friction over distance. Consumers will not travel more than a short distance to
secure common staples like bread and milk, as the cost and character of these items
creates a great deal of friction beyond a relatively short distance, typically only a
few miles. On the other hand, goods that are more expensive and rarely purchased,
such as automobiles, have a much lower level of friction in proportion to distance.
A customer may travel a considerable ways to procure such high-value items,
because the additional investments in travel cost and time are viewed as acceptable
due to the qualities of the good being sought. For trade, migration, or tourism to
occur between two points, transport systems must overcome the friction of distance.
By increasing the efficiency of movement between locations, the expense and time
of transport are lowered, reducing the friction as well.
Fronts
A front is the boundary between air masses and a linear zone of low pressure that
can extend for thousands of kilometers. Fronts usually represent sharp horizontal
divisions between weather types as they move at varying speeds over theland-
scape. During frontal passage, a weather observation station will experience
changes in airtemperature, air pressure,humidity, cloudiness, wind speed, wind
direction, and might receiveprecipitation. How different mustair massesbe for a
front to be defined? The answer is “not much.” Sometimes, there are only small
differences in temperature and moisture on either side of a front. How cold must
it be to define an air mass change as a cold front? The answer is “not very.”
Summer temperatures might be in excess of 38°C before frontal passage and a
toasty 35°C after the front—there is no absolute rule for the temperatures involved
on either side of the fronts explained below.
The concept of fronts is attributable to Norwegian meteorologists who devel-
oped the concept of air masses andmiddle latitude cyclonetheory. They recog-
nized that air mass boundaries were the locations of the most vigorous weather
activity and made the analogy between these boundaries and the military fronts
of recently completed World War I. Essentially, air masses were likened to oppos-
ing armies engaged in trench warfare. At various times the military fronts were
stationary and, at other times, one army gained the upper hand and pushed the
enemy out of the way. Air masses, energized by the push of the strong flows of
air in the middle and upper troposphere, are able to mechanically force other air
masses out of the way. In the case of the atmosphere of the middle latitudes, the
air masses “fight” back and forth over time because the polar front jet stream is
arranged in a series of immense moving waves. The jet stream separates warm,
Fronts 135