the common characteristic of the year-round presence of subtropical highs.
Adjoining regions are not as dry because the subtropical highs are not entrenched
throughout the year.
Over the oceans, the subtropical highs are particularly pronounced and are
sometimes referred to as semi-permanent. They exert major control in the average
wind directions and ocean currents. Over the subtropical ocean basins, winds and
currents are clockwise in the Northern Hemisphere and counterclockwise in the
Southern Hemisphere.
Ironically, the presence of light, diverging anticyclonic flow in the upper tropo-
sphere can have a profound effect on the formation ofhurricanes. Hurricanes are
cyclonic storms built from the warm sea upward. To develop, the vertical change
of wind speed and direction with height (wind shear) must be minimal. Jet streams
and other fast winds aloft literally break the top off of a developing hurricane. The
winds of upper anticyclones provide minimal wind shear and the divergence pro-
vides outward evacuation of the air rising from the surface and helps to deepen
the surface low and increase hurricane wind speeds.Areal Differentiation
Areal differentiation represents one of the classic philosophical approaches to geo-
graphic inquiry. Some of the earliest geographical scholars, including Strabo and
Ibn Khaldun, sought to describe and catalog variations in the places and cultures
they encountered, or were informed about by others. A central concept of areal dif-
ferentiation is that the surface of the earth may be divided intoregions, which may
be distinguished and categorized using various spatial criteria. Thus, areal differ-
entiation provides the theoretical foundation for regional geography, by conceptu-
alizing space as consisting of identifiable units that may be distinguished from one
another on the basis of a set of phenomena or criteria. For much of the historical
development of geography as a science, this was the approach followed. Geogra-
phers partitioned the world as they encountered it into sections based on differ-
ences they codified, formulating these distinctions in a descriptive narrative
designed to provide a sense of place. During thequantitative revolution,this
view was derided by its fiercest critics as simplistic, static, and sterile, but in recent
decades an emphasis on areal differentiation has reappeared in some subdisci-
plines of geography. For example, arealdifferentiation has offered a basis for
new directions in human geography, especially postmodern analyses that focus
on the social and cultural processes that construct a sense of place for a given loca-
tion; and studies in the geography of economic development, which seek to16 Areal Differentiation