Edmonton has a humid continental climate with cold, dry winters and warm, sunny summers, prone to extremes and large swings at all times of the year. It falls into the NRC 4a Plant Hardiness Zone.
Summer in Edmonton lasts from June until early September, while winter lasts from November until March and in common with all of Alberta varies greatly in length and severity. Spring and autumn are both short and highly variable. Edmonton’s growing season on average lasts from May 9 to September 22; giving an average 135-140 frost-free days each year. At the summer solstice, Edmonton receives 17 hours and three minutes of daylight, with an hour and 46 minutes of civil twilight, and on average receives 2,299 hours of bright sunshine per year, making it one of Canada’s sunniest cities.
The city is known for having cold winters, though its weather is milder than Regina, Saskatoon or Winnipeg, all of which lie south of Edmonton. Its average daily temperatures range from a low of −10.4 °C (13.3 °F) in January to a summer peak of 17.7 °C (63.9 °F) in July, with average maximum of 23.1 °C (73.6 °F) in July and minimum of −14.8 °C (5.4 °F) in January. Temperatures can exceed 30 °C (86 °F) for an average of four to five afternoons anytime from late April to mid-September and fall below −20 °C (−4 °F) for an average of 24.6 days in the winter. The highest temperature recorded in Edmonton was 37.2 °C (99.0 °F) on June 29, 1937 and on July 2, 2013, a record high humidex of 44 was recorded due to an unusually humid day with a temperature of 33.9 °C (93.0 °F) and a record high dew point of 23 °C (73.4 °F). The lowest temperature ever recorded in Edmonton was −49.4 °C (−56.9 °F) on January 19 and 21, 1886.
Edmonton has a fairly dry climate. On average, it receives 455.7 mm (17.94 in) of precipitation, of which 347.8 mm (13.69 in) is rain and 111.2 mm (4.38 in) is the melt from 123.5 cm (48.6 in) of snowfall per annum. Precipitation is heaviest in the late spring, summer, and early autumn, with the wettest month being July, having a mean precipitation of 93.8 mm (3.69 in), and the driest months being February, March, October, November and December. Significant snowfall accumulation typically begins in late October and tapers off by early April. Dry spells are not uncommon and may occur at any time of the year. Extremes do occur, such as the 114 mm (4.49 in) of rainfall that fell on July 31, 1953. Much of the precipitation that Edmonton receives in the summer comes from late-day thunderstorms, which are frequent and occasionally severe enough to produce large hail, damaging winds, funnel clouds, and tornadoes.
The summer of 2006 was particularly warm for Edmonton, as temperatures reached 29 °C (84 °F) or higher more than 20 times from mid-May to early September. The winter of 2011–12 was particularly warm:from December 22, 2011 through March 20, 2012 there were 53 occasions when Edmonton saw temperatures at or above 0.0 °C (32.0 °F) at the City Centre Airport, and even warmer in the city proper.
A massive cluster of thunderstorms swept through Edmonton on July 11, 2004, with large hail and over 100 mm (4 in) of rain reported within an hour in many places. This “1-in-200 year event” flooded major intersections and underpasses and damaged both residential and commercial properties. The storm caused extensive damage to West Edmonton Mall; a small glass section of the roof collapsed under the weight of the rainwater, causing water to drain onto the mall’s indoor ice rink. As a result, the mall was evacuated as a precautionary measure.
Tornadoes in and around Edmonton happen occasionally. Twelve tornadoes had been recorded in Edmonton between 1890 and 1989, and eight since 1990. An F4 tornado that struck Edmonton on July 31, 1987, killing 27, was unusual in many respects, including severity, duration, damage, and casualties. It is commonly referred to as Black Friday due both to its aberrant characteristics and the emotional shock it generated. Then-mayor Laurence Decore cited the community’s response to the tornado as evidence that Edmonton was a “city of champions,” which later became an unofficial slogan of the city.