It can be as changeable, wet, and dismal as the rest of the UK and Ireland in Northern Ireland. The temperate climate is like this in winter due to the effect of the Gulf Stream, which usually keeps temperatures above zero in winter. With its northerly locale, it is typically cooler than the Irish Republic and much of England, though days are long in summer.
Northern Ireland is seasonal—spring, summer, fall, and winter—as with elsewhere in the region. Summer averages in June, July, and August are 60-64°F and hot days can be in the 90s. Summer is the best time to visit anywhere in the British Isles, but be aware that everyone else visits at this time, too, plus school holidays are late July through early September. It also rains most in summer, usually in July when most days have rain. On average, year-round rainfall is about three inches per month.
Off-season bargains on travel and cheaper Northern Ireland hotels can typically be had outside summer, with November through March having the best deals (apart from Christmastime/New Year). Temperatures can drop to the 30s (°F) at this time, although it rarely gets very cold due to the tempering effects of the ocean. The weather is generally better in early spring than late fall.
Best Time to Visit Northern Ireland
June through September has the best weather and, if you can put up with hordes of tourists, July and August have the warmest temperatures. It tends to get muggier as the summer wears on, so look to early August for the best chance of optimal comfort. June is one of the best overall months as the temperatures are up, rainfall is lower, and crowds are fewer.
Note: Whatever time you plan on visiting, always pack for cooler weather and have an umbrella close to hand. Belfast can be breezy any time.