The mercury in Delhi shot past a 12-year record this month as northern India baked under a severe heat wave. But it was reports of unprecedented highs in Canada that added to the alarm over rising temperatures as North America counted hundreds dead and emergency actions were rolled out to offer respite to the public. As experts point to unmistakable recent weather trends to urge rapid action against global warming, here’s what has been driving temperatures up.
Clear Skies And Favourable Winds: What Makes The Heat Dome Tick
Close to 500 people had been reported dead in Canada’s British Columbia province alone as the heat wave in the country broke all-time records. The village of Lytton registered 49 degrees Celsius last week, the highest for Canada, as the country set a national record for the third consecutive day.
Experts are blaming the heat dome over North America for this spell of extreme temperature in a region that is hardly used to experiencing such sweltering weather. The latter is being particularly linked to the high death toll in areas where houses are built to trap heat and dwellings are not known to have fans or air-conditioning. It is this kind of cascading effect that can serve to explain how the heat dome is causing such terribly high temperatures.
Several factors have to come together for the heat dome effect to come into play. First, of course, is high temperatures. We know that when air heats up, it tends to rise. However, there is a high pressure zone over Canada that is not letting the hot air rise. Rising hot air generally creates a vacuum that winds from the sea rush in to fill. But in this case a temperature gradient from west to east in the Pacific Ocean is first blowing the hot air eastwards, which then is trapped by northern shifts of the jet stream. Jet streams are currents of cold, fast winds that move high in the atmosphere. This jet stream moves the hot air over northwestern North America and, as it is pushed down by the high temperature, heat dome conditions are created.
“A heat dome is basically that trapping dome. The heat event itself is the heat wave, lasting several consecutive days and nights that are well above normal,” climate expert Andrea Bair told the National Geographic magazine.
What Happens In A Heat Wave?
The Indian Meteorological Department (IMD) has an elaborate system for tracking and declaring heat waves. It says that “heat wave is considered if maximum temperature of a station reaches at least 40 degrees Celsius or more in the plains. For hilly regions, the same outlook is triggered if the temperature exceeds 30 degrees Celsius.
Based on the actual maximum temperature, a heat wave occurs when the mercury is at or above 45 degrees Celsius. A severe heat wave is called when actual maximum temperature equals or exceeds 47 degrees Celsius. IMD declares a heat wave on the second day when at least two stations in a sub-division report at least two consecutive days of such conditions.
For a heat wave to strike, IMD says, there has to be prevalence of hot dry air, that is, “there should be a
region of warm, dry air and appropriate flow pattern for transporting hot air over the region". That, along with an absence of moisture in the upper atmosphere, which enables temperature rise, and a practically cloudless sky provide the recipe for a heat wave. Last but not the least, there has to be “anti-cyclonic flow over the area" which prevents winds from blowing in.
IMD says that heat waves “generally develop over northwest India and spread gradually eastwards and
southwards but not westwards", given the prevailing westerly to northwesterly winds during the season. However, heat waves “may also develop over any region under the favourable conditions", it adds.
Does Global Warming Have A Part To Play?
Establishing a direct link between climate change and a specific weather phenomenon can be complicated, experts say, but suggest that global warming can lead to an increase in such events. Some evidence towards that may already be at hand.
According to the US weather National Oceanic and Atmospheric Administration (NOAA), 2020 was the second-hottest year on record for the planet, and 2019 was the third hottest. And the hottest year? Well, that distinction belongs to 2016. In fact, “the world’s seven-warmest years have all occurred since 2014, with 10 of the warmest years occurring since 2005".
Given that the main impact of climate change will be to lead to more extreme weather across the planet, experts say that rising temperatures resemble something of that anticipated impact. And, like all climate patterns have a spillover effect, more frequent heat waves will only worsen seasonal cycles and trigger the occurrence of drought and dry weather.
Canadian Prime Minister Justin Trudeau summed up the fears. “We’ve been seeing more and more of this type of extreme weather event in the past years. So realistically, we know that this heat wave won’t be the last,” he said.