There are a lot of superstitions that surround Leap Day...but how does February 29 affect science? Every four years we herald the arrival of an extra day in February. However, it’s a day that is both celebrated and loathed in the lives of climate scientists. Don’t worry, we’ll tell you why—Leap Day disrupts the regularity of the climate observation time series and the calculations that are derived from them require special attention. This is especially true for the determination of the U.S. Climate Normals. 

Leaping Right Into It

The current U.S. Climate Normals are calculated for climate stations using the period of 1991–2020. During this period, 365 days of the year have up to 30 observations if none are missing. Here’s the jump: there are only eight observations available for February 29. Because of this, scientists are not as sure about the normals for that date. To make things even more complicated, February 29 is part of the February monthly normals, the winter seasonal normals, and the annual normals. So, how do the climate scientists at the National Centers for Environmental Information (NCEI) resolve this conundrum?

Monthly Temperature and Precipitation

There’s an easy part and a complicated part of the explanation at hand. The easy part is that observations on February 29 are always part of the averages and totals for each February, so they are automatically part of the February monthly normals. For the monthly temperature averages, scientists add up all the days of the month and divide by 28 during regular years, and by 29 during leap years. For a cumulative value like precipitation, scientists add up all of the precipitation values in February, be they from 28 or 29 daily totals. Therefore, Leap Years are not a big issue for February monthly normals, which are made from all 30 Februaries from 1991 to 2020, nor is it a problem for winter or annual normal, which are based on the monthly normals.

Snagging Onto Issues

It gets more tricky when calculating the daily U.S. Climate Normals. Here is the main sticking point—when the daily normals for temperature and precipitation are averaged for a month, they have to equal the monthly normal average for that month. It’s impossible to complete these very complex procedures when the number of days changes from regular years to leap years. Therefore, we have a secret—NCEI scientists do not directly calculate daily normals for February 29. Instead, all of the daily normals are calculated for the typical 365-day year. 

Despite this shameful (albeit necessary) secret, it’s still important to know if temperatures are above or below normal, what the chances of precipitation are, etc. These values are not just left to the wayside. Daily U.S. Climate Normals are simply calculated the way most folks would: variables are averaged by the daily normal values from February 28 and March 1—but with one special exception. 

Precipitation and snowfall daily climate normals are not calculated on Leap Day since this statistic would introduce a lot of rounding errors in the effort to ensure daily and monthly totals are equal. 

Instead, NCEI scientists provide month-to-date and year-to-date cumulative values which are much more robust.

When calculating daily precipitation and snowfall normals, NCEI provides only month-to-date and year-to-date normals because that is how they usually use them: are we above or below normal for the accumulation through today? 

Just An Extra Day

Despite numerous science-related complications, Leap Day is a necessary part of our lives every four years. The arrival of February 29 means that our calendar will remain aligned with the orbit of the earth around the sun, which takes about 365.25 days to complete. Without Leap Day, the seasons would slide forward over time, and Climate Normals would get even more sticky than they already are. So use this day to celebrate, relax, perform an act of kindness, and/or regale your friends with tidbits from this web story. Whatever you choose, leap headfirst!