Final answer:
The ratio of the water vapor actually present in the atmosphere to the amount that would be present if the air were saturated is referred to as relative humidity. It is expressed as a percentage, with saturation being 100%. The dew point is the temperature at which the air is saturated with water vapor.
Step-by-step explanation:
The ratio of the water vapour actually present in the atmosphere to the amount that would be present if the air were saturated at the prevailing temperature and pressure is known as relative humidity. This is the measure of how much water vapor is in the air relative to the maximum amount the air can hold at a given temperature. When the air holds the maximum amount of water vapor, we say that the air is 'saturated', and the relative humidity is 100%. The dew point is the temperature at which air reaches this level of humidity, translating to an environment where the air can no longer 'hold' all of its water vapor, and some of it condenses into liquid.
For example, on a day when the temperature is 25.0°C and the air contains 9.40 g/m³ of water vapor, by using a reference such as Table 13.5 that lists the saturation vapor densities at different temperatures, we can determine the relative humidity. If the saturation vapor density at 25.0°C is known, we can calculate the relative humidity percentage using the ratio of the actual vapor density to the saturation vapor density.
The absolute humidity is a different concept, which is the total mass of water vapor present in a given volume or mass of air.