The mild seas that surround Portland act to keep night-time temperatures above freezing, making air frost rare: on average 9.6 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.