Blowed-up Real Good

Blowed-Up-Real-GoodBy The Metric Maven

Bulldog Edition

I very much like to check-out Phil Plait’s Bad Astronomy blog. His measurement presentation causes me to generally wince, but its topics are often rather interesting. On 2016-02-20 he wrote about the Largest Fireball Since Chelyabinsk falling over the Atlantic Ocean. The object appears to have possibly exploded (or just burned up) about 30 Kilometers above the ocean surface. Plait assures us there is no cause for alarm; this was a small meteorite. He states:

For comparison, the Chelyabinsk explosion, which was strong enough to shatter windows and injure more than 1,000 people (due to flying glass), had an equivalent yield of 500,000 tons of TNT, 40 times the energy of this more recent impact.

Plait provides a link to the NASA/JPL Near-Earth Object Fireball page, and presents a cropped graphic from the site:

NASA-ClippedI was quite pleased to see NASA using international dating for the entries. It all is nice and ISO 8601. The total radiated energy from the meteor is given in joules. It is presented in scientific notation with an exponent to the tenth power. Here is a larger section of the NASA table with the data for the new meteorite in the first row at the top:

Blowed-Up-NASAThe total energy radiated from the meteor as it cruises through Earth’s atmosphere is about 685.3 x 1010 joules. Why an exponent of ten? We can see as we go down the list that all of the examples have this same multiplier. When presenting data, scientific notation is not good for numerical comparison. One could choose a metric prefix and have 6.85 Terajoules instead. As the values are typically smaller than this, one could switch to Gigajoules and write it as 6853 GJ. In my view presenting comparison data with scientific notation hinders intuitive understanding. I know that some computer languages have a setting that parses the exponents so they are presented in engineering notation by steps of 1000.[1] The exponent for all of the values given in the table can be expressed in Gigajoules. Below is a table with the recent meteorite and the Chelyabinsk radiated energies:


When Expressed in Gigajoules it is apparent just how much larger the radiated energy of the 2013 Chelyabinsk meteor was when compared to the recent one thought to have have exploded over the Atlantic Ocean.

When presenting the calculated total impact energy, NASA gets their Dr. Strangelove on and uses kilotonnes of TNT. As I’ve pointed, out the tonne is nothing more than the introduction of medieval measures into the metric system and should be eschewed. Worst of all, a tonne is a Megagram, and when you use a Kilo- prefix it produces a KiloMegagram. The last column is KiloMegagrams of TNT when actually expressed without hidden metric notation. It should be Gigagrams of TNT, but then metric is not NASA’s strong suit. The metric system has a well-defined unit of energy, the joule; the same one that was used with radiated energy.

One kilotonne of explosive is equal to 4.814 Terajoules. It appears that using Gigajoules is probably still a viable choice for presentation of the Calculated Explosive Energy:


In my view the change from joules to kilotonnes of TNT was just an anemic attempt at a “gee whiz” expression without making an actual comparison. An atomic bomb using nuclear fission ranges from about 4 000 Gigajoules to about 80 000 Gigajoules. The 2016-02-06 meteorite is in this range. The largest Hydrogen bomb exploded released an energy of about 210 000 000 Gigajoules. One can see the Chelyabinsk explosion of 2013 is about 23 times larger than a typical fission bomb, but is overwhelmed when compared to the amount of energy released by the largest hydrogen bomb.

How data is presented matters. I wish someone in a position to enforce change within NASA would create a position called a “numerical editor,” something like what in book publishing is called a copy editor, but concerned specifically with the presentation of numeric data for consistency and clarity. I leave you with a graph I’ve presented before. It shows that sometimes NASA presents data in an effective manner, they just need to make it consistent:

Asteroids-Joules[1] Fortran 90/95 for Scientists and Engineers Stephen J. Chapman McGraw Hill 1998 pp 534-535

The Liter is Not All Wet

Morgue-FileBy The Metric Maven

Bulldog Edition

My friend Pierre spends a lot of time browsing for backpacks and such. I suspect he has always wanted to runaway from home, but just never has found exactly the right luggage. One day he came across a backpack with a capacity of 1700 cubic inches or 28.7 liters. This caused him to think about a new unit which is appropriate for storing Jimmy Hoffa or other expired homo sapiens. Pierre saw no reason that he should not suggest a new unit for SI because he had discovered how compromised the liter is:

“So for that one moment in time, I thought about how we communicate volume to others. Moving hand gestures seem to work, but that doesn’t help in print advertising. Usually, we use “cubic inches,” or “cubic feet.”

But, the French get wet. They use quarts/liters/litres/litrons and cubic decimetres for everything, apparently. …”

Then Pierre goes for the jugular:

“Speaking of which, liters aren’t actually an SI unit? I’ve been lied to? Maybe you should get on that with your foreign pals. Or just toss it and use quarts like everybody else does.

As an example, note this bag on sale on Amazon this week, specifically the part I highlighted en rouge:


Unlike an insanely hot, but, hairy-armpitted, chain-smoking French girl, we smartly measure volume by linear methods cubically applied. They just go right to liquids. How funny would it sound for us to say this bag could contain 108 cups of coffee (real cups, not “coffee cups”) . One could kind-of picture that. But saying “this bag holds 27,000,000 cubic millimeters?” Not so useful.

Even a mostly dim marketer can immediately see that metric isn’t good for advertising AT ALL.

Unless this is a “wet bag” of some sort, isn’t the metric system inappropriate here?

Who uses wet measurements to measure dry things? Besides luggage and motorcycle/car engine manufacturers. Those goose-feeding, croissant-eating French, that’s who. Well, and baking measurements too. But that’s just wrong.”

Chat-WetThe good news is that Pierre’s understated, quiescent and measured questioning provides me with an excuse to explain the importance of the liter—otherwise known as the Rodney Dangerfield of the metric system. First one must realize that wet and dry volumes are equivalent, and no distinction is necessary. In cooking, wet measurement cups have a line below the top, and are generally clear. Dry measures are made so that the exact measure is at the rim of the cup. One can scrape them flat with a knife and have the exact same volume as the wet value, but in a way that works better for dry stuff. It was Isaac Newton who changed cooking forever by defining mass. After that point, much like the metric system, the English creation was adopted by the French. They realized that dry ingredients were best weighted in the Earth’s gravitational field, which allows one to back out the mass in grams. I think we know what happened to English versus French cooking at that point.

There is no distinction between wet and dry volume in reality; but in the imagination of English speaking people, somehow a magical change occurs. Exhibit A is the US Gallon (Wikipedia):

The US liquid gallon

The US gallon, which is equal to approximately 3.785 litres, is legally defined as 231 cubic inches.[1][2] A US liquid gallon of water weighs about 8.34 pounds or 3.78 kilograms at 62 °F (17 °C), making it about 16.6% lighter than the imperial gallon. There are four quarts in a gallon, two pints in a quart and 16 fluid ounces in a US pint, which makes a US gallon equal to 128 fl. oz. In order to overcome the effects of expansion and contraction with temperature when using a gallon to specify a quantity of material for purposes of trade, it is common to define the temperature at which the material will occupy the specified volume. For example, the volume of petroleum products[3] and alcoholic beverages[4] are both referenced to 60 °F (16 °C) in government regulations.

The US dry gallon

This gallon is one-eighth of a US Winchester bushel of 2150.42 cubic inches; it is therefore equal to exactly 268.8025 cubic inches or 4.40488377086 L. The US dry gallon is not used in commerce, and is not listed in the relevant statute, which jumps from the dry quart to the peck.[5]

The liter is fixed in value. It is a 100 mm x 100 mm x 100 mm cube. The gallon?—-not so much.

The liter was clearly designed by Father Nature (wait till Mother Nature finds out) as it is a cube with edges which are very close to the width of an average man’s hand. This allows an average man to estimate a liter of volume very quickly.

SI, in its semi-infinite wisdom, made the cubic meter the official unit of volume, and the liter was relegated to second class citizen status. When the Australians decided to become a metric nation, they were apparently far enough away from the bad influences of the US, Canada and the UK to realize (from Metrication in Australia):

Metrication In AustraliaYes, even applications that involve describing the volume of a backpack. The backpack could be described as 28 700 milliliters (or 28 700 – 10 mm cubes), but any person slightly acquainted with the metric system will immediately see 28.7 liters, and would not understand the importance of extra numbers for marketing purposes. When actually attempting to present numbers in an understandable way, the liter is excellent. Water has a density of 1000 grams/L. If any SOLID object has a density higher than this it sinks, if it’s lower it floats. Wet and dry coexisting in harmony, without an artificial separation, because of the liter.