How cold is it?

It’s a law of physics that there’s cold, and then there’s Korea cold. Korea cold is a whole ‘nuther thing. The cold is, not surprisingly, a common theme in the coverage of the winter Olympics. This provides a good excuse to rehash a favorite math trick. After all, these kinds of tricks stick better when there’s some immediate context for texture.

On the temperature front, Saipan is a Fahrenheit (often just abbreviated “F”) outpost in a Celsius (“C”) hemisphere. Being able to quickly make a rough equivalence between the two scales can come in handy. The rule of thumb we use for a quick approximation can also help us memorize, and use the formula for a precise calculation.

Here’s the rule of thumb to make an approximate jump from Celsius to Fahrenheit: “Double it and add 30.”

For example, one news report from Korea said the temperature at an Olympic event was minus 10 C.

To convert this to F, we first double the C. This gives us minus 20. And now we add 30 to it; this results in 10, so that’s our (approximate) answer: 10 F.

Easy, eh? Yes. And, at temperatures we usually encounter in normal life, it’s accurate enough for deciding whether you’re going to pack your mink coat or your Speedo. For example, minus 10 C, which we just approximated as 10 F, is really 14 F if precisely calculated. So our approximation was off by four degrees F. That’s good enough for many casual contexts.

But some contexts require more precision. We’ll now consider how our rule of thumb can help here, too.

You might recognize the classroom formula: F = (9/5) C + 32.

True, that formula will give you the precise answer, but it’s a clunky format for mental math. The “9/5” ratio is awkward. Fortunately, we can just convert this ratio to its decimal equivalent, which is a more user-friendly 1.8.

You’ll note that 1.8 is pretty close to the multiplier of two specified by the “double it” step in the rule of thumb I mentioned. And, yes, indeed, the “double it” is just a rounded-off version of the 1.8.

If you’re a quick study, you’ll apply this same logic to the “32” in the classroom formula; you’ll see that it’s also reflected in the rule of thumb in rounded form, namely, 30.

With this in mind, it’s easy to use the rule of thumb, “Double it and add 30,” as a template to remember the precise formula, which is to say, “Multiply by 1.8 and add 32.”

Now let’s get fancy with this stuff. We know that water boils at 212 F. How can we put this point into its Celsius equivalent?

When going F-to-C instead of C-to-F, we just climb down the ladder in reverse order of how we climb up. We’ll use the exact formula (as opposed to the rule of thumb) as an example. Facing this 212 F, first we subtract 32, which leaves us with 180. Then we divide that 180 by 1.8. This results in 100. So our answer is 100 C. Right? Right.

I have one last observation about “Double it and add 30.” The same phrase, using units of distance (as opposed to units of temperature), can be used as a basis for estimating descent points in high-altitude jet flying. We need not detain ourselves with the aviation details here, but I just wanted to mention why “Double it and add 30” has such a special place in my brain.

Rules of thumb are often used in dynamic environments such as aviation, seafaring, navigation, and military operations. Using a rule of thumb often enhances situational awareness since it gives our brains some traction in the muddy dynamics of the real world.

Of course, by contrast, blindly following electronic screens has become an affliction of the common mind. Never before has the road-most-traveled been so widely paved.

But wherever our roads lead us, it’s useful to know what the weather will be and what we should pack. After all, if it’s 30 degrees, we’ll have to figure out if we’re talking “F” or “C” before reaching for the coat, or, perhaps, the zoris.