
It's tropical and hot. During the main monsoon (Jun/July) there is heavy rain. It can be wet and misty in the mountains during the second monsoon (Oct/early Nov) but at other times of the year the rain mainly falls during the night.
Kerala's summer is April to May and the temperature rises to 33°C, 91°F; the average winter temperature is 25°C, 77°F , Kerala is lush and green throughout the year. The monsoons provides an annual rainfall in excess of 100in in several districts. The southwestern monsoon arrives in late May with the wet period continuing until September. The northeastern monsoon brings a period of (much lighter) rain during October, November and December.
The weather does vary very slightly throughout the region (because of height and proximity to either the highlands or the coast. we mention this purely for honesty but proovide a fairly typical chart below.

Kerala's proximity to the equator means that the weather is pleasantly tropical. Temperatures are fairly stable throughout the year, with average maximum highs between 29º and 33º. The Kerala coast tends to see more sunshine and is less humid than inland areas, and because of the cooling effect of the western Ghats and the Arabian sea breeze Kerala avoids the really scorching dry weather of much of India. The Arabian Sea breezes make Kerala an ideal winter sun destination for watersports like windsurfing, though you may be the first there to do it.
Undoubtedly, the best weather in Kerala is from November to late February, which are the months of lowest rainfall. Yes the temperature may be marginally lower, but you'll still feel very warm so don’t rule out British late winter and the British spring. Kerala temperatures begin to rise in late February and there is a small increase in humidity. March to the end of May will still see plenty of pleasant weather and the warmest Kerala temperatures, but occasional thunderstorms. Whilst all Kerala holidays may experience tropical rain, there is a huge difference between a tropical shower and the heavy monsoons. These are very dramatic but Kerala still sees plenty of sunshine, however most travellers try to avoid this main Kerala rainy season, especially June and July. Another less intense rainy period can arrive around October.
and Kerala does have some highlands.
The temperatures are significantly cooler here and in the hill stations and the higher Western Ghats. We suggest you take light clothes, an umbrella or cagoule and a sweater or something for the cooler winter nights in the highlands. Serious trekkers will already be well equipped and adhere to the maxim "there's no such thing as bad weather, only inadequate clothing" - the average tourist won't go that high (without a coach).