When you’re on vacation, it’s often important to know what the weather is going to be like. International travelers from the United States often become confused because in almost any other country (apart from the USA), weather forecasts are given using Celsius rather than the Fahrenheit scale that is so familiar to us. We are almost conditioned to understand Fahrenheit but not Celsius!

Using the Fahrenheit scale, we know that 70s means fairly warm and pleasant. 20s means really cold — and we don’t have to think about it. But imagine waking up in an Irish hotel and you’re told that the temperature “is expected to reach a high of 16 today.” Are you going to wear that heavy sweater or is it a day to stroll in the countryside in your shorts and T-shirt?

Most people know the formula to convert Celsius to Fahrenheit:

°C  x  9/5 + 32 = °F

So let’s say you have 16°C.  You’d convert it as follows:

Multiply by 9:  16 x 9 = 144

Divide by 5: 144/5 = 28.8

Add 32: 28.8 + 32 = 60.8

Three steps! It’s not very difficult, but hardly the kind of activity I like to do when I am waking up on a vacation morning. So I have a quick and dirty way of estimating the temperature. I keep two “magic” numbers in my head: 16 and 28. I simply reverse these numbers to convert. So, 16°C = 61°F, and 28°C = 82°F. That’s pretty easy. Not totally accurate because I am .2 degrees off at 61°F and .4 degrees off at 82°F. But it’s certainly accurate enough for vacation time!

So, how about the numbers in between: 21°C, for example? Well, I know what 16°C is 61°F. 21°C is five more than 16°C. I double the difference (5 x 2 = 10). So, my quick and dirty method gives me 71°F. It’s only about one degree less accurate than doing it the old fashioned way, but simple to do while I am half asleep listening to the weather forecast in my Irish hotel.

Or if I have my handy smart phone with me, I simply type “21 C to F” into Google. The answer pops up as 69.8! Perhaps the fastest and most accurate way of all!