The climate of the tropical areas
The concept that tropical areas are palm fringed beaches, protected by a coral fringed lagoon is only partly true. The backdrop of dark green forest covered mountains and mist clouds rising from the base of high waterfalls, is often replaced by dry savannahs and deserts beyond the escarpments. There is great diversity of ecosystems that leads to an incredible number of species that inhabit the region between the Tropics of Capricorn and Cancer in the southern hemisphere.
The cultivation of ‘tropical plants’ is often misunderstood resulting in many species regarded as being temperamental. Light is as important as temperature and humidity, and some climatic knowledge is useful to successfully cultivate tropical plants.
The climate of the tropical areas is the result of a number of meteorological events and the passage of the earth around the sun. While geological features influence microclimates, it is the bigger influences of the Tropical Monsoons that dictate the seasons and rain.
Light composition, daylight length, and to some extent temperature, are a result of the orbit of the earth around the Sun. South of the Tropic of Capricorn and North of the Tropic of Cancer the sun is never directly overhead and this combined with the angle of the day/night line on the earth’s surface dictates daylight length and light composition.
The East coast of Australia is chevron shaped with the most easterly point at Point Danger. This shape is a good guide to understanding daylight length. When the sun is directly over the Tropic of Capricorn it is as far south and closest to the Southern Hemisphere as it gets, this time in the Southern Hemisphere is the Summer Solstice. In the Northern Hemisphere it is the Winter Solstice as it is furthest away. At this time of year the night to day line or sunrise follows the southern angle of the Australian coastline, while the opposite day to night line or sunset follows the angle of the northern coastline. When the sun is in the Northern Hemisphere, over the Tropic of Cancer, the opposite occurs in the Southern Hemisphere with the night to day line following the northern coastline and the day to night line following the southern coastline. The distance between these two lines determines daylight length and is always less between the tropics than outside them in summer and greater in winter. This is why the tropical areas have similar day and night hours all year round, whereas those areas outside the tropics have variable hours between day and night, with summer having a longer period of daylight than winter.
As the sun moves between the tropics it passes overhead twice a year, and the angle the sun’s rays strike the earth’s surface is never particularly acute. The distance the rays travel through the atmosphere is less than outside the tropics. Because the filtering effect of that part of the atmosphere called the Troposphere is less in tropical areas than those outside the tropics, the temperature between the tropics as well as light strength and composition varies little throughout the year. By comparison outside the tropics the angle of the sun’s rays is variable and more acute travelling further through the atmosphere. The atmosphere reflects the infrared rays more than the ultraviolet rays so light composition and heat changes; this change is reflected in the four seasons.
The topography and the monsoon determine the type of climate in the tropics, as the temperate seasons don’t apply. A monsoon once a year, as in Northern Australia, results in definite wet and dry seasons, however an area such as Borneo is under the influence of two monsoon events and may have a short period of little rain, but generally rains occur year round. The plants in a single monsoon climate require a rest period where as those from areas of year round monsoonal rain, do not, and so the life cycles are quite different. |
