It’s Strangely Difficult to Measure Big Explosions

On Thursday morning, sequential explosions at a toxic chemical warehouse in Tianjin, China sent fire into the air. Hundreds are injured, and at least 50 people were killed, reports Andrew Jacobs for The New York Times. But just how big was the blast?

Not the size of the bomb dropped on Hiroshima. It doesn’t even come close to some of the largest non-nuclear explosions in history. But the rumble did trigger the U.S. Geological Survey’s seismometers 100 miles away in Beijing, writes The Guardian’s Amanda Holpuch, registering between 2 and 3 on the Richter scale. Other outlets noted that the blast could even be seen in space.

Nevertheless, at the time of the blast, many were confused about its size. That's a problem, writes Chris Mills for Gizmodo — one that has a lot to do with how scientists measure explosions.

Explosions are measured in terms of how much TNT (or trinitrotoluene) you would need to create an explosion of equivalent size. But that's where things get complicated, writes Mills. Here's why: At its core, an explosion is a big chemical reaction that releases energy. But, writes Mills, depending on the quality of TNT, that energy might range from 2000 to 6000 Joules. For the sake of measuring explosions, scientists use a constant 4184 Joules per gram to represent that range.

That's all pretty arbitrary, says Mills. Though he suggests scientists abandon the imperial system of measurement altogether and adopt a standard explosion measurement like Joules instead, a solution doesn't seem likely any time soon.

So how powerful was the explosion when measured in the less-than-perfect terms available to scientists today? The Guardian's Emma Graham-Harrison reports that scientists estimate that the first blast came in around 3 tons of TNT, and the second hit 21 tons of TNT (not kilotons, as some suggested). Comparatively, the nuclear bombs that fell on Hiroshima and Nagasaki were equivalent to 13 and 21 kilotons of TNT.

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Helen Thompson writes about science and culture for Smithsonian. She's previously written for NPR, National Geographic News, Nature and others.