Yes… and no.
Of course the sunny season! … Ehmmm! not all the time. Depending on which volcano you want to see the sunny season is not always sunny and the rainy season is not always rainy.
Let’s take for instance September… Best time to see Arenal, Turrialba and Irazú. Rainy in the Central Valley and the Pacific Coast. December, on a clear day, great to see Poas and Irazu. Forget about Arenal… You would have to wait until March to see it again…
And let me explain you why…
In the sunny season we have the Caribbean trade winds coming in the country. They come in filled up with humidity and pour over the Caribbean plains… the cloud gets to the mountains (Volcanoes) gets stucked there… and then, the rest of the country, remains sunny… with no clouds (They are stuck at the tops of the volcanoes… fog up on the tops).
In the rainy season a different phenomena happens, and the humidity turns into rain quickly… and then, it turns out that the tops of the mountains look clear and beautiful.
So, let’s see… Out of the volcanoes, the best time to visit is:
Poas. March through April, October, November. (December is a question mark… but usually at the end of the year it looks great!)
Irazu: January to April, September and October. (Just like Poas… great at the end of the year!)
Turrialba: January to April, September and October.
Arenal: March through July, September and October.
Rincón: December through May.
MIravalles: December through May.
Remember though, this is just a guideline… We are in the tropics, the most classical conundrum for any weather forecast!