the mechanization of agricultural production and shifts in land tenure seen in
Europe and North America over the last two centuries are only now taking place.
For these regions, the revolution is only beginning. In the developed world, evolu-
tion of the agricultural structure continues apace as well. This most recent stage of
the revolution continues many of the trends of previous centuries, with profound
changes in the geographic structure of production, land ownership, and innovative
techniques designed to enhance production. The advent of agribusiness, especially
the growing influence of corporate farmingor so-called industrial agriculture,
along with changes in livestock breeding and husbandry may be taken as indica-
tors that the “revolution” has yet to run its course. Just as in the past, these changes
are not without controversy, and are not universally accepted as beneficial. Some
may even point to a “counter-revolution,” as reflected in the popularity of organic
farming, rising emphasis on sustainable farming practices, problems with losses of
farmland to urban sprawl, and concerns with the decline of the family farm, par-
ticularly in the United States. It is likely that the forces of modernization and glob-
alization will only intensify the revolutionary nature of contemporary agriculture.Air Masses
An air mass is a huge, identifiable body of air possessing a relative homogeneity of
temperatureand moisture characteristics within compared to the air surrounding it.
Most air masses have dimensions of hundreds of thousands of square kilometers and
are sometimes pushed thousands of kilometers from their source regions. Boundaries
between air masses are known asfrontsand the various types of fronts are described
elsewhere.
Although air is a continuous fluid, its properties are such that it frequently
organizes into air masses having differences substantial enough to cause signifi-
cant weather changes as they pass. When air of the lower troposphere passes over
Earth’ssurface, it exchanges energy and moisture characteristics with the surface.
Air slowly moving large horizontal distances starts to take on the characteristics of
the surface, be it tropical ocean or polar tundra. Some areas are well known as air
mass source regions. Source regions occur in all latitudes and are characterized
by non-mountainous terrain and frequent dominance of high pressure. High
pressure is associated with light winds and this allows air to take on the surface
temperature and moisture characteristics over the course of several days. As air
masses move out of their source regions, they are modified. It is not rare for Arctic
air to leave Siberia and travel over central North America to the Gulf Coast. While
its original temperature might be− 40 °C, it might moderate to temperatures8 Air Masses