It’s winter in the northern hemisphere. That fact, combined with the arctic blast that’s reddening many cheeks in North America, means that many of us will enjoy a white Christmas. Since it’s a time to be thankful, here’s five reasons you should thank astrophysics for this year’s Winter wonderland. 

1. Axial Tilt
   Earth’s axis is tilted about 23 degrees relative to its orbital plane. As the Earth orbits the Sun, the northern hemisphere is tilted toward the Sun for about half the year, and tilted away from the Sun for half the year. When it’s tilted away from the Sun, the sunlight that reaches us is at a lower angle, meaning there’s less energy and heat reaching the Earth’s surface. That, combined with the fact that the Sun is visible for fewer hours in the day means the northern hemisphere enters a period of cold winter. Of course, for the southern hemisphere it’s reversed, meaning it’s a summer Christmas for those down under.
2. Elemental Abundance
   The most abundant element in the Universe is hydrogen, making up about 74% of cosmic matter by mass. Helium comes in second, making up about 24%. These two elements are so common because they were the elements formed soon after the big bang. Other elements of the periodic table are formed through astrophysical processes such as fusion in the heart of a star. This is true of the third most common element, oxygen, which makes up only 1% of the elemental mass in our galaxy. Helium is a noble gas, and doesn’t combine with other elements to form molecules, which leaves only hydrogen and oxygen. One of the most readily formed molecules from these two elements is H₂O, which is the primary ingredient of snow. Water is so common in the Universe because it’s formed from the two most common molecule-forming elements.
3. Heavy Bombardment
   Although water is common, it can evaporate away from a small, warm planet that forms close to its star. This is why Venus and Mars are so dry. We often think of Earth as a watery world, but it actually has less water than many moons of the outer solar system. We’re still not entirely sure how Earth came to have much more water than its planetary cousins, but one popular idea is that water was brought to Earth by asteroids and comets that bombarded our world during its youth.
4. Dalton Solar Minimum
   When you think of the winter season, you might think of Charles Dickens, whose stories such as A Christmas Carol have become a holiday staple. Dickens often wrote of a wintery Britain covered in snow, which may have helped drive the nostalgia we have for a snowy holiday. But interestingly, Britain doesn’t often have a snow-covered Christmas. In the 1900s, it was a white Christmas only seven times. But things were very different in the early 1800s, when Dickens was a child. Six of his first nine Christmas holidays were snowy. This period also corresponds to the Dalton solar minimum, which is a period between 1796 to 1820 when sunspot activity was unusually low. There’s some evidence to show that solar minima are correlated with colder temperatures on Earth. For example, the Maunder minimum spanning 1645 to 1715 is associated with the “little ice age” of Europe. It might just be a coincidence, but sunspot activity could be the source of Dickens’ holiday nostalgia.
5. Global Warming
   Although it is more properly called global climate change, the warming of Earth in recent decades could be the reason why it’s so cold in the northeast. It might seem paradoxical, but our cold temperatures are caused by the polar vortex dipping farther into North America than it usually does. The polar vortex is bounded by the jet stream, which typically moves in a smooth circle around the Earth. In recent years the jet stream has had a more wavy flow, and this may be driven by record warm temperatures at the north pole. As the arctic ice continues to melt and polar temperatures rise, the polar vortex can be pushed southward more often. It’s too soon to tell if this shift in the polar vortex is driven by climate change, but it is a possibility.

So there you have it. But whether your solstice holiday is wintery white or summery green, I wish you a joyful and peaceful season.

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