Heat waves have been laying siege to regional temperature records around the world since the last week of June, as large areas of heat pressure, or heat domes, strewn across the hemisphere, have increased temperatures in relatively mild climates such as Canada, Ireland, Scotland and Siberia, and is being blamed for at least 33 deaths in Canada’s city of Montreal.
The eastern two-thirds of North America was covered in a massive heat dome since the closing days of June, toppling old heat records: on June 28, Denver hit a high of 105°F (40.5°C), tied with the Mile-High City’s all-time high set in 2012; on July 2, Canada’s capital of Ottawa recorded an all-time humidex reading (short for "humidity index", a measurement of heat plus humidity used in Canada) of 116.8°F (47.1°C) — this is the same city that, just six months ago, cancelled their New Year’s celebrations due to the extreme -11.2°F (-24°C) cold; on the same day, Montreal recorded a temperature of 97.9°F (36.6°C), the highest since temperature records began being kept in the city 147 years ago. All-time record warm temperatures for the daily lows for Burlington, VT (80.0°F/26.7°C) and Mount Washington, NH (60.0°F/15.6°C) were recorded on July 2 as well.
Across the hemisphere, the Scottish town of Motherwell hit an all-time high for the country at 91.8°F (33.2°C) on June 28, and Ireland’s town of Shannon saw a record-breaking high of 89.6°F (32.0°C); Georgia’s capital of Tbilisi saw a record high of 104.9°F (40.5°C) on July 4; Armenia’s capital of Yerevan saw a monthly record of 107.6°F (42.0°C) on July 2; on June 28, Russia’s Krasnodar (102.7°F/39.3°C), Primorsko-Arkhtarsk (102.7°F/39.3°C) and Rostov-sur-le-Don (101.1°F/38.4°C) regions either broke or matched previous record highs.
In the Middle East, Oman’s town of Quriyat set a world record for the hottest low temperature ever recorded on June 28, at 108.7°F (42.6°C). Bear in mind that that was the town’s daily low temperature; the daily high, while not a record, still rose to 121.6°F (49.8°C).
On his climate blog, meteorologist Nick Humphrey remarked on the 90-95°F (32-35°C) temperatures forecast to hit northern Siberia at the end of the first week of July, saying that "there appears to be general agreement over the intensity and timing of this extreme event. It is absolutely incredible and really one of the most intense heat events I’ve ever seen for so far north. Climate change has sent temps skyrocketing in the far north of the planet over just the past 20 years."
"Arctic Amplification is causing an abrupt weakening of the polar jet stream (on timescales of just the past decade or two), the main feature which steers and intensifies weather patterns in the mid-latitudes, Humphrey explains. "The weakening is causing the polar jet to become much wavier, with greater wave “breaks” and blocking patterns where waves sit in the same place for weeks promote extreme weather." This extreme north-south meandering of the polar vortex is what has been pushing warm air masses farther north than usual, and colder air south, generating the extreme temperature swings that have been experienced in both summer and winter over the past few years.