Clouds form when air with a relative humidity close to 100% cools. They are composed of that air and micro-droplets of liquid water, with either larger droplets of liquid water, centered around some type of solid particulate, or ice crystals, sometimes both. It is the larger water droplets or ice crystals reflecting or refracting light that provide the visual aspect of the cloud that we see. The ice crystals may have formed on solid particulates or micro-droplets of water that have themselves frozen. But what conditions are required to cause this to occur?
Relative humidity is the type used in weather forecasting. It is a comparison of actual water vapor content in the air against the maximum possible, the saturation vapor pressure of water for the current air temperature. The higher the local air temperature, the higher the possible maximum. In air with a relative humidity of 100%, any additional water vapor will condense, forming micro-droplets of liquid water. In air with a relative humidity of 100%, any drop of temperature will cause some water vapor to condense, forming micro-droplets of liquid water. So a relative humidity close to 100% is essential for cloud formation.
The micro-droplets will adhere to solid particulates in the air; these may be dust particles, the type you can notice in the air when a beam of sunlight catches them just right, salt carried up by the evaporation of sea or ocean water, ash from a volcano or burning forests, or increasingly, lighter than air byproducts from human industrial processes. Depending on the local air temperature, these will form as larger liquid droplets or as ice crystals. In sufficiently cold air, some of the micro-droplets may themselves freeze and form the nucleus for the development of larger ice crystals.
It is these larger water droplets or ice crystals that we see as clouds. The light from the sun, or to a lesser extent the moon and stars at night, being refracted through the lenses the droplets and/or crystals form, giving us the visual impression of white fluffy clouds floating in the sky; the greater the density, the darker the cloud will appear, moving through gray to black.
The molecular mass of water vapor is 18.02, lower than that for the average of the other components of air based on their respective percentages; dry air, the term used for air excluding its water vapor content, having a molecular mass of 28.98. What this means is that the higher the water vapor content (humidity) of air, the lower its mass, and air pressure is directly proportional to its mass.
As the water vapor content of air increases, the air becomes “lighter” and rises as higher pressure air pushes into the low pressure area. The rising air spirals because it is above a curved rather than flat surface; in tropical regions this can result in the spiraling form of hurricanes or cyclones when the disparity between low and high pressure areas is particularly great. The lighter region of air pressure that is a cloud, results in an inbound airflow that brings additional water vapor to the forming cloud. Clouds continue to expand until the mass of their water content, liquid or solid, becomes too heavy for the local air pressure to hold up against the force of gravity. When that point is reached, we get precipitation. If the cloud is composed of large, liquid water droplets, it will fall as a shower or rain. If it has a significant amount of ice crystals, they may melt on the way down and fall as rain, partially melt and fall as sleet, or fail to melt significantly, and fall as hail or snow.