On average what’s the coldest day of the year? In the northern hemisphere, winter begins on the winter solstice, which is the shortest day of the year. Actually, it’s not the shortest day, but rather the day where the sun is above the horizon for the fewest hours. FWIW, this year it’s on 21 December 2022. Since the sun is responsible for heating our planet (ignoring the small contribution of volcanic activity), the coldest day should be near the day with the least solar exposure, right?
Not Exactly
While solar radiation is certainly the main driver, it’s not the only factor impacting temperatures. Water plays an important role, too. Several aspects of water contribute. First, water cools more slowly than air or the surrounding land. Thus large bodies of water act as heat reservoirs keeping nearby land warmer. In the great lakes region, temperatures near the shore are typically several degrees warmer than further inland. Another factor is that snow and ice reflects sunlight. On sunny days, snow can reflect as much as 90% of the ultraviolet radition. Which is why skiers should protect exposed skin with sunscreen. So even as the days get longer, areas of the country with snow cover continue to get colder. Finally, areas with high humidity cool more slowly than drier areas. How many times have you heard someone say, ‘It’s 100 degrees, but it’s a dry heat.’? The same thing, but in reverse applies to cold days.
Net Result
So when can you expect the coldest day of the year? Well, fortunately, the National Oceanic and Atmospheric Administration (NOAA) has collected the data and produced a map for us.
As always thanks for reading.
Armen
Note to Pay the Bills: Celtic holidays and seasonal transitions play a key role in the Misaligned series. In Book #1 of the series, Penny Preston and the Raven’s Talisman, climaxes on the winter solstice. How does that fit in with a YA fantasy story about the intersection of theoretical physics and Welsh mythology? You can learn more here or find links to purchase books here.
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