Generally, the plateau region enjoys a mild and equitable climate favourable to vegetation and in particular to sandal trees. The slopes and plains are subjected to hot and dry climate. The average minimum and maximum temperatures are; 21.54o C and 27.02o C in the plateau and the average minimum and maximum temperatures are 26.24 0 c and 32. 84oC in the plains. Thus, the climate of the Sathyamangalam Wildlife Sanctuary is moderate. There is a rainfall gradient from east to west of the sanctuary. The eastern part of the sanctuary falls on the leeward side of the Western Ghats; hence this part receives low rainfall.
The Plateau forests are subjected to strong and gusty winds blowing from south westerly direction, during the south west monsoon, period of June to September. As the wind is mostly dry, it only hastens the process of evaporation. In May thunder storms with the occasional cloud bursts are common in the plateau.
The average annual rainfall of the sanctuary over a ten year period is 824 mm. But considerable variation could be noticed from place to place. Though the sanctuary lies in the rain shadow region, the bulk of the rain fall is derived (70%) from the north east monsoon during September, October and November. The period from January to April is usually dry, though occasional showers may occur. From May onwards, intermittent rains occur till August. Rain increases slightly between August and September and becomes heavy during October-December and tapers off in January.
Lack of water and green fodder to wild animals are the major limiting factors with regard to severity of dryness in Sathyamangalam Wildlife Sanctuary. As mentioned earlier this sanctuary lies in the rain shadow region, and only limited water is available in all seasonal waterholes for animals. During the months of March and April water is available only in perennial streams such as Moyar and all the seasonal waterholes and streams dried up. This drought period also coincides with accumulation dry leaf litter and fire.
There are two perennial Rivers in the Sanctuary, namely a) River Bhavani and b) River Moyar.
It originates from western catchments of Nilgiris, running through Kerala and again enters Tamil Nadu. This river runs through Coimbatore Division and enters into Sathyamangalam Forests in Bhavanisagar Range. Till it reaches Bhavanisagar dam, it runs along the sanctuary area. This is the main source of drinking water to the wildlife, especially elephants.
It is another perennial river, which also originates from Nilgiris and runs through sanctuary area and finally flow into Bhavanisagar reservoir.