Counting the Moisture Drops in a Fog
MEASUREMENTS of fog have hitherto been crude. But an example of more refined measurements of fog has recently been afforded by experts of the United States Bureau of Standards. The measurements were made in the most notoriously foggy region of the world—the Grand Banks of Newfoundland.
The tiny drops that constitute a fog are smaller than raindrops. They are formed by the condensation of the gaseous water in the air, known as water vapor. Each drop condenses about a “nucleus” —consisting of some substance other than water. The air always
contains an immense number of these nuclei, ready to form centers of condensation, when the conditions of temperature and moisture are right for this process. A method of counting these invisible nuclei was devised by John Aitken. It consists in causing a drop to form around each nucleus in a sample of air, and then counting the drops through a microscope. Another process, devised by Carl Barus, makes it possible to determine the size of
the drops. When a light is viewed through a cloud or fog it is seen to be surrounded by a colored ring, called a “corona.” You have seen such rings around the moon and around street lamps at night. The angular diameter, or aperture, of these rings depends up-
on the size of the |drops. Small drops produce big rings, and vice versa.
The apparatus of Barus was installed in the pilot-house of the Seneca, and the number of nuclei present in a given volume of air was measured three times, a day, whether the weather was foggy or otherwise. A sample of air was drawn through a pipe, projecting from the port bow. It was admitted to a “fog chamber,” saturated with water vapor, suddenly expanded, to condense water on the nuclei,
forming an artificial fog. The corona around a source of light, viewed through this fog, was measured, and the size of the drops was determined from the known amount of moisture in the chamber.