Weather Explainers

Why Minus 40 Is A Magical Number In Weather

At a Glance

Minus 40 degrees Fahrenheit is exactly the same as minus 40 degrees Celsius.
Alaska hosts unique celebrations when the temperature drops into the minus 40s.
Some wind chills will drop to this number in the next few days across the country's northern tier.

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If you've ever tried to tell your friends about the weather in another country, there is likely one number that becomes a point of confusion: The temperature. For the Fahrenheit-faithful, 50 degrees sounds chilly, but for the Celsius club it might sound like you're going to roast your you-know-what off.

But there's one number in meteorology that lines up exactly between Fahrenheit and Celsius: minus 40 degrees!

When the actual temperature dips down to minus 40, you'll often see people lining up at the university sign in Fairbanks, Alaska, to catch the occasion on camera. There's even an official "40 Below Club" for the tradition.

Women joining the 40 Below Club in Fairbanks, Alaska, on January 12, 2019.
UAB via Facebook (University of Alaska Fairbanks) — Women joining the 40 Below Club in Fairbanks, Alaska, on January 12, 2019.
(University of Alaska Fairbanks via Facebook)

Why the celebration when it is so cold?

The two temperature scales line up directly at that frigid number due to the math conversion between the two. We apologize in advance for the upcoming math!

If you're American and trying to figure out what your friend in France is talking about, you're likely to use the following formula:

(Temperature in Celsius * 1.8) + 32 to get the temperature you're more used to.

But why is minus 40 still minus 40 in both scales? Sorry, but we're going back to the math! We'll start by plugging in minus 40°C:

(-40°C * 1.8) + 32 = ??. Let's show our work...

(-72) + 32 =

-40.

If you want to go the other way, you can use this formula: (F - 32) / 1.8 = C, but for now, we've had enough math.

Upcoming -40s In The Forecast

If you really want to impress your friends and family, you can use the next few days as an example. While temperatures in the Lower 48 won't get quite that cold, a few states in the North will see conditions feel like it will be -40 degrees when you factor in the wind.

(MORE: Record-Cold Temps On The Way | The Wind Chill Explained)

You can track the current wind chill below.

Jonathan Belles has been a graphics meteorologist and writer for weather.com for 8 years and also assists in the production of videos for The Weather Channel en español. His favorite weather is tropical weather, but also enjoys covering high-impact weather and news stories and winter storms. He's a two-time graduate of Florida State University and a proud graduate of St. Petersburg College.