When you feel a heat wave, you know it-the hot weather makes you crave iced drinks, or better yet, air conditioning.

But determining what counts as a heat wave is very tricky. Temperatures vary greatly from place to place-the normal summer weather in Las Vegas would be considered hot in Seattle. Then there is the issue of climate change. When high temperature records are erased (sometimes over a weekend), how do you even say what the “normal” temperature is?

This is not a question of semantics. This is a matter of life and death.Overheating, according to National Weather Service, Is the number one weather-related killer in the United States. Defining heat waves helps cities understand when and how to respond to lethal temperatures—issue high temperature warnings, share tips for surviving high temperatures, and open cooling centers.

The recent heat wave across the west has fueled the wildfires now raging in Oregon, Washington, and California. Summer is much earlier than usual. Southwest China is in its The third heat wave In the summer of last weekend, the temperature hit a record high.Death Valley has what is considered to be the hottest 24-hour period on earth, with an average 118.1 degrees on Sunday.

Last month overall Hottest June In the recorded history of North America. Two weeks ago, the unprecedented “hot dome” turned the Pacific Northwest into a furnace, causing more than 200 deaths in Washington and Oregon, and hundreds of deaths in British Columbia. The high temperature in Portland reached 116 degrees and the tram cable broke.Almost melting 3 feet Snowfall near Mount Rainier in Washington broke Canada’s record high 121 degrees Fahrenheit, Incited and burned Leiden Town In British Columbia.

“The temperature is incredible,” said Christie Ebi, a professor of global health at the University of Washington in Seattle. She said that the heat wave actually exceeded what climatologists predicted might happen now in the Pacific Northwest, and she said it was changing the future shown by the model. A study last week showed that this particular heat wave was originally “almost impossible“There is no climate change.

Researchers have found that if the world warms by 2 degrees Celsius (3.6 degrees Fahrenheit) above the pre-industrial level, this severe heat wave may occur every 5 to 10 years and become a recurring nightmare. The skyrocketing temperature raises the bar for “really hot”-the extreme heat of the past may one day become your average summer weather.

“There are hundreds of heat wave warning systems around the world. Each one has done a lot of work to define what a heat wave is,” Ebi said. “A question that many others and I keep asking is: How will you decide when to change this definition?”

A cooling station in Portland during the record heatwave on June 28, 2021.
Catherine Elcether/AFP via Getty Images

Until recently, high temperatures have become the primary consideration for public health experts. In 1993, Philadelphia launched the nation’s first high temperature warning system, sounded the alarm and assigned health workers and neighborhood leaders to check the elderly and the homeless. This issue received further attention in 1995, when Chicago suffered a triple-digit high temperature that caused more than 700 people Within a week.

There are some general definitions of heat waves, although they are somewhat technical.This World Meteorological SocietyFor example, say that the daily high temperature for five consecutive days is 9 degrees Fahrenheit higher than the normal high temperature.However, according to other definitions, a heat wave may be just Two days Long, or there is a different threshold to determine what is really “hot”. Ebi said that for the study of heat, scientists sometimes analyze up to 95% of the temperature in a given area.Just looking at the high points will ignore other important factors: summer For example, the night warms faster Compared with the daytime, leaving home at night has almost no chance to cool down, which is a fatal risk for people without air conditioning.

Scott Sheridan, a professor of geography at Kent State University, who helped build a high temperature warning system around the world, said that the first step in setting a local heat wave threshold is to study which weather conditions will increase hospitalization and mortality. . The human body can adapt to higher temperatures to a certain extent, which means that the definition of dangerous high temperatures may fluctuate with the seasons.Heat waves in the spring or early summer can be very deadly without reaching the temperature in August-evidence suggests that more people die from the heat Earlier in the season.

Heat wave boundaries are sometimes used to determine what counts as heat-related deaths.For example, in New York City, a Extreme high temperature event Defined as the time of at least two days when the heat index reaches 95 degrees Fahrenheit, or one day at least 100 degrees.Propaganda organization says What are the different definitions of “hot day” This is one of the reasons the city may underestimate the death rate associated with high temperature.

Heat waves are difficult to define because they are not only determined by a set of meteorological conditions, but by these conditions. Stingy for health. Ebi said that she hopes to see a hierarchical warning system where meteorologists begin to issue early warnings of high temperature for vulnerable groups (such as adults over 65, infants, outdoor workers, and people with underlying diseases). When in danger, “So this is not a message for everyone.”

To add complexity, heat interacts with the way the town is built. A phenomenon called “Urban heat island effect“It means that the city is hotter than the surrounding areas such as fields or forests, because tall buildings, dark roofs and sidewalks absorb sunlight. Historically, areas with red lines have more highways, fewer parks and trees, and up to 19 degrees Fahrenheit Hotter than the coolest neighborhood.

Sheridan said that cities are at least better prepared for heat waves than before, although many cities are not yet prepared for the unexpectedly hot weather brought about by climate change. For example, in Seattle, only 31% of homes were air-conditioned in 2013; in 2019, 44% Did it, that number is Fast climb.

So, how do you set a local standard for heat waves when the average temperature keeps rising? Currently, most local definitions remain unchanged. Sheridan said that he didn’t know where the threshold had been raised, but it was often talked about as a possibility. He said that one thing you don’t want to do is to announce that the heat waves are coming so frequently that no one pays any more attention.

“If you issue a high temperature warning every time the temperature in Phoenix exceeds 90 degrees, you know, you will do it for half a year.”


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