The temperature at which air becomes saturated with water vapour is known as the:

The dew point is the temperature to which air must be cooled, at constant pressure and constant water-vapour content, for it to become saturated, that is, to hold the maximum amount of water vapour it can at that temperature. If the air is cooled further, the excess water vapour condenses into tiny droplets of liquid water.

Warm air can hold more water vapour than cold air. So when moist air cools, for example at night or when it rises, its relative humidity increases. When its temperature falls to the dew point, the relative humidity reaches 100%, and condensation begins.

Condensation at or below the dew point produces several familiar phenomena:

• Dew: water droplets that form on cool surfaces, such as grass and leaves, during clear, calm nights when the ground cools quickly by radiation.
• Frost: if the dew point is below 0°C (freezing point), water vapour changes directly into ice crystals on surfaces.

Fog and mist: when the air near the ground cools below the dew point, condensation occurs on tiny dust and smoke particles in the air, forming a cloud at ground level. Fog reduces visibility to less than 1 kilometre; mist is lighter.

Clouds: when rising air cools to its dew point at higher altitudes, clouds form.

Water droplets on the outside of a glass of cold water: the air next to the cold glass is cooled below its dew point.

The closer the air temperature is to the dew point, the more humid the air feels. A high dew point makes the weather feel muggy and uncomfortable, because sweat does not evaporate easily.

Humidity is measured with an instrument called a hygrometer. Relative humidity is the ratio of the actual amount of water vapour in the air to the maximum amount it can hold at that temperature, expressed as a percentage.

Why not the other options?

• ✗The boiling point is the temperature at which a liquid turns into vapour throughout its mass.
• ✗The freezing point is the temperature at which a liquid becomes solid (0°C for water).
• ✗The triple point is where solid, liquid and gas coexist.