water. Occurs when warm air layer overlies a sub-freezing layer near the ground — raindrops solidify into small ice pellets.
- Hailstones: Small rounded solid pieces of ice formed when raindrops pass through colder layers. Have several concentric layers of ice one over the other.
- Convectional Rain: Rainfall caused by rising convection currents of heated air. Common in equatorial regions and interior of continents, especially in the afternoon.
- Orographic (Relief) Rain: Rainfall caused when saturated air is forced to ascend over a mountain. Windward slopes receive more rain; leeward slopes are dry (rain-shadow area).
- Rain Shadow Area: The area on the leeward side of a mountain that receives less rainfall due to the descending dry air. Also called the lee side.
- Cyclonic (Frontal) Rain: Rainfall caused by cyclonic activity and fronts — when warm and cold air masses meet.
- Transpiration: The process by which plants release water vapour into the atmosphere. Along with evaporation, it adds moisture to the atmosphere.