WATER exists in the atmosphere in various forms: it falls from the clouds as rain, snow, or hail; it collects on cold surfaces as dew or frost. We cannot see very far into or through a cloud or fog, because the minute spheres of water or crystals of ice are large enough and close enough together to obstruct the beam of light, but we may see quite well through a mile of hazy air when the particles of water are very small or far distant from each other. In proportion as the particles are smaller and more distant the hazy appearance diminishes. So long as any visible particles of water or any fog are present we consider the air containing the particles or fog to be saturated.
How Fog and Cloud are Sustained in the Air. --Water globules are too heavy to float in the air at a constant elevation; they settle downward slowly against a resistance that depends on their size and on the viscosity of the air. The velocity of fall is such that this resistance is equal to the product of the mass of the falling drops by the acceleration in vacuo due to gravity. Therefore, all particles of water of a given diameter tend to fall at about the same rate, and for very small particles, such as occur in clouds and fog, this rate is so small that the gentlest upward movement of the air keeps them from falling to the ground.
The Amount and the Weight of Water Vapor and its Pressure. --If no particles of water or ice are present in the air the latter may still be just barely saturated by aqueous vapor, but in general the air is more likely to be below saturation. We speak of the air as "ordinary dry air" when some aqueous vapor is present but not enough to saturate it. By using chemical dryers we may absorb practically all of this vapor from a small mass of air and make it almost perfectly dry, and then we say that its absolute humidity is zero grains of water per cubic foot of space. Its relative humidity is also zero, meaning thereby zero per cent as compared