The Isle of Man has a temperate maritime climate — mild, damp, and changeable. Locals will tell you that you can get four seasons in a single day, and they are not far wrong. Summer highs typically reach 17–19°C; winter temperatures hover around 5–7°C. Snow is rare at sea level. Wind is a near-constant feature, especially on the uplands.
May and June bring the longest days and the most dramatic event on the Manx calendar: the TT Races. If motorcycling is your thing, this is an unmissable experience. If not, avoid the island entirely during these two weeks — accommodation is largely pre-booked months in advance, prices are high, and the roads are either closed or clogged. The Manx Grand Prix in August–September is a smaller version of the same experience.
July and August are the warmest months and the best for walking, cycling, and beach days. The island is busy but not overwhelmed outside of race events.
September and October are quieter, the light is good, and the heather on the uplands is in bloom. This is an underrated time to visit.
Winter (November–March) is quiet, some attractions reduce their hours or close, and ferry crossings can be disrupted by bad weather. That said, the island has a real off-season character and prices drop considerably.