Defining Climate Normals in New Ways

Photo of three tourists pointing at a map


You’re looking forward to taking a much deserved vacation in four months, and you want to visit a U.S. city you’ve never been to before. So, how do you decide where to go? Perhaps you’ll consider destinations where you can expect your ideal temperatures, whether you prefer highs below 70°F, lows above 60°F, or something else entirely. But, at four months out, your trusted weather forecast probably won’t be much help in finding locations that meet your temperature criteria.

Fortunately, you won’t need to make an educated guess or grab some historical weather data and drag out the calculator. Our scientists have calculated the “normal” temperatures you can expect to see each month at a multitude of locations across the contiguous United States. We call them Climate Normals, and we compute a new installment every decade.

What Are Climate Normals?

Scientists traditionally define a Climate Normal as an average over a recent 30-year period. Our most recent installment covers the period from 1981 to 2010. Why 30 years? Close to a century ago, the International Meteorological Organization—now known as the World Metrological Organization—instructed member nations to calculate Climate Normals using 30-year periods, beginning with 1901–1930. Also, a general rule in statistics says that you need at least 30 numbers to get a reliable estimate of their mean or average. So, our scientists have traditionally defined Normals as averages over 30 years simply because that is the accepted convention—not because a 30-year average is the only logical or “right” way to define a Climate Normal.

Doesn’t Climate Change Affect 30-Year Normals?

Some users of our traditional Climate Normals products have expressed concerns about using a 30-year average in an era of observed climate change. Why? Because if climate conditions are shifting upward or downward, rather than fluctuating above and below the same constant level, it may make sense to calculate an estimate of the current state of the climate in a different way. Based on extensive feedback from user groups, particularly from the energy industry, we are now releasing Supplemental Monthly Temperature Normals that define “normal” in alternative ways.

What Information Do Supplemental Monthly Temperature Normals Provide?

Some of the new calculations are what you might expect: averages over 5-, 10-, 15-, and 20-year periods. We also compute “normal” in two additional ways: the Optimal Climate Normal (OCN) approach and the Hinge Fit approach. We won’t get into the heavy statistical details here, but you can think of the OCN as a “smart” average where the data values tell you how many years to average over. The Hinge Fit is a different animal altogether—in fact, it’s not even an average but a statistical fit through the data values. Generally speaking, the Hinge Fit is relatively sensitive to recent shifts upward or downward versus the 30-year normal, while the OCN reflects the effects of recent shifts more moderately.

Image of Supplemental Climate Normals for Minneapolis / St. Paul Airport, Minnesota

How Can You Use the Supplemental Monthly Temperature Normals?

Our scientists designed all of these alternative ways of defining “normal” to provide a better estimate of current or future climate conditions in an era of climate change. You should consider the range of estimates provided by the seven different options when using these Normals in your specific applications. But, they can be especially helpful in long-term planning applications like determining where to build a power plant. You can also use them to help ensure you find your ideal destination for that vacation you’re looking forward to in a few months.

Maps Comparing Supplemental Climate Normals to 30-Year Climate Normals