Ryanair offer travel insurance policies as an add on to a flight booking, or they can also be purchased as stand-alone products. Ryanair travel insurance is offered in two levels, so you can choose the option that's right for you and your wallet.
Jamaica's rainy season coincides with ours here in Florida (I am in the Tampa area) for the most part. We have anywhere from 30-50% chance of rain daily until hurricane season is over. I didn't mean you have rain every day, all day from May to December in Jamaica, but I assume it is a lot like here in Florida.
The chance of wet days in Nairobi varies significantly throughout the year. The wetter season lasts 6.9 months, from October 22 to May 18, with a greater than 26% chance of a given day being a wet day. The chance of a wet day peaks at 48% on April 21. The drier season lasts 5.1 months, from May 18 to October 22.