(Spring blooms at Longwood Gardens. Photo by Andrea Poe, Washington Times Communities.)
When several grand sugar maples at the famous Longwood Gardens in Pennsylvania succumbed to disease recently, grounds manager Shawn Kister wondered if warm winters were to blame. Some plant diseases are stopped by cold, he knew, and others are encouraged by longer warm periods. Sugar maples like the cold, and Longwood is already at the southern edge of their historical range.
A few hundred miles south, Charlotte-based TV meteorologist John Ahrens noticed that area rainfall had remained lower than normal for several years. The farmers and landscapers around Charlotte were compensating with more irrigation, but those systems are expensive. What was going on? Was this going to be the new normal?
Defining normal
Gardeners, meteorologists, businesses, weather junkies and others will get answers to some of these questions in July, when NOAA’s National Climatic Data Center (NCDC) releases the latest version of an official weather product called the U.S. Climate Normals.
Updated each decade, the U.S. Climate Normals are 30-year averages of many pieces of weather information collected from thousands of weather stations nationwide. Each time they are updated, an old decade is dropped, and a new one added. Starting in July, when you hear that a day was hotter, or colder, or rainier than normal, that ”normal” will be a little different from what it was in the past.
Across much of the country, overnight low temperatures in January are as much as several degrees warmer in the 1981–2010 Normals than they were in the 1971-2000 version (left). Meanwhile, the average maximum temperatures in July are actually cooler across some parts of the country (right). (Maps by NOAA.)
This time around, the 30-year window for the U.S. Climate Normals is 1981-2010: the decade 1971-1980 was dropped, and 2001-2010 was added. Since the ’70s were an unusually cool decade, while 2001-2010 was the warmest ever recorded, it is not surprising that the average temperature rose for most locations. For the United States as a whole, it was not daytime highs (maximum temperatures) but overnight lows (minimum temperatures) that rose the most compared with the 1970s.
Many areas of the country—parts of the Great Plains, the Mississippi Valley, and the Northeast—actually had cooler July maximum average temperatures in the 2001-2010 time period compared to 30 years earlier; summer picnics in Missouri may have been less sweltering. At night, though, except for the southeasternmost part of the country, minimum temperatures in January were warmer. When temperatures were averaged for the entire year, every state came out warmer overall.
On a state-by-state basis, the annual average minimum (left) and maximum (right) temperatures across the United States are warmer in the 1981-2010 Climate Normals than in the 1971-2000 version. (Maps by NOAA.)
Assembling the Normals
The observations that go into the U.S. Climate Normals come from the National Weather Service’s network of Cooperative Observer Program (COOP) stations, numbering in the thousands; a smaller network of “first order” stations maintained by professionals in the National Weather Service or Federal Aviation Administration, numbering in the hundreds; and more than one hundred NOAA Climate Reference Network stations established in the past decade.
Rainfall data are collected from than 8,000 stations, while temperature comes from about 6,000 stations. Some of the data are collected and reported automatically by instruments, while most of the measurements are collected manually each day by volunteers. Once a decade, a team at NOAA’s National Climatic Data Center calculates the U.S. Climate Normals from these millions of bits of raw data.
The U.S. Climate Normals draw on precipitation measurements from several thousand stations located across the country. (NOAA map by Ned Gardiner, based on data from the National Climatic Data Center.)
NOAA scientist Anthony Arguez is the U.S. Climate Normals project manager. “Our job,” he says, “is to take the data values as collected by the National Weather Service, apply robust quality control and standardization to improve the fidelity in our products, and use sophisticated statistical techniques that result in high-quality normals of temperature, precipitation, snowfall, and derived quantities such as heating and cooling degree days.”
Based on feedback from user groups such as power companies, National Climatic Data Center scientists have added some new information into the U.S. Climate Normals product suite, such as hourly temperatures and month- or year-to-date precipitation. The data can be accessed by anyone who wants to use them, such as television stations, water managers, insurance and energy companies, meteorologists, or emergency planners.
The general methods for calculating climate normals were established back in the 1930s by the World Meteorological Organization (WMO), and they are still followed by the United States and many nations all over the world. The consistency means that the U.S. Climate Normals are part of a reliable, long-term data record that can be used for studying natural climate patterns and climate change. The information also helps people understand what conditions they can expect wherever they may live.
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