A Tropical Night occurs when the overnight temperature remains above 20 °C.
Tropical Nights are exceptionally rare phenomena across most of Prairies. In fact, both Calgary and Edmonton experienced a grand total of zero Tropical Nights between 1981 and 2010. Winnipeg experiences an average of one Tropical Night per year.
Water is a Greenhouse Gas.
The reason places close to the Rockies experience so few Tropical Nights has a lot to do with water–or rather the lack of it. Across southern Alberta and Saskatchewan, the air is much drier, mainly because the moisture was squeezed out as the air was forced up and over the mountains. Consequently, at night the lack of water vapour–a gas that readily absorbs and re-releases heat energy–results in cooler overnight temperatures.
In Southern Manitoba there is more moisture because the region is more frequently influenced by air traveling north from the Gulf of Mexico (think of southern Manitoba as the northern tip of Tornado Alley). This moist air traps more heat, preventing the mercury from dipping too fast at night.
A warmer climate means more Tropical Nights, right?
While the mean temperature across the Prairie Provinces ticks upwards at a relatively even pace, the projected changes in Tropical Nights tells a much more interesting geographical story, especially for Southern Manitoba. By 2051-2080 (what we refer to as the ‘Far Future’), the average number of tropical nights in Winnipeg will jump from just 1 to over 16.
|Where do you live?||Baseline Tropical Nights per Year||Far Future Tropical Nights per Year|
What does it all mean?
When it comes to heat and human health, it’s all about persistence. After a few days in a row with daytime temperatures above 30 °C and nighttime temperatures above 20 °C, the risk of heat stroke and exhaustion increases dramatically. In places like Toronto and Montreal, urban heat warning systems are used to prepare cooling centres for those who do not have regular access to air conditioning. Perhaps it’s time for communities in Southern Manitoba to consider developing similar systems.
The map at the top of this page shows the projected change in the mean annual number of Tropical Nights by 2051-2080, compared to the most current climate normal period (1981-2010). This map was constructed using the RCP8.5 emission scenario, commonly referred to as the ‘business-as-usual’ carbon scenario; the data itself is derived from latest-generation CMIP5 Global Circulation Models (GCMs). This data is provided for free from the Prairie Climate Impacts Consortium (PCIC) on their website: http://pacificclimate.org. Twelve models running RCP8.5 were used to create an ensemble, or average, which was then mapped. Also studied, but not shown here, was output from the more optimistic RCP4.5 emission scenario.
Climate Change Researcher, Prairie Climate Centre
University of Winnipeg