American air mass properties

In this paper the term Air Mass is applied to an extensive portion of the earth's atmospherewhich approximates horizontal homogeneity. The formation of an air mass in this sense takesplace on the earth's surface wherever the atmosphere remains at rest over an extensive area ofuniform surface properties for a suffciently long time so that the properties of the atmosphere(vertical distribution of temperature and moisture) reach equilibrium with respect to the surfacebeneath. Such a region on the earth's surface is referred to as a source region of air masses.As examples of source regions we might cite the uniformly snow and ice covered northernportion of the continent of North America in winter, or the uniformly warm waters of the Gulfof Mexico and Caribbean Sea. Obviously the properties of an air mass in the source region willdepend entirely upon the nature of the source region.The concept of the air mass is of importance not only in the source regions. Sooner orlater a general movement of the air mass from the source region is certain to occur, as one of thelarge-scale air currents which we find continually moving across the synoptic charts. Becauseof the great extent of such currents and the conservatism of the air mass properties, it is usuallyeasy to trace the movement of the air mass from day to day, while at the same time any modificationof its properties by its new environment can be carefully noted.Since this modification is not likely to be uniform throughout the entire air mass, it mayto a certain degree destroy the horizontal homogeneity of the mass. However, the horizontaldifferences produced within an air mass in this manner are small and continuous in comparisonto the abrupt and discontinuous transition zones, or fronts, which mark the boundaries betweenair masses. Frontal discontinuities are intensified wherever there is found in the atmosphereconvergent movement of air masses of different properties.Since the air masses from particular sources are found to possess at any season certaincharacteristic properties which undergo rather definite modification depending upon thetrajectory of the air mass after leaving its source region, the investigation of the characteristicproperties of the principal air mass types can be of great assistance to the synoptic meteorologistand forecaster. We owe this method of attack on the problems of synoptic meteorology to theNorwegian school of meteorologists, notably to T. Bergeron. Investigation of the propertiesof the principal air masses appearing in western Europe has been made in particular byO. Moese and G. Schinze. The purpose of this paper is to give the results of a similarinvestigation of the properties of the principal air masses of North America, and to commenton some of the striking differences which appear between conditions here and in Europe.