Not right for the season — used above all about weather. If the sun is burning in January, or you need a jacket in July, that weather is unseasonable. The word does not say 'bad' and it does not say 'good'; it says 'wrong for this time of year'. An unseasonable warm week in winter can be lovely, and people will still call it unseasonable. It is a slightly formal, written word — you meet it in weather reports, newspapers, and travel writing more than in talk between friends, who would just say 'it's really warm for December'. It has a second, older use about behaviour or timing: an unseasonable request, an unseasonable hour, meaning it comes at a bad or unsuitable moment. That use is old-fashioned now and mostly appears in books; if you hear the word today, weather is almost always what is meant. Note also that it is not the same as 'out of season', which is about food or sport or hotels, not about the sky.

不合時節的（天氣）；反常的。注意：中文常說「氣候異常」「反常的天氣」，但 unseasonable 不帶「災難」的意思，只是說「跟這個季節該有的樣子不一樣」，可能是異常暖，也可能是異常冷。