Cold.  Skin lacerating cold.  Bone gnawing cold.
The cold that frosts the mitochondria, cracks open continents, liquefies oxygen and behaves as though everybody in the street murdered its father just to marry its mother and govern the state of Denmark.   How can a Universe benign enough to give us poppy fields and calculus problems possibly be responsible for such inhumane conditions?   While this question might stump philosophers, astronomers can furnish us with a direct answer.  While this answer will do little to ameliorate our misery, it will at least lend us the comforting assurance that the fierce frigidity will be of finite duration.

We'll begin with the solar system's prodigious energy source, the blazing Sun.   Like its stellar brethen, Sol is ferociously energetic!  Every second, the Sun emits enough energy to power Earth's civilizations for 500,000 years!  Or, if you're a violence and destruction admirer, it is also equivalent to the energy release of one trillion megaton nuclear bombs.   Earth receives a minuscule fraction of this energy output.  However, that fraction is equal to 400 trillion trillion watts: a value known as the solar constant.   One would think that such a copious inflow would make the world a sweltering sauna.  But, remember, this energy is distributed unevenly over the planet.  It is tantamount to winning the power ball lottery along with every inhabitant of greater Los Angeles, Chicago and New York City.  Your portion becomes quite minute through dilution.

The uneven distribution is principally responsible for the winter's chill. Earth is titled on its axis by approximately 23.5 degrees, a value called the "obliquity."    As Earth revolves around the Sun, the orientation of the poles constantly changes.  At the June solstice, the north pole is aligned as closely toward the Sun as possible.  Summer begins in the Northern Hemisphere and winter starts in the Southern Hemisphere.   Six months later, at the December Solstice (Dec 21st this year) the south pole is aligned most closely with the Sun and the north pole is pointed away.   Winter begins in the Northern Hemisphere; Summer starts in the Southern.     

Why would a tilt cause the weather conditions to change so profoundly: from the beautiful summer of verdant landscapes caressed by warm zephyrs to the austere winter where razor sharp gusts rake across the iron hard Earth?   Ascribe the difference to the atmosphere and its tendency to absorb some of the sun's incidental radiation.  We live under  protective blanket of gases and, to be fair, this blanket also retains a lot of heat and makes the planet warmer than it would otherwise be without it.   However, the absorption rate depends directly on the amount of atmosphere separating the land and outer space. (To become briefly meteorological, the Sun heats the ground which imparts this energy into the air.)  The amount of absorption atmosphere decreases with increasing altitude.  For instance, there is forty times the amount of atmosphere between you and the horizon as exists between you and the zenith (the point directly overhead).  So, in the winter, when the Sun doesn't attain a high altitude in your sky, it will heat you less efficiently that it does in the summer.   

The notion that the Sun is closest in the summer and we're warmer as a result is logical, but incorrect.  In fact, Earth is closest to the Sun in January and farthest away in July.     Though the distant difference between the closest point (perihelion) and furthest (aphelion) amounts to about three million miles, the effect on our weather is slight, perhaps even negligible.  The reduced solar altitude resulting from Earth's tilt and causing the increased atmospheric absorption of the Sun's energy lowers our temperatures and chills our part of the world.

One interesting note related to perihelion.    Since Earth is closest to the Sun in the Northern Hemisphere's winter, the planet is moving fast in its orbit.  (Remember, Kepler, that the closer a planet is to the Sun the faster it revolves.)   Consequently, winter is the Northern Hemisphere's shortest season; summer is the longest.    The difference is a matter of a few days, but the fact that winter is brief compared to the other seasons is at least some comfort.