that assumed a static relationship between the observed and the observer, and
which devalued the importance of spatial perception and “sense of place.” Human-
istic geography emphasizes the importance of human agency, especially in the
construction oflandscapes, which, it is argued, are socially constructed from a
variety of possibilities and do not necessarily represent the same ideas, concepts,
or even historical experiences of all observers. Moreover, borrowing heavily from
the constructivist approach to literary criticism then emerging in the humanities,
especially in art and literature, humanistic geographers evolved a perspective on
cultural landscapes that viewed them as possessing a textual quality. The concepts
of landscape and place could be interpreted as dynamic spatial entities that might
be “read” in various ways, depending on a wide spectrum of factors influencing the
reader: gender, social position, race, and others. More recently, possibilism has at
least partially provided the theoretical basis of the generalized discipline of cul-
tural studies, social and feminist geographies, and post-modernist geographical
perspectives.
Precipitation
As part of thehydrologic cycle, water vapor is transformed intoclouds.Clouds
are common and, although in constant motion, usually cover about half of the
planet at a time. Even so, precipitation is occurring over a small portion ofEarth
because most clouds do not produce precipitation. How and why precipitation
occurs is vital in understanding the physical geography of Earth, because the pre-
cipitated water affects the surface lithosphere, biosphere, and hydrosphere.
Precipitation is triggered by one of two processes. The first is known as the
warm cloud process and involves only water droplets (including supercooled
water) above tropical locations. Cloud droplets collide and some of them coalesce
to become large enough to fall out of the sky as rain. The cold cloud process (also
known as the Bergeron process) takes place in middle latitude and polar clouds.
The temperatures are below freezing andsupercooled droplets and ice crystals
coexist at temperatures between 0°and− 40 °C. The saturation vapor pressure is a
bit higher over liquid water than over ice crystals and the net effect is that ice crys-
tals grow by adding mass from the nearby droplets. The enlarged ice crystals
become large enough to fall as snow.
The general term for water precipitated out of the sky is “hydrometer.” There are
several types of hydrometers and their occurrences are governed by moisture sup-
ply, rise of air, and the environmental lapse rate underneath the clouds. Snow starts
as snowflakes in clouds and falls through below-freezing air to the surface.
Precipitation 263