 The Northern Isles have a cool , temperate climate that is remarkably mild and steady for such a northerly latitude , due to the influence of the surrounding seas and the Gulf Stream . In Shetland average peak temperatures are 5 ° C ( 41 ° F ) in February and 15 ° C ( 59 ° F ) in August and temperatures over 21 ° C ( 70 ° F ) are rare . The frost @-@ free period may be as little as 3 months . 
