Which type of humidity is most critical for assessing cloud formation?

Relative humidity is critical for assessing cloud formation because it represents the amount of moisture in the air relative to the maximum amount of moisture the air can hold at a given temperature. This measurement indicates how saturated the air is with water vapor, which is essential for cloud development. When relative humidity reaches 100%, the air is fully saturated, and any further cooling or addition of water vapor can lead to condensation, forming clouds.

Additionally, while absolute humidity measures the actual mass of water vapor in a specific volume of air, and specific humidity expresses the mass of water vapor per mass of air, neither provides insight into the saturation level of the air in relation to temperature. Similarly, the mixing ratio, which is the mass of water vapor to the mass of dry air, does not directly account for temperature variations that affect the air's capacity to hold water vapor. Therefore, relative humidity is the most relevant measure when evaluating the potential for cloud formation.