Defining a Heat Wave

By Linda Waner, Climate and Society ’13

How hot is too hot?  How much does humidity matter? Is a heat wave in May worse than one in August?  These are the questions being worked out at the IRI, in pursuit of creating a heat wave map room for the Red Cross/Red Crescent Climate Centre. The questions are deceptively simplistic, but the answers could have major health implications.  And they’ve been on the mind of one Climate and Society student every day this summer.

Intuitively, it seems easy to diagnose a heat wave—it’s when it’s too hot outside to want to do anything!  But more than putting a damper on plans, extreme heat events can be dangerous. In the past few decades, heat waves have claimed thousands of lives in the United States, Europe, India, China, and Russia. And yet, according to the US CDC, heat-related deaths are 100% preventable. Research has shown that simple, common sense actions, such as staying in an air-conditioned building, can be enough.  However, the most important element in reducing heat wave-related mortality might be knowledge that a heat wave is coming. If saving lives is the goal, then researchers and communities need to work together to develop warning systems.

Giving the heads up that a heat wave is expected is complicated because heat waves themselves are complicated. Think about the reaction to the recent multi-day stretches of temperatures hovering around 100°F in New York. Know anyone in Phoenix?  Think of how they might respond to similar temperature levels. Now think about someone in Alaska. The temperature might be the same, but is the reaction?  Research suggests it’s not, which is why mortality levels during heat waves in the Northeast are higher than those in the South or Southwest U.S. This could be because regions more prone to heat waves are better prepared for them. It could be because the people living in such regions are better acclimatized.  Evidence points to a mixture of both (a new excuse to vacation in the tropics—I’m acclimatizing, honey!).

It would be simple to send out a warning when temperatures were expected to be above a certain point regardless of region. This is known as an absolute threshold.  But as these differing regional mortality numbers suggest, a heat wave in the mid-nineties could be deadly in Chicago; in Nevada the threshold for a deadly heat wave might be higher. To start a warning system that could be used throughout the world, the better answer might be: it’s all relative.Naturally, this refers to using a relative threshold.

Establishing a relative threshold algorithm to create a new heat wave map room is a job for a C&Ser. Just throw a ‘c’ and ampersand on a certain superhero’s uniform beforehand.

While costume might be optional, knowledge of the IRI’s data library is not. By being familiar with a massive source of climatological information, it is possible to begin establishing regional relative thresholds throughout the world. A relative threshold is inherently local, and involves temperatures exceeding a certain value or percentage above normal. Normal must be calculated over a baseline period, such as thirty years, not unlike what is done in the Climate & Society classroom.  This type of threshold can account for the region-to-region differences in physiological and behavioral responses, and therefore has the potential to maximize lives saved.

Unfortunately, the work has not been easy, although this is mostly because a certain junior researcher (and now blog post-author) has difficulty working in heat waves. Even on research on…heat waves.  Nonetheless, it’s incredible to have the opportunities and skills necessary to save lives.  Just another day in the life of a C&Ser?  Let’s hope so.

Feature image: NOAA