What measurement shows the weight of water vapor in a certain volume of air?

The measurement that shows the weight of water vapor in a certain volume of air is absolute humidity. This is defined as the mass of water vapor present in a given volume of air, typically expressed in grams of water vapor per cubic meter of air. Absolute humidity provides a direct measure of the amount of moisture in the air, regardless of temperature or pressure.

Relative humidity, on the other hand, is a percentage that compares the current amount of water vapor in the air to the maximum amount of water vapor the air can hold at a specific temperature, highlighting how close the air is to saturation. Specific humidity measures the mass of water vapor relative to the total mass of air, providing a ratio rather than a volume-based measurement. Saturation vapor pressure indicates the pressure exerted by water vapor in a system when the air is saturated, but it does not quantify the actual mass of water vapor present.

Thus, absolute humidity is the appropriate choice for understanding the weight of water vapor in a specific volume of air.