The best time to visit Munnar is October to February, after the monsoon has ended and the hills are lush and clear. Temperatures are cool — occasionally cold at night in December and January — and the visibility is good for the mountain views. This is also when Eravikulam National Park is most accessible.
The monsoon (June to September) brings very heavy rainfall and thick mist that can persist for days. Some visitors find the misty, rain-soaked hills dramatically beautiful; others find it frustrating. The park closes for calving from February to March. March to May is warm and dry — pleasant enough but the hills are less dramatically green than after the monsoon.
Munnar gets genuinely cold at night between December and February. Bring a jacket even if you are arriving from the Kerala coast in summer.