Agriculture Reference
In-Depth Information
generally greater than 25°C, with no month having an average less than
18°C. Others have described the tropics as areas with a mean temperature of
not lower than 21°C and where the mean annual range of temperature equals
the daily range of temperature. The latter boundary is very much infl uenced
by continentality. Another boundary is the isotherm where the mean sea-level
temperature in the coldest months is not below 18°C; though it can include
certain errors, these are relatively small on a world scale and reliable data are
available for its computation (Fig. 2.1). In the tropics, diurnal temperature
variation is greater than the seasonal change.
2.
Rainfall is usually abundant, seldom less than the semi-arid 750 mm to as
high as 4300 mm, indicating considerable variation (Fig. 2.2). The heaviest
rainfall occurs near the equator. Seasonality in rainfall increases with distance
from the equator. Where rainfall is marginal for agriculture, its variability takes
on greater signifi cance.
3.
Photoperiod varies little throughout the year; at the equator day length is
about 12 h (Table 2.1).
4.
The position of the sun is more directly overhead, giving a year-round
growing season (Fig. 2.3).
5.
Rainfall, temperature, and solar radiation lead to higher potential
evapotranspiration.
These characteristics describe the true tropics, on and near the equator, with
latitudinal changes toward the poles producing a variety of subclimates.
Even near the equator, mountain ranges and other geographical factors
can produce various subclimates. Since temperature, solar radiation and
photoperiod are fairly constant in the tropics, the variety of subclimates and
vegetation are frequently dependent upon rainfall.
A continuous succession of climates starts with a long season of well-
distributed precipitation and a short dry season close to the wet tropics. As
you move away from the equator and the latitude increases, there is a gradual
change to a short season of relatively low rainfall with a long dry season.
Some seasonal variation in mean daily temperature becomes apparent, with
cool temperatures increasing with increasing distance from the equator.
Table 2.1.
Day length extremes in hours and minutes at various latitudes in the
tropics and subtropics.
Latitude º
0
10
20
30
40
Longest day
12:07
12:35
13:13
13:56
14:51
Shortest day
12:07
11:25
10:47
10:04
9:09
