As per the tutorial, which I guess many don't read.
The island's weather has five types: 「Sunny」, 「Cloudy」, 「Rainy」, 「Heatwave」 and 「Typhoon」. Weather mainly affects factors such as status changes during island exploration and how easily drift materials can be found.
On Cloudy days, water consumption during exploration is lower than on Sunny days, making them ideal for exploring the island. However, rain may begin to fall while you're out.
On Rainy days, water consumption during exploration drops to zero, but stamina drains faster. Prolonged activity in the rain may cause colds, so be careful.
Additionally, rain automatically fills up water containers, slightly restoring your water reserves after sleeping.
On Heatwave days, water and stamina consumption both increase during exploration. Food spoilage also accelerates─expiration dates shorten more than usual when the day changes.
On Typhoon days, you cannot go outside at all. Outdoor facilities will suffer heavy durability loss.
While the disadvantages are severe, for the two days after the typhoon, rare drift materials will wash ashore and can be collected more easily.