Introduction

The story of weather begins with something you cannot see: water vapour. Air is not just oxygen and nitrogen — it carries water in a gaseous form, and that invisible ingredient drives clouds, rain, fog, and storms. This section sets the stage by explaining how water enters the atmosphere, in what forms it exists, and how we measure its presence.

Water vapour makes up anywhere from zero to four per cent of the atmosphere by volume. That range matters — the amount is never fixed. It shifts with place, season, and temperature, and it is the single most active player in weather phenomena. Without it, there would be no clouds, no precipitation, and no humidity to speak of.

Water exists in the atmosphere in three physical states:

• Gaseous — water vapour, the invisible form
• Liquid — tiny droplets that make up clouds and fog
• Solid — ice crystals in high clouds, hail, and snow

Where does all this moisture come from? Two main sources feed the atmosphere:

• Evaporation from water bodies — oceans, seas, lakes, and rivers
• Transpiration from plants — moisture released through leaves

These two processes set up a continuous exchange of water between the atmosphere, the oceans, and the continents. The cycle runs through four processes: evaporation, transpiration, condensation, and precipitation. Water evaporates from the sea, travels through the air, condenses into clouds, and falls back as rain or snow — only to evaporate again.

The water vapour present in the air is called humidity. It is not just a vague feeling of dampness; it is a measurable quantity, and geographers express it in several distinct ways.

Absolute humidity is the actual amount of water vapour present in the atmosphere at a given moment. It is measured as the weight of water vapour per unit volume of air, expressed in grams per cubic metre. This value is not uniform across the earth — it differs from place to place, depending on local conditions like proximity to water and temperature.

The key to understanding humidity is temperature. The ability of air to hold water vapour depends entirely on its temperature. Warm air can hold more moisture; cool air holds less. This single fact explains why humidity behaves so differently across the globe.

Relative humidity compares the moisture actually present in the air to the full capacity of the air at that same temperature. It is expressed as a percentage. When temperature rises, the air's capacity to hold moisture increases, so relative humidity falls even if the actual moisture stays the same. When temperature drops, capacity shrinks and relative humidity rises. This is why relative humidity is greater over the oceans and least over the continents — oceans supply abundant moisture and moderate temperatures, while continents experience wider temperature swings and drier air.

When air holds moisture to its full capacity at a given temperature, it is said to be saturated. At that point, the air is incapable of holding any additional moisture. The temperature at which saturation occurs in a given sample of air is called the dew point. This is the critical threshold — cool the air to its dew point and condensation begins, forming dew, fog, or clouds.