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Updated Statistics Show the “Normal” US Climate is Getting Warmer

If Philadelphia has a run of abnormally hot weather this summer, it will be considered less unusual than it would have been a year ago. That’s because “normal” now has a new definition. A set of updated national climate averages due to be released this week shows that average summer temperatures in the city are nearly 2°F warmer than the benchmarks meteorologists have been citing for the past decade.

The updates come from the National Oceanic and Atmospheric Administration (NOAA), which will impliment its new 30-year “Climate Normals” on July 1. When weather forecasters use terms like “above normal” or “below normal,” they’re referring to average temperatures and precipitation figures compiled from daily, monthly and yearly readings at thousands of locations across the country during the previous three decades. The normals that are about to expire are based on conditions from 1971–2000; the new ones start in 1981 — and unlike their predecessors, they include the first decade of the 21st century, which was among the hottest on record for the country (and was the hottest on record globally). They also exclude the 1970s, which was a particularly chilly decade in many areas. As a result, the new normal temperatures for Philadelphia and the rest of the nation are higher than the previous set.

Philadelphia residents splash in a city fountain during a heat wave last summer. Credit: spike55151/Flickr.

“What we think of as really hot days just aren’t as unusual as they once were,” says Gary Szatkowski, chief meteorologist at the National Weather Service’s Mount Holly, N.J. forecast office, which serves the Philadelphia metro area. “We’re having more of these really hot days, and that is now reflected by these new normals.”

Every ten years, NOAA’s National Climatic Data Center (NCDC) updates the climate normals for about 7,500 weather stations around the country. Because they span the previous three decades, the updated values reflect longer-term trends; a couple of hot, cold, or rainy years don’t sway the statistics one way or another. However, the new normals do reflect more persistent trends at the local, regional, and national scales. For example, one city might experience an overall decrease in average annual snowfall, or several neighboring states might see an increase in average winter temperatures.

Meteorologists rely on these normals to compare weather conditions on any given day to "average" conditions. This matters to ordinary people who want to know how to dress on a given day, but it also has implications for city planners, who need to project snow-removal costs for next winter, say, and to insurance companies who need to know what premiums to charge to cover future climate-related disasters. Energy companies also use the figures to help set base rates for customers.

At any particular weather station, the change in temperature or precipitation often doesn’t look very large, or even like it could have a significant impact. But when you take a step back, and look at the direction many of these changes are going in, some trends really stand out.

For example, now that the new normals include the first decade of the 21st century, the average annual temperatures for all of the lower 48 states has gone up. On average, temperatures from 1981-2010 are 0.5°F warmer than they were between 1971-2000.

“The normals aren’t predicted, they are a reflection of the record [of past climate],” says Szatkowski, “and the new normal is warmer than the old one.”

The new normals reveal that, for much of the country, the winter months are warming up more quickly than the summer months. In the Midwest and across the Northern Plains, new average January temperatures are between 2°F and 4°F higher than they were in the previous, 1971-2000 normals. Also, nighttime lows are increasing more rapidly than daytime highs.

“This doesn’t mean that there won’t still be cold weather records broken. That’s still going to happen,” says Szatkowski. “But in recent years, across the country there have been roughly twice as many record heat events as record cold events. We’re clearly more likely to be on the warm side than the cold.”

The new normals for each of the lower 48 states shows that the period of 1981-2010 was warmer than 1971-2000. Credit: NOAA.

The increase in record heat events resonates for the City of Philadelphia’s Department of Public Health. In 1995, Philadelphia developed its Hot Weather–Heat Watch Warning system, the first of its kind in the country. When a particularly oppressive heat wave is forecast, the city issues television, radio and newspaper alerts that warn the public of the hot weather. They also notify the public about good ways to stay cool in the heat, and how best to get help in a heat-related emergency.

Philadelphia’s heat warning system is coordinated with the Mount Holly forecast office, and Szatkowski says his office also works with a similar severe cold weather warning and response system in Philadelphia. “The program is used when it gets really cold in the area,” he says,  “but in recent years, they have had to activate the heat system much more often. There’s a lot more worry about the warm days than the cold ones.”

NOAA’s new climate normals reflect this drop in the number of excessively cold days. Previously, in Philadelphia, the thermometer dipped below freezing on an average of 87 days per year. According to the new 1981–2100 statistics, though, there are now typically fewer than 80 days each year when temperatures drop below freezing at some point.

The new normals don’t show a comparable increase in the number of extremely hot days (above 90°F) for Philadelphia, but studies show that summers like last year’s, which was the hottest in Philly’s recorded history, are a harbinger of what’s to come. Several climate models predict that within 40 years, July alone could see an average of 20 days above 90ºF — twice as many as is typical today.

“Looking over the long haul, at Pennsylvania or New Jersey, or even Philadelphia specifically, the trend is that it is going to get warmer and wetter,” says Szatkowski. Studies already show that warming temperatures are increasing the frequency and intensity of heavy rainfall events in the Northern Hemisphere.

Philadelphia’s Director in the Office of Sustainability, Katherine Gajewski, says the city is preparing for more hot days during summer, and more heavy rainfall.

“We’re planning to implement a system that is going to be better equipped at handling that first critical inch of water that comes during a heavy rainstorm,” says Gajewski. By slowing and redirecting the drainage of floodwater, she says the city can prevent sewer water from contaminating adjacent rivers. Like dozens of other cities across the country, Philadelphia’s outdated storm water drainage system is currently in violation of the Clean Water Act. But a proposed $2 billion update will bring the city into compliance and help manage the predicted increase in intense rainstorms, according to city officials.