“The brighter you are, the more you have to learn.” -Don Herold
With the seasons about to turn to their extremes — summer in the Northern Hemisphere and winter in the Southern — you’re very likely looking forward to one of two things:
- Warm, long, bright, sun-filled days, or
- cold, short, dreary, good-to-spend-indoor days.
But just what is it that makes the days this way? And are they really this way, or are parts of this just us fooling ourselves? You’ve continued to send in your questions and suggestions, and this week’s Ask Ethan comes courtesy of Jim Joyce, who asks:
In summer, it always “seems” that the intensity of sunlight is brighter. […] However, given the relative change of distance of a location from the sun during the year, is there any meaningful difference in intensity? What about distance changes along our orbital elliptical path?
No doubt, the difference between what the Sun is doing during the Summer Solstice and the Winter Solstice is incredibly pronounced.
For those of us who live outside the tropics — with latitudes greater than 23.4° — we’re used to the Summer Solstice as the one day of the year where the Sun passes closer to directly overhead than any other. On the Winter Solstice, on the other hand, the Sun’s maximum height over the horizon is a full 46.8° lower than it is at Summer Solstice.
That’s not only a difference you can see, it’s a difference you can clearly feel in terms of warmth!
There’s no doubt that the Sun, when it’s directly overhead, feels more intense than it does when it’s lower in the sky. This is why — even though there’s a lag as the atmosphere heats up — it’s almost always warmer near and after noon than it is in the morning or after the Sun sets.
But is the light from the Sun really more intense?
Hardly. You’ve very likely heard an awful lot about the Sun’s variability, and variations in solar irradiance over time. What most people don’t realize is that this is true: the Sun does vary in its brightness over time, but those variations are tiny! On average, at the top of the Earth’s atmosphere, the energy-per-unit-area we receive from the Sun is 1,365.5 Watts/m^2 when the Sun emits at its minimum energy, and 1,366.5 Watts/m^2 when the Sun is at its maximum.
In other words, it’s true that the Sun’s energy output varies, but only by ~0.1%! That’s far too small to cause any change in what we feel.
On the other hand, you might start asking yourself about Earth’s proximity to the Sun. Our orbit isn’t a perfect circle; quite to the contrary, we’re an ellipse! Is it possible that, when the Earth is closer to the Sun, that extra radiation due to our proximity causes us to feel that extra solar intensity?
This, too, is a very small effect. When we’re at our closest approach to the Sun, we receive light that’s about 6% more intense (because intensity scales as the inverse distance squared) than when we’re at our farthest. It’s true that 6% is a much bigger number than 0.1%, but it’s still negligible; the Earth is nearly at its farthest point from the Sun, but the Northern Hemisphere is as warm as it’s been all year!
Instead, there are two effects that dominate, and they’re not only very closely related, they come about because of the same phenomenon: axial tilt!
It’s true that, no matter when we’re talking about, the Sun’s light incident on Earth is around 1366 Watts/meter^2, with that 0.1% variation thanks to the Sun’s intrinsic properties and that 6% variability due to the Earth’s distance. But consider this: when the Sun’s light strikes the Earth, if the Sun is directly overheard, all of those 1366 Watts-per-square-meter will strike the Earth’s atmosphere where you are. But if the Sun is at an angle, you’ll have to contend with that energy being spread out over an even larger area.
For those of you who remember your trigonometry, the amount of sunlight-per-unit-area striking the top of Earth’s atmosphere where you are right now is that initial number — 1366 ± 0.1% ± 6% — multiplied by the cosine of the Sun’s angle from the zenith!
That 6.1% variation comes out to the equivalent of only a 3.5° difference at most when you’re talking about the Sun’s angle in the sky. By far, this is the dominant effect: axial tilt means the energy from the Sun is spread out over a larger region of the Earth’s surface, and so less of it strikes you.
It isn’t enough to make the Sun appear significantly dimmer, but as far as how it feels? That’s easy to notice.
But I said that there were two effects, and the Sun’s energy being spread out over a larger surface area of the Earth is only one. The other is something that happens every time the Sun is in the sky: it has to travel through the atmosphere!
The atmosphere isn’t just effective at scattering away radiation, it’s more effective at it as the light passes through more and more of it! When the Sun is directly overheard (at a 90° angle), it “only” has to pass through our roughly 62 miles (100 km) of atmopshere. But if the Sun’s down at only a 45° angle, it passes through 88 miles (141 km) of atmosphere, further decreasing the intensity.
In fact, at my mid-northern latitude of roughly 45°, sunlight only passes through 67 miles (108 km) worth of atmosphere at noon on the Summer Solstice, but a whopping 168 miles (272 km) at noon on the Winter Solstice: nearly three times as much!
This is the same reason why sunrise and sunset might be beautiful, luminous sights, but are lousy for experiencing the Sun’s warmth. So it’s not our proximity to the Sun, nor variations in our parent star itself that cause the light to feel different, but rather how direct the Sun’s rays are when they strike our portion of the Earth, as well as how much of our atmosphere they must pass through.
Thanks for a great question, Jim, and don’t forget to send in your questions and suggestions here; the next Ask Ethan could be all because of you!
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