The mild seas which almost surround the tied island produce a temperate climate (Köppen climate classification "Cfb") with a small variation in daily and annual temperatures. The average annual mean temperature from 1981 to 2010 was . The warmest month is August, which has an average temperature range of , and the coolest is February, which has a range of . Mean winter temperatures are amongst the highest in the British Isles, and by far warmer than the United Kingdom average. However, due to the islands proximity to the sea, summers are cooler than the national average, with temperatures rarely climbing to the extremes seen in in-land areas further north. As a result of its coastal extremity and mild winter minimum temperatures, Portland is suitable for plants with the Royal Horticultural Society's hardiness rating H2. Mean sea surface temperatures range from in February to in August; the annual mean is .