Which is the ratio of the water vapor actually present in the atmosphere to the amount that would be present if the air were saturated at the prevailing temperature and pressure?

The ratio of the water vapor actually present in the air to the amount if the air were saturated is called relative humidity.

Explanation:

The relative humidity allows:

To measure the actual amount of moisture in the air at a given temperature, and compares it to the maximum amount of moisture the air can hold at the same temperature (saturation conditions of the air at the specific conditions). Also, is allows to give an idea of the comfort that a specific condition gives. This is, because the comfort level under a specific humidity conditions, depends more on relative humidity than in the actual humidity in the air.