Obviously, it’s winter – it’s February, and I live in the northern hemisphere. And while my son loves the snow, he’s not so certain about this cold nonsense. Particularly first thing in the morning. So one morning he comes lumbering into the living room, still wearing his red flannel footie pyjamas and dragging half of the blankets from his bed. He climbs up on the couch, snuggles down under all the blankets, and peers at the snow through the sliding glass door.
“Why is it cold?” he asks.
“It’s winter,” I answer.
“But why is it cold?”
Well, you stumped me there son. I don’t really know. I know that the Earth tilts on its axis, but the 23 degree tilt doesn’t seem like it moves us far enough away from the sun to make things cold. So, I have no idea.
Fortunatly, the internet does. And the answer, it seems, answers a second question he had: “Is winter over?”
Axial Tilt Is The Reason For The Season
Let’s talk about orbital mechanics for just a moment. The good folk at Hyperphysics have a lovely little set of explanations about where seasons come from, and it all starts with something called the celestial sphere. This is an imaginary sphere with the Earth at its center, a “north celestial pole” above our own geographical north pole, and a “south celestial pole” above our geographical south pole. The sun appears to trace a path through the sky that is tilted at 23.5°to the equator of the celestial sphere, and this path is called the ecliptic.
In truth (and I hope I don’t have to explain this one), the Earth orbits about the sun on a plane and the Earth’s axis is tilted relative to that orbital plane. But seasons seem to be explained in terms of the ecliptic plane. Here’s how they work:
- The Summer solstice occurs when the Sun’s path on the celestial sphere draws as close to the celestial pole for the hemisphere you live on as possible.
- The Winter solstice occurs when the Sun’s path on the celestial sphere draws as close to the celestial pole for the hemisphere you don’t live on as possible.
- The autumn and spring equinoxes occur when the sun’s path on the celestial sphere intersects the celestial equator, with the apparent direction of travel determining which equinox it is.
So, is winter over? Not until the sun “travels” far enough along the ecliptic plane to move from the winter solstice to the spring equinox.
Axial Tilt Is The Reason You’re Cold
Despite my assumptions, axial tilt is why you’re cold in winter and hot in summer. But it has less to do with “distance from the sun” and more to do with “energy per unit of area received from the sun”. See, the Sun generates 6,300,000 mW/cm2 (milliwatts per square centimeter) at it’s surface. This energy spreads out, however, and by the time it reaches the Earth we receive only 137 mW/cm2 at the Equator, on average. This is known as the solar constant.
The Earth is curved, however. As a result, something called the cosine effect kicks in.
Without delving too much into the math – because it’s been forever since I took trigonometry – the actual amount of energy the Earth receives above the equator is decreased, because the Earth’s axial tilt causes the 137 mW/cm2 to spread out over more than a single cm2. To see this in action, turn a flashlight on and point it at a piece of cardboard. Angle that cardboard away from you, and watch how the light spreads out. Since we’re kept warm by the energy received from the sun, the less of it we receive the colder we are.
There’s A Few Other (Grossly Simplified) Factors To Consider
The sun is clearly the big dog in this fight – after all, if the sun went out we would all freeze and die.
There are other factors, though. Altitude above sea level matters, because the thinner the air gets the less heat the air can retain (grossly simplified). Cloud cover plays a role as well – daytime clouds reflect solar energy (reducing the amount absorbed by the Earth) and nighttime cloud cover helps retain heat (grossly simplified). The amount of surface water is important, because water holds more heat than the air (grossly simplified). Ocean currents help distribute this warm water around the globe as well, altering tempertures to a significant degree (grossly simplified).
Really, at the end of the day, anything we say about the causes of winter cold – whether it’s me or a climate scientist – is going to be grossly simplified. Somewhat more grossly simplified, in my case, but simplified no matter what. The Earth is a complicated set of interacting moving parts, after all. But it all starts with the Sun, and our axial tilt.