Converting from Celsius to Fahrenheit

By now, perhaps you've noticed that the temperatures are generally reported in degrees Celsius, but we'll be forecasting temperatures in degrees Fahrenheit. We'll need to do a lot of converting.

I bet you still remember the formula for converting from Celsius to Fahrenheit, or at least recognize it when you see it:

F = C * 9/5 + 32

A simple enough formula, right? WRONG!!! No meteorologist uses this formula to convert from Celsius to Fahrenheit unless they want the answer to the nearest tenth of a degree or something. We prefer something that's a lot easier and just requires a few key numbers to be memorized.

The first key number, you already know. It's that 32 F = 0 C. See, that wasn't so hard. The next two numbers to remember are 10 and 50. A temperature of 50 F corresponds to a temperature of 10 C. Two nice round numbers, easy to remember.

What do we have so far? 32F = 0C, 50F = 10C. Note that a change of 18 degrees Fahrenheit (from 32 to 50) corresponds to a change of 10 degrees Celsius (from 0 to 10). That's true no matter what the temperature. With this information, we can quickly construct a conversion table:

      Celsius      Fahrenheit

        -20           -4

        -10           14

          0           32

         10           50

         20           68

         30           86

         40          104 

Aside from 0=32 and 10=50, another easy one to remember is 20=68: 20 C is room temperature.

The next rule you could figure out from the first one: a 9 degree change in Fahrenheit corresponds to a 5 degree change in Celsius. You can use this to expand the table a bit. Here's an expanded section:

Celsius      Fahrenheit

        10            50

        15            59

        20            68

Now the trick. To do a specific Celsius to Fahrenheit conversion, you guess! That's right, guess. Don't worry about doing any math. Take a look at the table again. Every one degree Celsius is about 2 degrees Fahrenheit. So find the nearest 5 degree mark, and go up or down from there.

If 11 C = 52 F

12 C = 54 F

13 C? Work down.

15 C = 59 F

14 C = 57 F

13 C = 55 F

Once you've got every five degrees figured out or memorized, the biggest number you ever have to add or subtract is four. That's pretty easy to do in your head.

Let's try another one. What's 31 C? Answer: since 30 C is 86 F, 31 C must be about 86 + 2 = 88 F.

For converting backwards from Fahrenheit to Celsius, just do the same thing in reverse, rounding away from the benchmark numbers every 5 Celsius. For example, what's 76 F? Well, 25 C is 77 F, so 76 F must be about 24 1/2 C. Round down to get 24 C.

With a little practice, you should get good at this pretty quickly. Every time you see a Celsius number, convert it to Fahrenheit. Every time you see a Fahrenheit number, convert it to Celsius. Soon you'll be multithermal.

Those of you who are mathematically inclined (and if you're not you'd better become so in a hurry in this major) should, with some thought, be able to figure out your own way of being able to convert down to the nearest tenth of a degree. I can do it some of the time.

Convert the following Fahrenheit and Celsius numbers:

1) 50°F

2) 48°F

3) 7°F

4) 30°C

5) 15°C

6) 37°C