It's fair to say that the weather can very, depending where you are on the island, largely due to the topography and the proximity to the Albanian mountains.
If the wind is from the north, the weather will usualy be good, with clear skies etc. However, if from the south, the moist warm air(usually all the way from the Sahara, but having travelled over a long reach of sea) rises as it meets the mountains of Corfu and Albania, cools and creates clouds and rain. Usually a prolonged blow from the south will result in a fairly spectacular thunder storm, which clears the air.
If you notice the wind blowing from the south over a period of time, you will notice that the visibility out to sea gradually deteriorates over a few days,with clouds building up over the mountains - a sure sign rain is on its way.
I think it's also fair to say that Corfu's climate has changed in the last 20 years or so - from being "Mediterranean" (hot and dry) to "sub-tropical" (hot and humid, with more rain)
The good thing about Corfu though (if you have a car) is that if it's raining where you are, the sun will be shinning on the oter side of the island (mountain).