The Washington, D.C., climate is changing... again

Here we are enduring another July of weather records and you, like others, may think it must be because the climate is changing. And that is true, the climate is changing. But it won’t happen until August.


A word of explanation: I and other meteorologists in D.C. rely on the National Weather Service for data on the region’s “normal,” or average, temperatures. (If you want to see my problem with the word “normal” in meteorology, click here.) The NWS averages are based on temperatures over 30 years, which in today’s world means from 1971 to 2000. But that three-decade range shifts as time goes by, and on Aug. 1 the NWS will begin to use a new set of averages based on observations from 1981 to 2010. So this year the climate for all cities, states and most countries will be “changing,” although you probably won’t feel it in any substantial way.

You might wonder, Why 30 years? Why not pick 20 years, 40 years or even a century to form a “climate average”? Well, a story I’ve heard that I can’t verify for certain (but it is a good story), is that the 30-year period was chosen by the German government in the early 1900s for health resorts that claimed to have beneficial weather and needed to show climate data for evidence. For some reason, the Germans decided upon 30 years. Anyway, that amount of time has now been used for many years as an internationally agreed-upon climate-average standard.

So what do the updated averages mean for Washington?

Not much. The hottest time of the year is still mid-July. The average highest high is still 89 degrees (not 90 degrees… yet). But the number of days with an average high increases a little. The new average high from July 7 to July 22 is 89 degrees, while the old average high of 89 degrees spanned from July 16 to July 27.

The strangest item in the new Washington climate records is the fewer number of 90-degree-plus days. Last summer, we tied the record with 67 days in the 90s, and we’re going to be well above average this summer. But the new climate average is only 31 days in the 90s, whereas the average for years 1971 through 2000 is 37. That’s partially because the record year of 67 days in 1980 is being removed from the new climate averages and because we recently had a few “cool” summers. But it sure will be interesting in the coming decade to see how many years Washington only has 31 days of 90-degree heat or higher.

On average, the nights in D.C. are getting milder. The average overnight low for mid-summer will be 71 degrees (it is now 70), and the average mid-winter morning low will be 28 degrees and that for only 11 days. The current climate average is a mid-winter low of 27 degrees for 23 days. But boy, the winter nights have quite a change in the country’s northern plains.

Spots like Minneapolis will have mid-winter lows almost 4 degrees above the current climate, but the summer highs in the Great Plains on average will be lower. My goodness! Minneapolis in the middle of January will have an average low of only 7 or 8 degrees, rather than a numbing 3 degrees on average now.{ }

The new average temperatures for the states also show an increase in annual overnight lows and daytime highs, even if mid-summer might not sizzle as much on average as this summer has.

Any snow lovers out there? The average snowfall for Washington will show a decrease of almost 1 inch of snow. Fret not: The averages for Baltimore and Dulles increase by about the same amount.

So what's the take-away from all this? In general, the nights are getting milder with the biggest impacts in the middle of the U.S. In climate, the higher annual range of temperatures in the middle of a land mass is called “continentality.” So maybe in a slowly warming and more moist world, some continentality is making itself known in the country’s midsection.

For utility budgets in our area, there will be slightly lower heating bills in the winter, plus not as much snow to shovel. But with a bit higher temperature in the summer and warmer nights, people may need to use more air conditioning. So we probably break even.

For lots more information on the new climate averages, visit the National Climactic Data Center.