When is the best time to visit India - especially its waterfalls? Well based on our experiences, we can say that the climate is somewhat similar to Thailand's as most of the country's precipitation falls during the monsoon (their wet season) and the rains are either less frequent or non-existent at other times of the year. However, all year round, much of the country is very hot and humid with the coolest months being in December and maybe November and January depending on where in the country you're at.
The thunderstorms seem to be characterized by downpours accompanied with lightning and thunder at night. So when the day breaks, you should be able to see the falls throughout the country at their most vigorous state. The drawback is that you're likely to see more flooding, landslides, potholes, and washed out roads and bridges at this time of year.
The dry season is pretty much the rest of the year.
The end of the monsoon (November or even in October) and the onset of the coolest months, which are typically in December and January, are part of this dry season. Although you might still think it's hot in many parts of the country even at this time of year, it gets scorching hot (like consistently in the 40 degree Celsius range with humidity in the Spring months of say March through May.
So given all this information, we tried to time our trip to take advantage of the monsoon while being far enough away from that season to maximize the likelihood of better weather without getting stranded or risking danger from landslides, flooding, etc. you tend to get with lots of rain. At the same time, we wanted to make sure the country experienced its monsoon so the waterfalls would have the highest likelihood of flowing well.
That was the reason why we did our traveling in November.
For we think that is the month that is an optimal trade between the inconveniences of the monsoon while decreasing the chances of costly delays or change of plans and experiencing a fairly comfortable climate.
I'd say if you want to see all the waterfalls at their highest flow (not considering logistical uncertainties due to the weather), then by all means go during the monsoon season (say June through September) or one delayed by a month (say July through October) since you could argue that June might be too soon if the monsoon hasn't kicked in yet or the cumulative precipitation levels have yet to saturate the drainages with water.
Perhaps the worst time for waterfalling would be towards the end of the dry season (say March through May). At this time, the monsoons have yet to begin in earnest, and you experience some of the most worst heat and humidity in the process (imagine weeks at a time of over 40 degrees C along with stifling humidity almost throughout the country).
However, with Global Warming, the monsoon patterns have been changing (by delaying the monsoon more and making the rains less reliable in pockets of the country [i.e. it rains buckets in some regions and hardly rains at all in others]). Thus, the observations and advice given on this pace could eventually go obsolete (for better or worse, especially if your water supply and agriculture is dependent on these patterns).
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