Why Is It Cold?

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

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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

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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.

cosine

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.
sunless_freeze

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.

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What If The Oceans Froze?

My son has a fascination with ice. He loves it in drinks (something he gets from me, more than from his mother), and he loves to look at it. We play games with it, like the time I put an ice cube in a bowl for him so he could watch it melt. Or the time we left a cup of water outside to see it freeze. I think he likes the idea that water can turn into a solid, and the fact that it’s cold is just a bonus bit of entertainment.

So we’re driving to church one Sunday, and looking at the snow that’s covered everything – one of the few days this winter where we’ve actually had snow – and he asks me “what if the oceans froze?”

Well, that sounds like an apocalyptic scenario if I’ve ever heard one. “Froze solid?” I reply?

“Yes! So we could ice skate on them!”

Bear in mind that my son has never gone ice skating. So I have no idea where that came from. But the question is interesting. And, sadly, nowhere near as much fun as he’d hope.

When Does Salt Water Freeze?

To start with, ocean water has a much lower freezing point than freshwater. In “Can the ocean freeze?“, NOAA informs us that seawater freezes at 28.4 degrees Fahrenheit (which is -2 degrees Celsius), because of the salt. They also tell us that the average temperature of all ocean water is about 38.3 degrees Fahrenheit (3.5 degrees celsius). So, in theory, to freeze the oceans we’d simply need to reduce the average ocean temperature by 9.9 degrees Fahrenheit (5.5 degrees Celsius).

How cold would it have to get?

Interestingly, the simple truth is that all you’d have to do to freeze the ocean is get the air below the freezing point of the ocean. Then, eventually, you’d manage it. That would require bringing the average equatorial temperature down to that level, and the best figure I could find for that average temperature is 77 degrees f (25 degrees C). That’s a 38.7 degree F (21.5 degree C) difference. This would bring average global temperatures down to 22.3 degrees F (-5.5 degrees C), so things would be terribly cold. To put it in perspective, polar climates have an average temperature of 50 degrees F (10 degrees C).

Killing Frost

A killing frost is a temperature that will kill a plant entirely – not just damage the extremities. Corn and soybeans will die below 28 degrees F. Wheat is a little hardier, depending on the growth stage, but will pretty much die at 24 degrees F (-4 degrees C). So if it got cold enough to freeze the oceans, we’d be living in a permanent killer frost.  Which is another way of saying that we’d be in huge trouble.

Snowball Earth

Interestingly, this may have happened before. There is something called the Snowball Earth hypothesis, that says the Earth may have been frozen like this some 650 million years ago. Equatorial temperatures would have been around what present day Antarctic temperatures are like now – which means an average range of -67.18 degrees F (-55.1 degrees C) in Vostok to 22.46 degrees F (-5.3 degrees C) in the Antarctic Peninsula. There’s a whole lot of disagreement about this theory, though, so take it with a grain of salt until and unless more information comes along.