Science: Wets v. Drys

As summer died and autumn rains swept the Atlantic seaboard, jolly Professor Carl-Gustaf Arvid Rossby last week talked about weather in a Swedish accent to members of the Fifth International Congress of Applied Mechanics, at Cambridge. Mass. New facts had been obtained, said Dr. Rossby, from weather sounding balloons and airplane explorations of the upper atmosphere. These had been woven together into an original theory about the general circulation of the atmosphere, an elaborate theory still thin in spots, but one that raises scientific hopes for more accurate weather prediction.

The upper atmosphere contains huge curling streaks, or tongues of air, said Professor Rossby, which remain at fairly constant potential temperatures and specific humidities, are not considerably affected by winds. Their rates of drift are variable. Two types of tongues have been discovered: 1) dry, usually coming from the north; 2) wet, coming from the south. A large tongue may stretch for 1,500 miles across the U. S., and 20 or 30 smaller streaks may be observed in one day, forming a roof over the entire continent. Plotted on a meteorological map they resemble a mass of partly coiled snakes. Although the greater area of each tongue remains at uniform specific humidities, the extremities tend to undergo changes. At the southern ends of the dry tongues heat is lost and dry air descends from the upper regions. The northern ends of the wet tongues tend to condense, pass mois ture to the dry tongues. As the tongues exchange heat and moisture and as atmospheric currents follow the rotation of the earth, transverse currents of air are generated. Theoretically these currents cut across the more stable air tongues, dividing each air tongue into three parts or “cells” — a centre cell in which the circulation is counterclockwise, between two cells in which the circulation is clockwise — like three gear wheels revolving in series. Friction tends to develop kinetic energy, ultimately generating strong winds in the centre cell of each air tongue. The two outer cells tend to disperse this energy north and south. The effects of this dispersion reach the ground, and ground friction furthers the action of the brakes.

Professor Rossby promised that further study will reveal more significant facts on the origin and dispersal of storms. Mean while, he suggested that it would be more practical for the Weather Bureau to supplement its pressure maps with charts of the homogeneous air tongues. He has been using “isentropic charts” for over a year, he said, with excellent practical results.