A concomitant feature of the amount of precipitation is how often it comes.
Some places average less than one day per year of measurable precipitation while
others garner more than 340 days. The world tendency is for the places with
the least precipitation to have the most unreliable precipitation. London, United
Kingdom has an expected precipitation of somewhat over 500 mm (20”) per year
and Londoners are shocked if the precipitation varies more than 10 percent from
year to year. The arid parts of the planet have more than 50 percent interannual
variability. At Luxor, Egypt, the yearly precipitation averages only 2.5 mm (.1”)
so any precipitation is unexpected.
Precipitation rates are also measured viaremote sensingtechniques. Remote
sensing has great advantage in this realm because precipitation doesn’t need to fall
in a gage to be estimated and there is foreknowledge as to where the event headed.
Radar uses microwaves to measure precipitation and modern Doppler radars can
sense motions of precipitation within storms and pinpoint dangerous situations
such as mesocyclones. Since the middle of the 20th century, precipitation has been
artificially produced. This is usually accomplished by “cloud seeding,” consisting
of spreading silver iodide from aircraft. The silver iodide crystals act as nuclei
around which water can gather, thus enhancing nature’s methods. The precipitation
processes described above need a significant supply of water vapor and a signifi-
cant rise of air—factors that humans cannot control. Droughts occur when these
factors are not present so that the production of precipitation by artificial means
is relegated to possible enhancement of precipitation that might have occurred
on its own.Pre-industrial City Model
A concept in urban geography, sometimes also referred to as the Sjoberg model,
and now disputed by many urban geographers, that all cities prior to the industrial
revolution exhibited a common internal morphology and functionality. Prior to
Sjoberg’s work, several models of urban structure and development had been pro-
posed in the first decades of the 20th century, including Ernest Burgess’s Concen-
tric Zone Model, and theSector Model, proposed by Homer Hoyt. Several
decades after the appearance of these models, the sociologist Gideon Sjoberg
developed the pre-industrial city model, and laid out his argument in the book
The Preindustrial City: Past and Present, published in 1965. The model empha-
sizes the role of social forces in determining the spatial characteristics of the urban
place. Sjoberg’s basic thesis was that virtually all cities that had emerged in the
pre-industrial era, a stretch of somewhat over two millennia, shared common266 Pre-industrial City Model