|dc.description.abstract||In this paper the term Air Mass is applied to an extensive portion of the earth's atmosphere
which approximates horizontal homogeneity. The formation of an air mass in this sense takes
place on the earth's surface wherever the atmosphere remains at rest over an extensive area of
uniform 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 surface
beneath. 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 northern
portion of the continent of North America in winter, or the uniformly warm waters of the Gulf
of Mexico and Caribbean Sea. Obviously the properties of an air mass in the source region will
depend entirely upon the nature of the source region.
The concept of the air mass is of importance not only in the source regions. Sooner or
later a general movement of the air mass from the source region is certain to occur, as one of the
large-scale air currents which we find continually moving across the synoptic charts. Because
of the great extent of such currents and the conservatism of the air mass properties, it is usually
easy to trace the movement of the air mass from day to day, while at the same time any modification
of 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 may
to a certain degree destroy the horizontal homogeneity of the mass. However, the horizontal
differences produced within an air mass in this manner are small and continuous in comparison
to the abrupt and discontinuous transition zones, or fronts, which mark the boundaries between
air masses. Frontal discontinuities are intensified wherever there is found in the atmosphere
convergent movement of air masses of different properties.
Since the air masses from particular sources are found to possess at any season certain
characteristic properties which undergo rather definite modification depending upon the
trajectory of the air mass after leaving its source region, the investigation of the characteristic
properties of the principal air mass types can be of great assistance to the synoptic meteorologist
and forecaster. We owe this method of attack on the problems of synoptic meteorology to the
Norwegian school of meteorologists, notably to T. Bergeron. Investigation of the properties
of the principal air masses appearing in western Europe has been made in particular by
O. Moese and G. Schinze. The purpose of this paper is to give the results of a similar
investigation of the properties of the principal air masses of North America, and to comment
on some of the striking differences which appear between conditions here and in Europe.||en||