The relative humidity is the ratio of the water vapor to vapor pressure at a given temperature, the same amount of water vapor leads to the higher relative humidity in cooler air than in warm air i.e dew point.
As clouds are composed of water droplets particularly ice crystals. As typical when the clouds are formed in a cool air the temperature equals the dew point. Which is 100% humidity resulting in condensation that produces clouds.
Over the oceans, the ability to dissolve the salt and the ability to hold on molecules of water is called a Solute effect. This allows the clouds to form at a humidity of less than 100%. Nucleation is an initial part of a cloud droplet around any type of particle.
