thrive which do not on the mainland. The limestone soil has low nutrient levels; hence smaller species of wild flowers and grasses are able to grow in the absence of larger species. Portland sea lavender can be found on the higher sea cliffs; unique to Portland, it is one of the United Kingdom's rarest plants. The wild flowers and plants make an excellent habitat for butterflies; over half of the British Isles' 57 butterfly species can be seen on Portland, including varieties that migrate from mainland Europe. Species live on Portland that are rare in the United Kingdom, including the limestone race of the silver-studded blue. Climate 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 1991 to 2020 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 island's 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 . The mild seas that surround Portland act to keep night-time temperatures above freezing, making air frost rare: on average 6.3 days per year — this is far below the United Kingdom's average annual total of 55.6 days of frost. Days with snow lying are equally rare: on average zero to five days per year; almost all winters have no more than one day with snow lying. It may snow or sleet in winter, yet it almost never settles on the ground—coastal areas in South West England such as Portland generally experience the mildest winters in the UK. Portland is less affected by the Atlantic storms that Devon and Cornwall experience. The growing season lasts for more than 310 days per year, and the borough is in Hardiness zone 9. Weymouth and Portland, and the rest of the south coast, has the sunniest