Water Vapour: The Variable Part of Air

Introduction

Tables of the composition of air nearly always describe dry air . Real air is never completely dry: it always contains some water vapour, and the amount changes from hour to hour and place to place. Over a hot rainforest it can be about 4% of the air; over a frozen desert it may be almost nothing. On this page you will see why water vapour is the variable part of air and how it gives us humidity, dew and mist.

Core explanation

Water vapour is a gas. Water vapour is water in the gas state: separate H₂O molecules moving freely among the nitrogen and oxygen molecules. It is invisible. The white "steam" above a kettle and the white of a cloud are not water vapour at all — they are tiny droplets of liquid water that have condensed from it.

How much is there? The proportion of water vapour ranges from almost 0% in very cold, dry air to about 4% by volume in hot, humid air. A typical figure is 1 to 2%. Because it changes so much, scientists remove it before quoting the "standard" composition of air, and add it back separately as humidity.

Where it comes from. Water vapour enters the air by evaporation from oceans, lakes, rivers and wet ground, and by transpiration from the leaves of plants. Warmth speeds up evaporation because more water molecules at the surface have enough energy to escape.

Why the amount varies — temperature. The maximum amount of water vapour that air can hold depends strongly on temperature. Warm air can hold much more than cold air: roughly, the maximum doubles for every 10 °C rise. Approximate maximum values are:

Air temperature Maximum water vapour (g per m³ of air) --- --- 0 °C about 5 10 °C about 9 20 °C about 17 30 °C about 30

Humidity. The humidity of air is the amount of water vapour it contains. Weather forecasts usually give the relative humidity :

relative humidity = (actual water vapour ÷ maximum possible at that temperature) × 100%

Air at 100% relative humidity is saturated : it cannot hold any more water vapour at that temperature.

Condensation and the dew point. If air cools, the maximum it can hold falls. At a certain temperature, called the dew point , the air becomes saturated. Cool it further and some water vapour must condense into liquid. This explains:

- dew on grass after a clear, cold night; - mist and fog , when air near the ground cools below its dew point; - condensation on a cold window or on the outside of a glass of iced drink; - clouds , when rising air cools as it expands.

Why humidity matters. Our bodies cool by evaporating sweat. In very humid air, sweat evaporates slowly, so hot, humid weather feels much more uncomfortable than hot, dry weather.

Formulae

relative humidity (%) = (mass of water vapour present ÷ maximum mass that could be present at that temperature) × 100

Step-by-step reasoning

To predict whether dew will form overnight:

1. Find the actual amount of water vapour in the evening air. 2. Find the lowest temperature expected during the night. 3. Look up the maximum water vapour the air can hold at that temperature. 4. If the actual amount is greater than that maximum, the air will cool past its dew point and dew will form.

Visual explanation

Picture air as a set of theatre seats for water molecules, where the number of seats depends on temperature. On a warm afternoon there are many seats, and only half are filled — 50% relative humidity. As the evening cools, rows of seats are removed. Once every remaining seat is taken, any extra water molecules have nowhere to sit and "drop out" as liquid droplets: dew.

Real-world analogy

Warm air is like a large sponge and cold air like a small one. A sponge only half full is not dripping. Squeeze it smaller while keeping the same water inside, and it eventually starts to drip. Cooling air does the same to its water vapour.

Real-world example

On a cold morning the inside of a car windscreen often mists up. The warm, moist breath of the passengers meets the cold glass, cools below its dew point and condenses as tiny droplets. Blowing warm, dry air from the heater raises the glass temperature and evaporates the mist.

Why?

Why is desert air often dry even though deserts are hot? Warm air could hold a lot of water vapour, but in a desert there is very little water available to evaporate. Relative humidity is low because the actual amount is far below the large maximum possible at that high temperature.

Common misconception

"The cloud above a boiling kettle is water vapour." Water vapour is invisible. The visible cloud is made of tiny liquid droplets formed when the vapour cools in the air. Look closely at the spout: the gap just in front of it, where the vapour is still hot, appears clear.

Worked example

Question: Air at 20 °C contains 8.5 g of water vapour per m³. The maximum it could hold is 17 g per m³. What is the relative humidity?

Reasoning: relative humidity = (8.5 ÷ 17) × 100% = 50%.

Answer: 50%.

Quick check

1. Why does the proportion of water vapour in air vary so much? Answer: It depends on temperature and on how much water is available to evaporate, both of which change from place to place and over time.

Exam focus

Remember that standard compositions are for dry air and that water vapour varies from about 0% to about 4%. Be able to explain condensation, dew and mist using the idea that cooler air can hold less water vapour. State clearly that water vapour is invisible.

Advanced insight

Water vapour is actually the most important greenhouse gas by amount of heat absorbed. However, its level is controlled by temperature rather than by human emissions directly: extra water vapour rains out within days. As carbon dioxide warms the air, the air can hold more water vapour, which adds further warming — an example of a feedback.

Summary

Air always contains some water vapour, from almost 0% to about 4% by volume. The amount depends on temperature and on available water. Humidity measures water vapour in air, and relative humidity compares it with the maximum possible. When air cools to its dew point it becomes saturated and water condenses, forming dew, mist, fog and clouds.

Practice questions

1. What is meant by a relative humidity of 100%? Answer: The air is saturated: it holds the maximum water vapour possible at that temperature. 2. Explain why dew forms on grass on a clear, cold night. Answer: The ground and the air next to it cool below the dew point, so the air can no longer hold all its water vapour, and the excess condenses on the grass. 3. Air at 30 °C holds 15 g of water vapour per m³. The maximum at 30 °C is 30 g per m³. Calculate the relative humidity. Answer: (15 ÷ 30) × 100% = 50%. 4. Why do tables of air composition usually describe dry air? Answer: Because the water vapour content varies so much; removing it gives figures that are nearly the same everywhere. 5. Why does hot, humid weather feel more uncomfortable than hot, dry weather? Answer: Sweat evaporates more slowly in humid air, so the body cannot cool itself as effectively.