Yoshida - 1981 - Fundamentals of Rice Crop Science

CLIMATIC ENVIRONMENT AND ITS INFLUENCE 67
Geographers define the tropics as the region between the Tropic of Cancer
(23.5°N) and the Tropic of Capricorn (23.5°S). It is commonly said that the tropics
is where winter never comes, implying that the temperature remains high throughout
the year. However, at high altitudes the weather is cool and the common
definition no longer holds. Thus, it would be more meaningful to define the tropics
in terms of temperature. One such definition proposed by Koppen in 1936 uses the
threshold monthly mean temperature for the coldest month of the year (Trewartha
1968). The monthly mean temperature of 18°C for the coldest month is used for a
climatic boundary of the tropics.
Another temperature-based definition, proposed in 1945 by the plant ecologist
Kira, uses a heat index — a summation of monthly mean temperature. The heat
index, which involves the minimum temperature for plant growth as a threshold
temperature, is defined as:
(2.1)
where t i is the monthly average temperature in centigrade, and the heat index (W)
is for 12 months. The subtropics and tropics are defined as regions where the heat
index value is greater than 180° and 240°C, respectively. When such classification
is applied to the subtropics and tropics, the heat index can be converted to the
annual mean temperature because monthly mean temperatures in these regions are
always higher than the threshold temperature. For instance, an annual mean
temperature of 20°C gives:
(2.2)
Similarly, an annual mean temperature of 25°C gives a heat index value of 240°C.
Thus, heat index values of 180° and 240°C correspond to annual mean temperatures
of 20° and 25°C, respectively. This modification defines the tropics as the
region where annual mean temperature is above 25°C.
Vegetation characteristics can also be considered as a convenient basis by which
to identify the tropics, subtropics, and temperate regions of Asia. In Asia, an
annual mean temperature of 20°C is considered as the northern limit for vegetation
that includes palm (palmae) and screw-pines (pandanus), and 25°C is the northern
limit for dipterocarps (Dipterocarpaceae).
Karachi, Pakistan (25°N), and New Delhi, India (28°N), are north of the Tropic
of Cancer but have annual mean temperatures of 25.8°C and 25.3°C, respectively.
Hence, they have a tropical climate by Kira's definition. By Koppen's definition,
however, New Delhi does not have a tropical climate because the monthly mean
temperature for the coldest months (December, January, and February) is below
18°C.
2.2. EFFECTS OF CLIMATE
The average rice yields in rice-growing countries range from less than 1 to more
than 6 t/ha. There are a number of biological, environmental, and socioeconomic