A study of interdiurnal pressure and temperature variations in the free atmosphere over North America

The three-dimensional distribution of pressure and its variation with time are intimately associated with the principal weather phenomena. However, this association is indirect rather than direct and the link connecting the two is the horizontal and vertical field of motion. The relationshipbetween the field of motion and the variation in time and space of pressure is one of interdependence.On the other hand, the field of motion seems causally related to most weatherphenomena. Neither of these relationships is fully understood so it is not surprising that current attempts to utilize pressure distributions as tools for predicting weather phenomena do not meetwith complete success. In this connection, the variation of pressure with time is of prime interest,inasmuch as it reflects physical processes currently in operation in the atmosphere and as it presagesfuture developments. Study of these processes through an analysis of pressure variations is complicatedby the compressibility of the atmosphere and by the observed fact that mass variationsof either sign may be taking place within any layer in the atmosphere quite independently ofvariations of the mass within any other layer.Since the instantaneous distribution of pressure and its variation with time are hydrostaticallyrelated to the temperature field and its changes, it is advantageous to study the two elementssimultaneously. Any consideration of the temperature field in the atmosphere inevitably requiresthat some attention must be devoted to the tropopause as a major first order discontinuity oftemperature. Moreover, the significance of this general reglon of the atmosphere as a location ofprocesses important in determining tropospheric pressure variations has been stressed by theAustrian School of Meteorologists.The problem of achieving a better understanding of the nature of pressure changes may beattacked in one or more of three different, though complementary ways - theoretically, descriptively,and/or, statistically. The theory has been discussed by Stüve, Defant, V. Bjerknes,J. Bjerknes, Solberg, and Bergeron, and more recently by Wulf and Obloy, J. Bjerknes andHolmboe, and Petterssen, as well as by others. The descriptive approach by means ofdetailed analyses of selected situations has been demonstrated in many investigations all overthe world, among which are several particularly interesting studies in North America and Europe. The statistical treatment was first attempted by Dines and Schedler and considerablyextended and refined by Haurwitz and others and by Penner.The statistical studies mentioned above have been based upon an analysis of mean conditionsand mean changes in vertical columns of air assumed to be in hydrostatic equiiibrium. Since theproblem is essentially dynamic, such analyses can never present the complete picture of thephysical processes involved in pressure changes. However, in view of the present unsatisfactorystate of knowledge concerning these processes, such statistical studies play an extremely valuablerole in improving the general understanding of the complex systems of mass variations which areintegrated into pressure changes by the atmosphere. They present mean conditions against whichdescriptive and theoretical studies may be evaluated with regard to representativeness and applicability,respectively. Furthermore, they are systematic summaries of actual conditions which may properly serve as a guide to the direction in which further theoretical and descrptive investigations may most profitably proceed.The paucity of upper air data has restricted the scope of previous studies along these lines andso it is the aim of this investigation to utilize the information recently available as a result of thewell organized network of radiosonde observation stations in North America to extend these studies. In particular, it is desired to investigate the geographical and seasonal differences in themean values of pressure and temperature variation at all levels and the related upper air conditions.It is hoped that the greater number of observations in the stratosphere may throw some light onits true importance with respect to tropospheric pressure variations and that mean conditionsthroughout the lower atmosphere may be so defined that it will be possible to set up certain requirementswhich any proposed mechanism for pressure changes must satisfy in order to be thoroughlyconsistent with reality.The observational material and the methods of analysis will be discussed in Chapter I.Chapters II, III, and IV will be devoted to a description of the results of the statistical analysis.Since the interpretations of one aspect of the study depend upon the results of some of the otheraspects, most of the interpretations and conclusions wil be discussed in Chapter V.