Sri Lanka lies in the tropical zone of the world. Its position between 5 and 10 north latitude endows the country with a uniform but high year-round temperature, moderated by ocean winds and considerable moisture. The mean annual temperature ranges from a low of 16°C in Nuwara Eliya in the Central Highlands to a high of 32°C in Trincomalee on the northeast coast (where temperatures may reach 38 °C). The average yearly temperature for the whole island ranges from 28 to 30 °C (Figure 3). Day and night temperatures may vary by 4 to 7, however temperature does not determine the seasons. January is the coolest month, (where frost could occur for few days to weeks) May, the hottest period, precedes the summer monsoon rains.

Seasons are determined by the rainfall pattern. The rainfall pattern is mainly influenced by the monsoon winds and is marked by 2 major seasons. The first is from mid-May to September. When these winds encounter the slopes of the Central Highlands, they unload heavy rains on the mountain slopes and the southwestern sector of the island providing southwest (SW) monsoon rainfall. Some of the windward slopes receive up to 2500 mm of rain per month, but the leeward slopes in the east and northeast (NE) receive little rain (Figure 4). During the second season, November to February, monsoon winds come from the NE, bringing moisture from the Bay of Bengal. The northeastern slopes of the mountains may be inundated with up to 1250 mm of rain during these months. However, since the north east wind is passing only a small piece of ocean, it does not carry a whole lot of moisture as the SW monsoon, therefore the rainfall from the NE monsoon is much smaller than the SW monsoon winds.

In between the two monsoons we receive intermonsoonal rains mainly as convectional rains. The first season occurs in October. During this season, sometimes tropical cyclones bring overcast skies and rains to the SW, NE, and eastern parts of the island. Another intermonsoonal period occurs in March until mid-May, with light, variable winds and evening thundershowers.

On the basis of the annual rainfall and its distribution, the island is demarcated into 3 major divisions and one sub division; wet, dry, intermediate and arid zones (Figure 5). Wet zone (2500-5000 mm/yr) receives SW monsoon rains as well as convectional rains, while dry zone get NE monsoon rain (1250-1900 mm/yr). Intermediate zone (1900-2500 mm/yr) receives NE monsoon as well. Two arid zones, NW and SE coastal fringe receives very little rainfall (>900 mm/yr) only in few months of the year and latter part of the year remains dry. Several agroecological zones have been set on the basis of rainfall, temperature and soil type (Figure 6).

Humidity is typically higher in the SW and mountainous areas and depends greatly on the seasonal patterns of rainfall. For instance, daytime humidity at Colombo stays above 70 % all year, rising to almost 90 percent during the monsoon season in June. Anuradhapura experiences a daytime low of 60 % during the intermonsoonal month of March, however, a high of 79 % during the November and December rains. In the central highlands, Kandy's daytime humidity usually ranges between 70 and 79 %.

Topography

Physiogrphy