The weather in Formentera is typically mediterranean, with a mild climate, and good average temperatures during all the year. Summers are quite hot, but as the island is mostly flat, there is always a little wind that cooles a bit the atmosphere (except when it comes from Africa!!). So if you visit the island in summer prepare to sweat. Spring and autumn are lovely seasons to travel to the island, as there are many hours of sunshine every day and it's possible to swim from April till November. Winters are also soft, but days are shorter and winter can make the atmosphere cooler.
To ilustrate the weather take a look at these averages:
Maximum temperatures (ºC)
Winter: 15ºC Spring: 21ºC Summer:29ºC Autumn:18ºC
Minimum temperatures (ºC)
Winter: 8ºC Spring: 13ºC Summer:20ºC Autumn:12ºC
Sea temperature (ºC)
Winter: 13ºC Spring:16ºC Summer:23ºC Autumn:18ºC
Sunshine hours day
Winter: 5 Spring: 8 Summer: 10 Autumn: 7
Which essentials should you take in your luggage?
Mostly cotton garments, and please, some scarfs and jackets (take into account that nights can be a bit fresh, and if you rent a motorbike you can feel freshness). Obviously bathing suits, sun protection (here the sun is really strong!) and anti-mosquito lotion!!