Index Measures and Quantifies Severity of Heat Waves in a Warmer World

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AccuWeather is launching what it says is the only scale that tracks the duration and severity of heat waves, which are becoming more common occurrences due to factors like climate change, deforestation and urban heat island effects.

AccuWeather’s HeatWave Counter and Severity Index promises to help people better understand the severity, intensity and duration of heat waves – the No. 1 cause of weather-related deaths in the U.S. Heat waves on average kill more Americans than any other type of severe weather, according to AccuWeather.

Historical temperature records show a trend with increasing number of days in a heat waves each year over time, especially in recent decades, as well as an increase in the severity of heat waves.

The world just experienced its warmest early June on record by a substantial margin. The past eight years were the warmest on record globally, according to the World Meteorological Organization. WMO says global temperatures are likely to surge to record levels in the next five years, fueled by heat-trapping greenhouse gases and a naturally occurring El Niño event,

Factors driving the rise in heat waves include global climate change, deforestation and urbanization driving the “urban heat island” effect resulting in cities experiencing a tendency for higher temperatures, according to AccuWeather.

Marshall Moss, AccuWeather’s vice president, operations quality and innovation, says “the number, globally, of heat waves has been getting worse,” an effect that is attributable to climate change and other factors.

“There’s a combination that goes into the fact that there are more and more severe heat waves,” Moss said.

The idea behind AccuWeather’s heat wave severity index is to “provide context for people,” said Dan DePodwin, director, forecasting operations for AccuWeather.

The index will quantify heat waves by duration and intensity. Heat waves are defined as three or more days in which the temperature exceeds a threshold. The threshold varies with the climate of a location.

To be considered a heat wave in New York, St. Louis, or Los Angeles requires the daily temperature to reach or exceed 90 degrees on three straight days. The threshold in Miami and New Orleans is 95 degrees. In Austin, Texas, it is 105, and in Phoenix the threshold is 110 degrees.

The index is also dependent on the duration of the heat wave and how much the temperature exceeds the threshold: a three-day heat wave in New York City measures a 3; a three-day heat wave with some of the days reaching temperatures in the middle or high 90s may reach a scorer of 8.

AccuWeather’s extensive historical weather data enabled index designers to quantify past heat waves for many of major cities. Comparing past and present heat waves will enable people and businesses to understand trends around the growing impact of climate change, and to make the best decisions when they are affected by life-threatening heat, the index designers say.

The index will be used on the AccuWeather Network, on some of the TV stations that carry the AccuWeather forecast, and it will be used as the basis for reports on heat waves on AccuWeather.com and the AccuWeather app.

The index follows in the footsteps of other AccuWeather scales, such as the AccuWeather RealImpact Scale for Hurricanes and comfort weather indices like the RealFeel Sun and RealFeel Shade temperatures.

The new heat index may all so be of interest to professionals in the workers’ compensation area.

According to DePodwin, AccuWeather will be providing the index to businesses for which heat waves are a vulnerability – contractors, agriculture and others.

“There’re a lot of businesses that might care about that,” DePodwin said. “It could be people who have workers outside, it could be school districts who have different heat-related thresholds for their student athletes. So, there is definitely a lot of application on the business side.”

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