The Grand Line is pretty damn strange... each island has its own magnetic field - that's why you can't use a compass - and that gives them a unique climatic zone. There's winter, fall, spring and summer islands, and they all have four different seasons. So, say that you're on a winter island: then you have winter-winter, winter-spring, winter-summer and so on!
[Someone paid attention to Nami. A lot of attention.]