Silent Valley National Park - Climate & Rainfall
There is considerable variation in climate due to change in elevation from plains to the Ghats where hills are drier and cooler; the plains are humid and hot. The prevailing winds are from the west and south west during the period April to September and from east during the period October to March. From March a light western sea breeze is experienced in the later part of the day, which gradually develops into a south west monsoon around the beginning of June.
From November to March, there are strong dry east winds blowing during early hours of the day, till past noon, leading to drying of forest area resulting in rapid spread of forest fires. There are occasional thunder storms in April and May. Both the south west monsoon and the north eastern monsoon cause rains in this area. The major share, however, comes from the south west monsoon, which sets in during the first week of June. The heaviest rainfall is during the months of June, July, and August.
Silent Valley gets copious amounts of rainfall during the monsoons, but the actual amount varies within the region due to the varied topography. In general, rainfall is higher at higher altitudes and decreases from the west to the east due to the rain shadow effect. Eighty per cent of the rainfall occurs during the south-west monsoon between June and September. The park also receives a significant amount of rainfall during the north-east monsoon between October and November.
The park being completely enclosed within a ring of hills has its own micro-climate and probably receives some convectional rainfall, in addition to rain from the two monsoons. In the remaining months, condensation on vegetation of mist shrouding the valley is estimated to yield 15 per cent of the total water generated in the rainforest.
The variation in the intensity of rainfall is observed across the area. An increase in rainfall is noticed as one goes from foot hills to the high ranges. The elevated hills on the western side of Silent Valley receive an average of 5045 mm of rain as recorded at Arthala at 1200 m elevation followed by Neelikkal receiving 4364 mm of rain at 1005 m elevation. At higher elevations, near Walakkad the rainfall even reaches up to 6500 mm. Thiruvizhamkunnu, which lies in the foot hills of the Silent Valley at an elevation of about 75m receives only 2717 mm of rainfall.
The first high hills west of Silent Valley receive the heaviest rainfall which progressively decreases as clouds crosses over successive ranges. The southern slopes of the ridge that bounds the Silent Valley the west are the first high hills to receive the South West monsoon clouds and get about 4550mm of rainfall. So the eastern side of the ridge lies in the Silent Valley, which receives about 3200mm of rainfall.
The parallel valley, the Attappady Valley east of this, receives about 2300 to 2800mm of annual precipitation. The northern portion of Silent Valley receives the highest rainfall in the environs and is about 7500 mm annually. This is due to the fact that this area is situated just behind the first high range of Ghats. Average minimum temperature ranges from 8 o C to 14 o C and average maximum temperature varies from 23 o C to 29 o C.
The annual pluvial average is over 5000 mm (Singh et al; 1984) and is contributed both by South west and East West monsoons. Over 80% of the rain is received during the South west monsoon (June to September), 12 % is contributed by north east monsoon (October to December), 6% through pre-monsoon thunder showers (during May) and the rest (2%) is received during the dry season (KFRI, 1990).
In 2006, the Walakkad area of the park received the highest ever annual rainfall of 9,569.6 mm. In 2000, the figure was 7,788 mm; in 2001, 8,351.9 mm; in 2004, 8465.3 mm; and in 2005, 9,347.8 mm. The annual rainfall received in the valley (at Sairandhri) was 7,788.8 mm in 2000; 8,361.9 mm in 2001. In 2002, 4,262.5 mm; in 2003, 3,499.65 mm; in 2004, 6,521.27 mm, in 2005, 6,919.38 mm; in 2006, 6,845.05 mm; in 2007, 6,009.35 mm; and in 2008 it was 4386.5 mm. The figure till October 2009 was 5,477.4 mm. Average annual rainfall in the park between 2000 and 2008 was thus 6,066 mm.
The mean annual temperature is 20.2 °C. The hottest months are April and May when the mean temperature is 23 °C and the coolest months are January and February when the mean temperature is 18 °C. Because of the high rainfall, the relative humidity is consistently high (above 95%) between June and December.