1. The Thornthwaite Classification SystemAs with any other climate
system, Koeppen’s has its shortcomings. One of the most important is
that it is based on vegetation boundaries that have been associated with
monthly values of mean temperature and precipitation. This is
problematic because these two variables alone do not directly determine
the geo- graphic limits of natural vegetation. A superior system would
also include potential evapotranspiration as a factor that plays a
direct role in determining the geographic limits of vegetation.In
tandem, the opposing effects of evapotranspiration and precipitation
determine the water balance, examples of which are shown in Figure
15-1-1. Wherever evapotranspiration exceeds precipitation, the amount of
moisture in the soil is reduced, as its loss is not offset by soil
moisture inputs. When precipitation exceeds evapotranspiration, the soil
moisture is replenished until it reaches the maximum amount of water
that can be retained by the soil against the force of gravity (field
capacity).Thornthwaite’s classification systemwas developed to
distinguish climates based on moisture availability. It is based on the
principle of the water balance and evolved through decades of work that
culminated in its final form in 1955. Its first criterion is a moisture
index that compares the amount of average precipitation each month to
the potential evapotranspiration. The latter value derives from a
formula using mean temperatures and the monthly values of average period
of daylight (a function of latitude) for each station to establish a
monthly moisture index. These monthly values are then summed to produce
an annual moisture index, the value of which distinguishes arid,
semiarid, sub- humid, humid, and perhumid (very humid) climates. These
divisions are based on arbitrary percentage changes in the moisture
index (20 percent changes for the humid climates, 33 percent changes for
the dry climates), not from associations with plants or other
nonclimatic phenomena.The second criterion is the thermal efficiency of a
location, or the total amount of potential evapotranspiration (the
amount of evaporation that would occur under given climate conditions
and assuming an unlimited supply of soil water), The remaining two
criteria are based on the seasonality of precipitation and potential
evapotranspiration. When combined, the four criteria create a more
physically based climate classification scheme than that of Koeppen.So
why hasn’t the Thornthwaite system supplanted Koeppen’s as the most
popular? This is partly explained by its greater complexity. Compared to
Koeppen’s system, the water-balance computations needed by
Thornthwaite’s method are laborious, and the resulting regions are
therefore quite removed from the underlying climatic data. Thus, the
pattern of climates that emerges is harder to interpret in terms of
large-scale processes. Also, although the basic concept of potential
evapotranspiration is widely accepted, the method developed by
Thornthwaite does not follow from physical principles, but instead
relies on data collected mainly in the eastern United States. The data
were used to construct empirical (observation-based) equations for
potential evapotranspiration. Unfortunately, Thornthwaite did not
publish details regarding how the equations were developed, and there
are questions as to how well they work in other areas. Thus, if
Thornthwaite’s method were applied to the entire globe, it is not clear
how meaningful the resulting regions would be.Of course, the Koeppen and
Thornthwaite systems are not the only systems that have been devised,
Most of the others are designed for more specific applications than
either of these two Some, for example, have identified regions of human
comfort, whereas others consider the dominance of different types of air
masses. The familiar map of plant-hardiness zones found on seed
packages is another example. More sophistícated classifications have
used energy budget considerations.Regardless of the premises on which
they are based, each has its own set of advantages and
disadvantages.FIGURE 15-1-1 Water Balances. The diagrams illustrate the
monthly balances of precipitation, potential evapotranspiration, and
actual evapotranspiration for Irkutsk, Russia, and Memphis,
Tennessee....In what way is the Thornthwaite system superior to the
Koeppen system? Get solution
2. The Thornthwaite Classification SystemAs with any other climate system, Koeppen’s has its shortcomings. One of the most important is that it is based on vegetation boundaries that have been associated with monthly values of mean temperature and precipitation. This is problematic because these two variables alone do not directly determine the geo- graphic limits of natural vegetation. A superior system would also include potential evapotranspiration as a factor that plays a direct role in determining the geographic limits of vegetation.In tandem, the opposing effects of evapotranspiration and precipitation determine the water balance, examples of which are shown in Figure 15-1-1. Wherever evapotranspiration exceeds precipitation, the amount of moisture in the soil is reduced, as its loss is not offset by soil moisture inputs. When precipitation exceeds evapotranspiration, the soil moisture is replenished until it reaches the maximum amount of water that can be retained by the soil against the force of gravity (field capacity).Thornthwaite’s classification systemwas developed to distinguish climates based on moisture availability. It is based on the principle of the water balance and evolved through decades of work that culminated in its final form in 1955. Its first criterion is a moisture index that compares the amount of average precipitation each month to the potential evapotranspiration. The latter value derives from a formula using mean temperatures and the monthly values of average period of daylight (a function of latitude) for each station to establish a monthly moisture index. These monthly values are then summed to produce an annual moisture index, the value of which distinguishes arid, semiarid, sub- humid, humid, and perhumid (very humid) climates. These divisions are based on arbitrary percentage changes in the moisture index (20 percent changes for the humid climates, 33 percent changes for the dry climates), not from associations with plants or other nonclimatic phenomena.The second criterion is the thermal efficiency of a location, or the total amount of potential evapotranspiration (the amount of evaporation that would occur under given climate conditions and assuming an unlimited supply of soil water), The remaining two criteria are based on the seasonality of precipitation and potential evapotranspiration. When combined, the four criteria create a more physically based climate classification scheme than that of Koeppen.So why hasn’t the Thornthwaite system supplanted Koeppen’s as the most popular? This is partly explained by its greater complexity. Compared to Koeppen’s system, the water-balance computations needed by Thornthwaite’s method are laborious, and the resulting regions are therefore quite removed from the underlying climatic data. Thus, the pattern of climates that emerges is harder to interpret in terms of large-scale processes. Also, although the basic concept of potential evapotranspiration is widely accepted, the method developed by Thornthwaite does not follow from physical principles, but instead relies on data collected mainly in the eastern United States. The data were used to construct empirical (observation-based) equations for potential evapotranspiration. Unfortunately, Thornthwaite did not publish details regarding how the equations were developed, and there are questions as to how well they work in other areas. Thus, if Thornthwaite’s method were applied to the entire globe, it is not clear how meaningful the resulting regions would be.Of course, the Koeppen and Thornthwaite systems are not the only systems that have been devised, Most of the others are designed for more specific applications than either of these two Some, for example, have identified regions of human comfort, whereas others consider the dominance of different types of air masses. The familiar map of plant-hardiness zones found on seed packages is another example. More sophistícated classifications have used energy budget considerations.Regardless of the premises on which they are based, each has its own set of advantages and disadvantages.FIGURE 15-1-1 Water Balances. The diagrams illustrate the monthly balances of precipitation, potential evapotranspiration, and actual evapotranspiration for Irkutsk, Russia, and Memphis, Tennessee....Explain why the Thornthwaite system is not more widely used than the Koeppen system. Get solution
2. The Thornthwaite Classification SystemAs with any other climate system, Koeppen’s has its shortcomings. One of the most important is that it is based on vegetation boundaries that have been associated with monthly values of mean temperature and precipitation. This is problematic because these two variables alone do not directly determine the geo- graphic limits of natural vegetation. A superior system would also include potential evapotranspiration as a factor that plays a direct role in determining the geographic limits of vegetation.In tandem, the opposing effects of evapotranspiration and precipitation determine the water balance, examples of which are shown in Figure 15-1-1. Wherever evapotranspiration exceeds precipitation, the amount of moisture in the soil is reduced, as its loss is not offset by soil moisture inputs. When precipitation exceeds evapotranspiration, the soil moisture is replenished until it reaches the maximum amount of water that can be retained by the soil against the force of gravity (field capacity).Thornthwaite’s classification systemwas developed to distinguish climates based on moisture availability. It is based on the principle of the water balance and evolved through decades of work that culminated in its final form in 1955. Its first criterion is a moisture index that compares the amount of average precipitation each month to the potential evapotranspiration. The latter value derives from a formula using mean temperatures and the monthly values of average period of daylight (a function of latitude) for each station to establish a monthly moisture index. These monthly values are then summed to produce an annual moisture index, the value of which distinguishes arid, semiarid, sub- humid, humid, and perhumid (very humid) climates. These divisions are based on arbitrary percentage changes in the moisture index (20 percent changes for the humid climates, 33 percent changes for the dry climates), not from associations with plants or other nonclimatic phenomena.The second criterion is the thermal efficiency of a location, or the total amount of potential evapotranspiration (the amount of evaporation that would occur under given climate conditions and assuming an unlimited supply of soil water), The remaining two criteria are based on the seasonality of precipitation and potential evapotranspiration. When combined, the four criteria create a more physically based climate classification scheme than that of Koeppen.So why hasn’t the Thornthwaite system supplanted Koeppen’s as the most popular? This is partly explained by its greater complexity. Compared to Koeppen’s system, the water-balance computations needed by Thornthwaite’s method are laborious, and the resulting regions are therefore quite removed from the underlying climatic data. Thus, the pattern of climates that emerges is harder to interpret in terms of large-scale processes. Also, although the basic concept of potential evapotranspiration is widely accepted, the method developed by Thornthwaite does not follow from physical principles, but instead relies on data collected mainly in the eastern United States. The data were used to construct empirical (observation-based) equations for potential evapotranspiration. Unfortunately, Thornthwaite did not publish details regarding how the equations were developed, and there are questions as to how well they work in other areas. Thus, if Thornthwaite’s method were applied to the entire globe, it is not clear how meaningful the resulting regions would be.Of course, the Koeppen and Thornthwaite systems are not the only systems that have been devised, Most of the others are designed for more specific applications than either of these two Some, for example, have identified regions of human comfort, whereas others consider the dominance of different types of air masses. The familiar map of plant-hardiness zones found on seed packages is another example. More sophistícated classifications have used energy budget considerations.Regardless of the premises on which they are based, each has its own set of advantages and disadvantages.FIGURE 15-1-1 Water Balances. The diagrams illustrate the monthly balances of precipitation, potential evapotranspiration, and actual evapotranspiration for Irkutsk, Russia, and Memphis, Tennessee....Explain why the Thornthwaite system is not more widely used than the Koeppen system. Get solution
