Ask Eric: Why Are Winter Skies Clearer For Star Gazing?

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Hi Eric - As I gazed up at the moon and adjacent planet on the coldest night we recently had, I wondered why it is that on the coldest nights, the sky seems to be the clearest.

What would cause a cold night sky to be clearer than any other?

Sincerely, Bruce Burchfield

Even on a cold and blustery January night, I'm sure you've looked up and been taken aback by the clarity of the heavens above. There are all sorts of treats in the night sky this time of year, from Jupiter and its moons, to the brilliantly shining Venus that graces the winter skies. And my personal favorite - a crescent moon that looks so crisp and clear it's like you can reach out and grab ahold of it.

So why don't we often notice this striking clarity as much in the summer, when it's a heck of a lot more comfortable to stand outside and take it in?! It's not just you - there are scientific reasons for this phenomenon.

The first reason may be an intuitive one. In winter, our skin is cracking and we're going through chapstick like it's bacon flavored. That's because winter air is often arctic air, which in turn is very dry. There isn't a whole lot of water vapor hanging in the winter sky (exception: this past December which was downright tropical). Tiny water droplets suspended in the air can help disperse or bend light, and in turn you get a fuzzier picture of objects farther away. The colder the night the better...it means your dew points are probably in the single digits, if not subzero.

Hazy Boston skyline from Blue Hills in summer - a combination of water vapor and some air pollution as well.

This is definitely a problem in the summer, when haze often hangs in the sky. Warmer air can in turn feature higher dew points, and we have more tropical air masses making their way northward from equatorial regions during June, July, and August. We also have a tendency to see stagnant air masses in summer when we go long stretches between fronts. Particulates and other bits of air pollution can collect in the air above us, adding further distortion to celestial viewing.

The other reason for your clear night sky is definitely not intuitive for most people. We all know about the Milky Way galaxy, which our solar system calls home. But did you know we live in the sticks? Earth is located out on a spiral arm of the Milky Way, called the Orion Arm (or Spur). Essentially, we're a rural location well outside the main city, which is the center of the Milky Way.

From this position, we view the big city during nighttime in the summer. The light of billions of stars fills our field of view. But in winter? We turn our heads to our own sparsely populated section of the Milky Way. In turn, there are fewer stars in our field of vision! Less clutter, more prominent stars and planets in the night sky, and deep space beyond instead of our galaxy's glare. All of this is reversed for those in the southern hemisphere.

It also just so happens that we have some very bright stars in our neighborhood, such as those that form Orion's Belt (also where our arm of the galaxy got its name). Lucky us! So don't waste these winter nights, even if it requires a few extra layers to go out and enjoy.