Half the population is suffering under ‘heat advisories’. These kick in when the heat index—a measure of perceived temperature that takes humidity into account—surpasses 105°F (about 40 °C), or when the nighttime low exceeds 80°F (about 27 °C) for two consecutive nights.

Here’s what the Capital Weather Gang has to say:

East of the continental divide, it’s difficult to escape today’s searing heat. NOAA reported that as of 1 p.m., heat advisories or excessive heat warnings affected 150 million Americans in 23 states. Washington, D.C. had been under a heat advisory earlier today, but it was canceled when it became clear temperatures would fall just below advisory criteria.

Almost all of the south central and southeast states have seen heat indices exceed 105 degrees Tuesday afternoon. Some sample readings at 3 p.m.: Little Rock 109, St. Louis 109, Raleigh 105, Memphis 111, Charleston 108.

In recent days, the searing heat has set scores of new record high temperatures across the eastern two thirds of the country. Yesterday alone, 41 record highs were set including Ft. Smith, Ar. (107), Indianapolis, In. (96), Louisville, Ky. (97), Watertown, Ny. (90), Altoona, Pa. (94), and Charleston, WV (95).

Record high minimum temperatures have been more even pervasive, offering little nighttime relief from the oppressive afternoon heat. On Monday, 132 record high lows were set.

In Louisville, Kentucky this morning, the low dropped to a mere 84 degrees. Meteorologist Eric Fisher at The Weather Channel tweeted: “That. Is. Filthy. Heat Index was still above 100 at 5am.”

Some of the most remarkable heat occurred on in central Plains on July 9 and 10. Oklahoma City reached 110 degrees on the 9th, tying its all-time high for the month. Wichita, Kansas rose to 111 degrees on the 10th, its hottest temperature in 30 years. See CapitalClimate for more on the records which extended into Arkansas and Missouri.

In both Oklahoma City (13 days) and Dallas (10 days), the mercury has reached 100 or better for at least ten straight days. Hot weather is predicted to persist there through the weekend, at least.

Across the country during the month of July, record highs have outnumbered record lows 349 to 68 (or more than 5:1).

Could any of this be related to, umm, global warming? Joe Romm has a blistering critique of the American media’s failure to mention this possibility:

• Joe Romm, After Story on Monster Heat Wave, NBC Asks “What Explains This?” The Answer: “We’re Stuck in a Summer Pattern”!, Climate Progress, 13 July 2011.

Inference is a tricky business. It’s easy to spot patterns where they don’t exist, especially when the patterns are as subtle as an increase in extreme weather events, also known as ‘global weirding’. If there’s a flood, or a drought, we can easily explain it this way. The human mind, after all, is programmed to seek out patterns: we can see faces in clouds.

But it’s also easy to fail to recognize patterns where they do exist—especially when acknowledging them would require difficult changes in behavior. “Am I an alcoholic? No, I just got really drunk last night… and, okay, the night before…”

Earlier this spring, Bill McKibben had a sarcastic editorial about this:

• Bill McKibben, A link between climate change and Joplin tornadoes? Never!, Washington Post, 24 May 2011.

It starts:

Caution: It is vitally important not to make connections. When you see pictures of rubble like this week’s shots from Joplin, Mo., you should not wonder: Is this somehow related to the tornado outbreak three weeks ago in Tuscaloosa, Ala., or the enormous outbreak a couple of weeks before that (which, together, comprised the most active April for tornadoes in U.S. history). No, that doesn’t mean a thing.

It is far better to think of these as isolated, unpredictable, discrete events. It is not advisable to try to connect them in your mind with, say, the fires burning across Texas — fires that have burned more of America at this point this year than any wildfires have in previous years. Texas, and adjoining parts of Oklahoma and New Mexico, are drier than they’ve ever been — the drought is worse than that of the Dust Bowl. But do not wonder if they’re somehow connected.

If you did wonder, you see, you would also have to wonder about whether this year’s record snowfalls and rainfalls across the Midwest — resulting in record flooding along the Mississippi — could somehow be related. And then you might find your thoughts wandering to, oh, global warming, and to the fact that climatologists have been predicting for years that as we flood the atmosphere with carbon we will also start both drying and flooding the planet, since warm air holds more water vapor than cold air.

It’s far smarter to repeat to yourself the comforting mantra that no single weather event can ever be directly tied to climate change. There have been tornadoes before, and floods — that’s the important thing. Just be careful to make sure you don’t let yourself wonder why all these record-breaking events are happening in such proximity — that is, why there have been unprecedented megafloods in Australia, New Zealand and Pakistan in the past year. Why it’s just now that the Arctic has melted for the first time in thousands of years. No, better to focus on the immediate casualties, watch the videotape from the store cameras as the shelves are blown over. Look at the news anchorman standing in his waders in the rising river as the water approaches his chest.

Luckily, scientists are busy at work on these questions. For example, these papers on floods came out in February:

• Pardeep Pall, Tolu Aina, Dáithí Stone, Peter Stott, Toru Nozawa, Arno Hilberts, Dag Lohmann, and Myles Allen, Anthropogenic greenhouse gas contribution to flood risk in England and Wales in autumn 2000, Nature 470 (17 February 2011), 382–385. Supplementary information available for free online.

• Seung-Ki Min, Xuebin Zhang, Francis W. Zwiers and Gabriele C. Hegerl, Human contribution to more-intense precipitation extremes, Nature 470 (17 February 2011), 378-381.

I believe that someday we will understand whether and how extreme weather events are linked to global warming—if not individually, at least statistically. Whether we’ll understand it soon enough for it to make much difference—I’m less sure about that.

Luckily, I’m back in Singapore now, so I don’t personally have to worry about the heat wave in the USA. No heat advisory here! The weather is quite normal, with the heat index a nice cool 100 °F (or 38 °C).