Science

When Is The Official Start Of Winter?

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The official start date for winter can be a bit confusing. If you say winter starts on December 1, you are correct. But if you say winter begins on December 21, you are also correct. So, what’s the difference, and which one is right? Both dates are valid, although the two dates reflect two different calendars: meteorological, which states winter starts on December 1, and astronomical, which states winter starts on December 21. Why should you care? For scientists, businesses and anyone looking for accurate statistical data about the seasons, the meteorological calendar offers the most accurate insights from which informed decisions can be made for the safety of people, products and property.

The primary difference between the calendars—besides the date—is that on the meteorological calendar, the seasons follow the annual temperature cycle. In the astronomical calendar, the seasons follow the position of Earth with the sun. Winter solstice, the start of winter according to the astronomical calendar, is when the top half of the Earth is tilted away from sun, while the bottom half is tilted toward the sun. This makes the noontime sun in the northern half of the earth the lowest it will be for the year. While humans have been using the astronomical calendar for thousands of years, the meteorological calendar was established in the early to mid-1900s and is separated into four even sections of three months each. It’s built on the premise that summer is the warmest time of year and winter is the coldest, with transition seasons in between.

Unlike the astronomical calendar, each season in the meteorological calendar is always 90 to 92 days long and always starts on the first of a month. This clear definition makes it easier to calculate seasonal statistics, ultimately creating a better understanding of weather events.

When we look specifically at winter, the reason December 1 is used as the start date for meteorological winter—besides being close to the start of the coldest 90-day period—is also because winter will always start on that date, whether it is the year 1925 or 2025.

In comparison, because the astronomical winter is based on the solstice, there is more variability. There are differences in season length between hemispheres and differences from year to year as well. For example, this year, the astronomical winter starts on December 21 and runs through March 19. But next year’s astronomical winter will end on March 20 in the U.S. Some years the winter starts can be different even within the different regions of the U.S. when the beginning of astronomical winter is close to midnight. Because there is so much variability with the start and end dates with this calendar, it can make accurate record-keeping difficult.

The meteorological calendar allows for accurate comparison of data for each season, whether it’s by month, partial months or even days. The accuracy of the seasonal statistics in this system allows for average temperatures to be conveyed to the public or business planning. It provides more accurate insights into understanding the risks and seasonal trends for businesses ranging from retail stores to agriculture.

When looking at the meteorological data, you can see some interesting temperature trends. For example, the coldest 90-day period in the U.S. begins before December 1 across the country’s Southwestern region and after December 1 across the Northeastern region.

The data also shows that for every location in the lower 48 states, the coldest 90-day period always begins within 12 days of December 1. If you average the temperatures in cities across the continental U.S., the coldest 90-day period starts on November 30. It’s also worth noting that there is no U.S. city in which astronomical winter is colder than meteorological winter.

This year is proving to be interesting as we head into winter with what was already a strong El Niño event getting even stronger. Whether it feels like winter on December 1 this year, know that the cold is coming. With El Niño firmly in place and remaining strong through at least the first half of winter, much of North America will see above-normal temperatures, with some very mild periods expected from the Northern Rockies all the way through the Northeast. The main storm track will be established from California across the Southern Plains into the Southeast states, with above-normal precipitation and bouts of strong to severe weather.

More importantly, with the meteorological calendar’s structure, we can accurately track that information for public safety and provide business insights thanks to this modern calendar system.

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