What’s Normal Weather, Anyway?

When a TV meteorologists says “temperatures will average ten degrees above normal for the next few days” or “rainfall this past month was below normal,” the word “normal” has a very specific meaning. Every ten years, the National Oceanic and Atmospheric Administration (NOAA) looks back over the previous three decades and calculates the average temperature or precipitation for a given day or month, in a given location, during that period. If the high temperatures on every August 10th in New York City averaged out to 83 degrees, that's defined as the normal high for that place for that date. If the average low for January 3 was five degrees above zero in Minot, ND, that becomes the normal low meteorologists will refer to.

The reason NOAA does the recalculations every decade is that long-term trends, either upward or downward, can push the numbers in either direction — but since the average goes back 30 years, only a long-term trend can make a difference. A couple of randomly warm years, or a couple of cold ones, won't change a 30-year average by much.

The latest average does show such a long-term trend, however. Every one of the lower 48 states in the U.S. has a new average overall temperature (winter and summer, day and night) that's higher than it was last time around, adding up to a national normal of 0.5°F higher than it was a decade ago. The reason is simple. Last time around, the decade of the 1970's was part of the calculation. But that's now dropped out: the new normals are based on the years 1981-2010 — and the 2000's were the warmest decade in the modern record. 

If long-term climate change is really happening, as most climate scientists say it is, the next set of normals in 2021 should be higher still, and so on for as long as we keep emitting heat-trapping greenhouse gases into the atmosphere. Half a degree doesn't mean a whole lot by itself, but keep adding them up, and we could see some significant changes in the American climate. 

Two things to keep in mind when playing with this graphic. First, some cities shown here have actually seen their new normals drop compared with the last version. That's because local conditions — wind patterns and offshore ocean currents, for example — can push small areas in the direction opposite to the general trend (and remember, every one of the Lower 48 states got warmer overall, even if some cities didn't).

Second, the cities that appear hear were chosen by NOAA as the first ones they released — not because they represent the most dramatic changes, but because they're among the nation's biggest and best-known. More numbers will be forthcoming; as they come out, we'll keep adding them. So it will pay to come back often to see what's new.

Also see: Climate Central news: Updated Statistics Show the “Normal” US Climate is Getting Warmer