Isle of Man, where it is located

The climate of the Isle of Man, a British Crown dependency, is oceanic, cool and humid throughout the year. The island is located between Northern Ireland and north-western England.
The average temperature ranges from 5.5 °C (42 °F) in February to 15.5 °C (59.5 °F) in August.
Here are the average temperatures.
Isle of Man - Average temperatures
Min (°C)4345810121211964
Max (°C)88911141618181614119
Min (°F)393739414650545452484339
Max (°F)464648525761646461575248

Precipitation amounts to 865 millimeters (34 inches) per year. It is well distributed over the seasons, and is frequent also in summer. However, the wettest period is from October to January. In the hills that lie in the interior of the island, rainfall is more abundant, so that at the top of Snaefell, at 621 meters (2,037 feet) above sea level, 1,900 mm (75 in) of rain fall per year.
Here is the average precipitation at the airport.
Isle of Man - Average precipitation

Winter, from December to February, is quite cold: temperatures are not so low, but wind and humidity can increase the sensation of cold. The sky is often cloudy, and rainfall is frequent. Sometimes, intense Atlantic depressions can cause wind storms.
Snowfalls are relatively rare and not abundant (at least at sea level); usually, cold spells are not intense and do not last long. The cold records from December to March are around -6/-7 °C (19/21 °F).

In spring, from March to May, the temperature slowly rises, and Atlantic fronts continue to affect this area more or less as in the rest of the year, although they are a bit less intense than in winter. In May, it is often still quite cold, especially in the first half of the month.

Summer, from June to August, is cool, wet and cloudy, with quite frequent rains. Hot days are rare: the temperature may never reach 25 °C (77 °F) for an entire summer. Even sunny days, or at least those in which the sky is completely clear from dawn to dusk, are quite rare.

Autumn, from September to November, is gray, cloudy and rainy as in the rest of the year, and with each passing week, it gets progressively colder. In November, the wind is often moderate or strong.

Peel Castle

The amount of sunshine in the Isle of Man is not good. However, the (relatively) sunniest season is spring, when the sun also shines a little more often than in Ireland or England. Entire days with totally clear skies are rare: it's more likely for the sky to be partly cloudy. Here are the average sunshine hours per day.
Isle of Man - Sunshine

The Irish Sea is cold all year round, in fact, it reaches at most 15 °C (59 °F) in August. Here are the average sea temperatures.
Isle of Man - Sea temperature
Temp (°C)87781012141515141210
Temp (°F)464545465054575959575450

Best Time

The best time to visit the Isle of Man runs from mid-May to mid-September; May and June are cooler months, but they are relatively sunnier. July and August are the warmest months, or the least cool, however, it's better to bring a jacket, a sweatshirt and a sweater also in midsummer.