LISS.398A TECHNOLOGY, ENVIRONMENT and HUMAN ADAPTATION:
PART I EARLY PEOPLES in
the NEW WORLD
PRECIPITATION TYPES
Most precipitation results from adiabatic (constant heat)
cooling of ascending air-water solutions. It is commonly
categorized as convective, orographic or cyclonic according
to the mechanism causing the assent. While not mutually
exclusive, each of these mechanisms (or some combination of
them) is characteristic of specific global / regional
circulation processes -- specifically, they are types of
turbulent disturbances
associated with these
processes. Different types of precipitation interact with
the
hydrologic system
in different ways. Thus,
circulation processes, the types of precipitation associated
with them, together with hydrologic processes ultimately
determines the amount of water locally available to support
human life.
CONVECTIVE PRECIPITATION
Convective precipitation is produced by adiabatic (constant
heat) cooling of vertically rising columns of air.
These rising columns occur mainly during the day in the
summer and are probably caused by increased solar heating of
the surface. They consist of numerous distinct rising
columns so that rainfall from convective precipitation is
usually local, brief and often intense -- in the form of
showers.
HYDROLOGIC INTERACTION
Convective precipitation is relatively less effective for
for increasing soil moisture since much of it runs off in
the form of surface drainage -- often carrying soil with it.
However, in mid-latitudes it occurs during warm periods of
active plant life and provides high ratios of moisture to
cloud cover.
OROGRAPHIC PRECIPITATION
Orographic precipitation is produced by the effect of
topographic uplift on moving masses of air. A typical
pattern of orographic precipitation is a mountain range in
the path of a moving mass of high moisture content air. As
the air rises to pass over the mountains precipitation
occurs on the windward slopes. The leeward slopes receive
less precipitation since because: 1) the air has less water
dissolved in it; 2) adiabatic heating of the downward moving
air increases its temperature and thus the amount of water
it can hold
(Fig. 1.1.7).
HYDROLOGIC INTERACTION
In mid-latitudes orographic precipitation frequently appears
as winter snow at higher altitudes. Thus, its entry into
the hydrologic cycle may be seasonally delayed until the
snow melts. Snow melt transported as surface flow is an
important factor in human occupation of arid regions like
the Greater Southwest (North America). Not only is water
transported into otherwise moisture deficient regions, but
it appears in these regions at the season of most active
plant growth.
CYCLONIC PRECIPITATION
Cyclonic precipitation is produced traveling cyclones and
anti-cyclones characteristic of mid-latitudes. These storms
produce moderate, continuous precipitation over extensive
areas as they move from east to west.
Typically, these storms consist of two precipitation belts
-- one associated with the forward sector of the storm
producing steady, slow precipitation for 6 to 12 hours and a
second associated with rear sector producing thunderstorm
activity.
HYDROLOGIC INTERACTION
Both the low intensity and long duration typical of most
cyclonic precipitation result in more of it entering into
the soil and ground water systems and less being transported
by surface flow. Also its capture in seasonal snow pack
reservoirs (see above under OROGRAPHIC) is frequently
significant.
Colorado School of Mines
Division of Liberal Arts and International Studies
Dr. Joseph D. Sneed
jsneed@mines.edu
