A look at weather extremes and the big-picture climate connections.

2012 Heat Wave is Historic, if not Unprecedented

Now that the heat wave has finally subsided, we can begin to take stock of how unusual it really was. There is no question that it was extraordinary in its intensity and geographical scope for so early in the summer season. But was it unprecedented in American history?

The short answer is no, it wasn’t unprecedented. The longer answer is that it will go down in history as one of the hottest and longest-lasting early summer heat waves in U.S. history, and by at least one measure, it was the record setter in every sense of the phrase.

It was also a clear taste of what’s to come as the climate continues to warm due to manmade global warming.

An illustration from the U.N. Intergovernmental Panel on Climate Change (IPCC), illustrating how a shift in mean temperature makes warm temperature extremes more likely to occur.

To dig deeper into the numbers, you have to compare the 2012 June-July heat wave with the epic U.S. heat waves of the past, two of which stretch all the way back to the Dust Bowl era.

So I compared how the record totals from this heat wave stacked up against data from some of those hottest summers — 1988, 1954, 1936, and 1934.

(Note that my number-crunching doesn't exactly compare one heat wave to another, but rather looks at the number of records set over a slightly longer timespan. It also omits record warm overnight low temperatures, since these records are not available via the NCDC's database for years prior to 1993.)

During June 2012, there were 3,282 daily record highs set or tied across the country, according to preliminary numbers from the National Climatic Data Center (NCDC). These figures are likely to rise in coming weeks as more records filter into NCDC. During the first week of July (July 1-7), there were 2,278 daily record highs set or tied.

These records include the sweltering 105°F high temperature in Washington, D.C. on Saturday, which was its second-warmest high temperature on record. Washington had a record 11 straight days with temperatures of 95°F or higher, and tied the record for four consecutive days of 100°F or higher.

It turns out that the total number of daily records this June were about the same as occurred during the month of June during two of America's hottest summers — 1936 and 1934, both of which occurred during the Dust Bowl.

However, June of 1988 blew this June away, with 9,800 record highs set that year. Not coincidentally, 1988 was the summer when massive wildfires charred Yellowstone National Park, and global warming first made national headlines when NASA climate scientist James Hansen testified before the Senate.

The tally of monthly and all-time record highs shows that June of 2012 was still an extraordinary month. Of the 3,282 daily records this June, 645 qualified as monthly record highs, and 173 were all-time highs. In 1988, there were 963 monthly records set or tied in June, and 163 all-time records.

So June of 2012 tied or broke more all-time records compared to the month of June during those previous hot summers.

During the period from July 1-7, 2012, there were 2,278 record daily highs set or tied in the U.S. This beat the same period during 1988, 1954, 1936, and 1934. That's not surprising since the time period of individual heat waves differ. More monthly record highs were set or tied during this period than during the other hot summers as well. Again, given the varying timeframes, that's not surprising. July 1-7 of 1936 came the closest, with 155 monthly records set or tied during the period.

Jet stream pattern during the height of the heat wave, on July 5, 2012.

Interestingly, in terms of all-time records, July 2012 came in second, with 119 all-time records set or tied, behind the 134 records set or tied during July 1-7, 1936. (Many of the all-time records set or tied this year with records that had been set in 1936.)

Because there is a much longer period of record today than there was during the previous hottest summers — especially when viewing the marks from the Dust Bowl era in the '30s — it is now more difficult to set monthly and all-time records. This argues in favor of viewing the 2012 heat wave as a more impressive event.

On the other hand, the number of records in 2012 may be inflated somewhat, since there are more weather observation stations now than there were during the previous hottest summers.

One thing I’m curious about is whether the atmospheric conditions that led to this heat wave were as unusual as what took place during past hot summers. We already know from studies that global warming makes it more likely that heat extremes will occur.

Given the background warming that has already taken place in the U.S. and elsewhere, it’s reasonable to think that while a record-breaking heat wave used to require a massive, unusually powerful dome of high pressure in order to occur, now the same record could be set with a weaker weather system.

The heat dome that was present during this year’s event was persistent and formidable, but it did not set records for its strength. In fact, it’s possible that with a stronger High Pressure center, temperatures could have shot up even higher than they did.

I'll leave you with Climate Central's Record Temperature Tracker, which allows you to investigate the records as they were set this year. I recommend clicking "play month" to see the heat records sweep across the country during late June into early July.

« Extreme Planet


By John Bolduc (Concord, MA 01742)
on July 9th, 2012

Thanks for this clarifying article.  How do you assess the strength of the heat dome?  I think you are saying that we have experienced a near record event but that it occurred with a weaker heat dome that occurred in 1936.  So that would be troubling.

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By Albert Hall (Gaithersburg, MD 20878)
on July 9th, 2012

What is especially amazing, I suppose, is that this 2012 heat wave broke records that had been set in those (and other less potent heat waves). The “bar” is now set even higher. So, actually, what would be nice to see is what the average new record temp this time around is compared to those of the older heat waves.

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By larry (akron ohio 44302)
on July 9th, 2012

we have had alot of sun spots recently, i believe that the sun is heating up because it is an old star and old stars get hotter as they age, but instead of looking at science, people are believing polititians that believe in greenhouse effects, back at the turn of the century when everyone burnt coal, there were more greenhouse gases in our atmosphere than there are now.  in the near future polititians are going to charge or tax people for the polutants that they create by travel ,heating,cooling your house or building, production of products, etc.
it is already happening at the country level,, by hydocarbon credits, it will soon happen at the local level. remember its just a way to tax you and you’ll be happy to pay it.

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By Richard (Amherst, NH 03031)
on July 12th, 2012

The sun is a middle aged start now, and that has not changed over the past century (no where near old yet). The sun is about 4.5 billion years old now. When it reaches about 7 billion years of age, it will begin to enter it’s next, older stage. At about 7 billion years old it begins to turn into a red giant., which will take another 3 billion years to happen. At an age of about 10 billion years, it will be a red giant, having expanded to a size that reaches almost the earth’s orbit.

There are more greenhouse gases in the atmosphere now than there has been for at least the past 600,000 years. The major greenhouse gas is water vapor. When water vapor reaches an upper limit (dew point), it rains, reducing the level. The other GH gases, mostly CO2, methane, nitrous oxides, and ozone, have all been added to the atmosphere by human activities.  They are all pollutants, and too much of any of them would make human life on this planet very difficult, or even impossible.

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By liz thornton (2096)
on July 15th, 2012

I am not a scientist however when I read that twaddle from By larry, I realise that he comes from a different planet and he and his peers want to give us all brain transplants. To say that we were all burning more coal than now is pathetic. The world has gone mad ripping out massive chunks of Forest.while at the same time Coal mines chug all day all night to supply the world with polluting fuel so that we can keep our lights on all day and all night.
As we warm up so we use more air conditioners and more coal. We did not have air conditioners in the good old days Barry Remember ?

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