Which meteorological parameter is expressed as the ratio between the air's actual water vapor content and the amount of water vapor required for saturation at that temperature?

The correct choice is relative humidity because it defines the amount of water vapor present in the air relative to the maximum amount of water vapor that the air can hold at a specific temperature. This is expressed as a percentage, allowing for an understanding of how close the air is to being saturated.

When temperature increases, the capacity of air to hold moisture also increases; thus, relative humidity helps to reflect changes in moisture availability in the atmosphere. In contrast, absolute humidity measures the total mass of water vapor in a given volume of air, regardless of temperature. Dew point indicates the temperature at which air becomes saturated with moisture, but it does not express the ratio of actual moisture to potential moisture capacity. The saturation point merely refers to the state when the air holds the maximum amount of moisture it can, but again, it does not provide the ratio needed for relative humidity. Therefore, relative humidity is the most relevant parameter that directly relates to the actual moisture content versus saturation capacity at a specific temperature.