Non-Technical Summary

Why cyclones deepen and fill up is one of the fundamental problems in meteorology that still largely resists theoretical treatment; but it is a matter of great practical importance because it does not seem that our daily forecasts will improve greatly until we find quantitative expressions for the factors leading to development as well as mere translation of pressure systems. Mr. Gringorten in this paper follows up a line of attack already laid down by several European investigators to see how well it may agree with reality. While he does not offer us a new principle the result is very interesting because he shows that the familiar conception of the isallobaric wind component, i.e. the small departure of the wind from the “gradient wind” which follows the isobars (except with surface friction), must play an important role in the pressure changes observed on the weather maps or the barograms every day. Qualitatively we have believed this for some time but Mr. Gringorten has computed the expected pressure change, from formulae proposed by Bjerknes and by Sutcliffe, for a particular time and place. This case was for the layers between 2000 and 8000 feet above Detroit between 7:30 p.m. Feb. 2 and 7:30 p.m. Feb. 3, 1940. The computed pressure change at 2000 feet was at the rate of 22.8 millibars in 12 hours but the observed change was only 5 mb. The practical meteorologist will not regard this as very good agreement but in view of the great difficulties in the theoretical treatment of such a problem the theoretician finds some consolation in that at least the computation gives results in the right direction. Of course, as Mr. Gringorten points out, it was obvious on Feb. 3 from the A. M. weather map that the pressure would fall at Detroit by that evening, and no one would suggest that the theory be used for forecasting.—R. G. S.

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*A shortened version of a paper read at the Seattle meeting, June, 1940; based on a study submitted in fulfillment of degree requirements in meteorology at California Institute of Technology.