Mykonos is a very pleasant place to visit April-October, with temperatures in the 70s F/21-26 C and 80s F/27-31 C, though in August it can edge up into the 90s F/32-27 C and even higher than 100 F/38 C in a hot summer. Little rain falls June-September, although you can still get the odd day here and there. It rains a few days a month in spring and fall, but most rain falls in December and January, when daytime temperatures drop into the 50s F/10-15 C.
Meltemi winds, known by the old Greeks as the Etesian northern winds, can bring about harsh sailing conditions, but also provide cooling, low humidity and good visibility. They are particularly fierce on Mykonos and can easily last three to six days. When they become too strong, ferries may no longer operate.
The island's south-coast beaches are reasonably well-sheltered from the summer meltemi winds, whereas the north coast is totally exposed.