Halifax has a humid continental climate , bordering on an oceanic climate (Cfb), with warm summers and relatively mild winters, which is due to Gulf Stream moderation. The weather is usually milder in the winter or cooler in the summer than areas at similar latitudes inland, with the temperature remaining (with occasional notable exceptions) between about −8 and 24 °C (18 and 75 °F). January is the coldest month, being the only month with a high that is slightly below freezing at −0.1 °C (31.8 °F), while August is the warmest. The sea heavily influences the climate of the area, causing significant seasonal lag in summer, with August being significantly warmer than June and with September being the third mildest month in terms of mean temperature. The January mean is only 1.1 °C (2.0 °F) colder than the isotherm for the oceanic climate.
Precipitation is high year-round. Winter features a mix of rain, freezing rain and snow with frequent freeze-thaw cycles. Snowfall is heavy in winter, but snow cover is usually patchy owing to the frequent freeze-thaw cycles, which melt accumulated snow. Some winters feature colder temperatures and fewer freeze-thaw cycles; the most recent of which being the winter of 2014–2015, which was the coldest, snowiest and stormiest in about a century. Spring is often wet and cool and arrives much later than in areas of Canada at similar latitudes, due to cooler sea temperatures. Summers are mild and pleasant, with hot and humid conditions very infrequent. Warm, pleasant conditions often extend well into September, sometimes into mid-October. Average monthly precipitation is highest from November to February due to intense late-fall to winter storms migrating from the Northeastern U.S., and lowest in summer, with August being the year’s warmest and driest month on average. Halifax can sometimes receive hurricanes, mostly between August and October. An example is when Hurricane Juan, a category 2 storm, hit in September 2003 and caused considerable damage to the region. Hurricane Earl grazed the coast as a category 1 storm in 2010. In 2019, Hurricane Dorian made landfall just south of Halifax as a post-tropical storm with an intensity equivalent to a category 2 hurricane and caused significant damage across Nova Scotia. Atlantic sea surface temperatures have risen in recent years, making Halifax and the coast of Nova Scotia somewhat more susceptible to hurricanes than the area had been in the past.
The highest temperature ever recorded in the city of Halifax was 37.2 °C (99.0 °F) on July 10, 1912, and the lowest temperature recorded was −29.4 °C (−20.9 °F) on February 18, 1922. The March 2012 North American heat wave brought unusually high temperatures to the city of Halifax. On March 22, the mercury climbed to 28.2 °C (82.8 °F) at the Halifax Windsor Park weather station, and 27.2 °C (81.0 °F) at Halifax Stanfield International Airport. In spite of the possibility of high temperatures, in a normal year there is only one day that goes above 30 °C (86 °F). Halifax also has a modest frost count by Canadian standards due to the maritime influence, averaging 131 air frosts and 49 full days below freezing annually. On average the frost-free period is 182 days, ranging from May 1 to October 31.