On a Method of Observing and Counting the Number of Water Particles in a Fog

Abstract

The phenomena known as haze, fog, mist, and rain are in a general way but the successive development of the same process, and the line which divides the one from the other is very indefinite. Dust in the atmosphere produces a haze, and the thickness of a haze of this kind depends principally on the amount of dust present when the relative humidity of the atmosphere is very low. But as the humidity increases the effect of the vapour increases also; the dust particles attract the water vapour which becomes deposited on them, thus increasing their size and their hazing effect, till at last when the air is nearly saturated it becomes very thick, and forms what we call a fog; when in this condition, the thickness of the atmosphere depends principally on the degree of saturation. Between the haze and the fog, however, there is no recognised distinction in kind, it is principally one of degree. After the air is saturated and the conditions are such as to tend to cause supersaturation, then a change takes place in the condensation. A few of the dust particles have water deposited on them, and after a time they grow and become little drops of water, in which the original dust nucleus bears a very small proportion to the total weight. At this stage it is still called a fog, but after more water is deposited on the small drops they grow and become what is known as mist, and when the mist drops combine and fall they are called rain-drops.