# If the earth is a globe, what causes temperature differences across lines of latitude?

If the earth is a globe and the sun is 93 million miles away, why is the equator the hottest place on earth?

In terms of distances to the sun, the distance to the equator is almost equal to that as the distance to the poles.

Around January 3, the sun is said to be about 91.4 million miles away, but on July 7, the sun is said to be about 94.5 million miles away. The earth is warmer on July 7 (summer time), and yet it is 3 million miles further than when it is in January, how is this possible?

• Ignoring such things as axial tilt, seasons, and atmospheric transport, it's because the Earth is a globe. At any given latitude (which I think you've confused with longitude: en.wikipedia.org/wiki/Longitude ) the insolation per unit area is proportional to the cosine of the latitude. So if you had a solar panel and wanted to get the maximum energy from it, at the equator you'd lay it flat on the ground, while at the poles you'd stand it upright. Commented Apr 7, 2017 at 18:07
• @bmende actually it explains it perfectly, but I think it's a tiny bit complex if you don't understand. A picture being worth 1000 words and all that: windows2universe.org/earth/climate/images/… It's all about the angle the sun is in the sky - hence, cosine of the latitude. Commented Apr 8, 2017 at 0:12
• @bmende No, both distance and angle matter. Winter is, in one sense, colder because the sun is lower in the sky. It's also in the sky less time. The angle to the sun is generally more important, unless you vary the distance enormously. The 3% distance variation between December and June is pretty small. The change in angle of the sun in the sky and the length of a day varies much more than 3% unless you live very near the equator. Both matter, the tilt or angle matters a lot more for most of the Earth. Commented Apr 8, 2017 at 2:08
• @bmende: The reason the varying distance from the sun has no obvious effect is that it's much smaller than the variation caused by axial tilt. If the Earth had no tilt, we'd possibly notice mild seasonal variation due to orbital eccentricity (like Pluto does). Commented Apr 8, 2017 at 4:51
• @bmende People often don't realize summer and winter are not global phenomenon, when it is winter in [most of] south america it is summer in north america. The earth is actually closer to the sun in northern winters [january].
– John
Commented Apr 8, 2017 at 14:15

Polar regions are colder than equatorial ones for a simple reason - geometry.

More specifically, it is caused by the greater angle of the Sun's rays to the surface of the Earth. As you go farther north, the Earth's (mostly) spherical surface bends back from the direction of the Sun's rays, and the same amount of photons are spread over a larger area.

What's more, the closer you get to the poles, this already-attenuated sunlight has to go through more of the Earth's atmosphere, and so more sunlight is scattered, and consequently less will reach the ground.

Here's an image (Thanks, Pont) to show the difference:
Source: Wikimedia Commons. License: CC BY-SA 3.0

The sun's angle is more important to climate differences than the modest variation in Earth's distance to the sun, because the atmosphere is kept warm by the greenhouse effect. It's not the direct light that matters: Air is more or less transparent to the visible light that makes up most of the Sun's spectrum. Instead, the ground absorbs that light and reradiates it as infrared and microwave radiation. That longer-wavelength radiation is absorbed by the atmosphere, but if the ground is not radiating as much infrared because it absorbed less sunlight, the atmosphere above it doesn't get warmed quite so much.

Granted, if the Earth's orbit were much more eccentric then the variation in distance to the Sun would matter more to climate variation. But it doesn't.

This would happen even if Earth's axis weren't tilted. Polar regions would still be cold and equatorial regions would still be hotter. In fact, the difference would be greater because there'd be less mixing.

The image below shows the what happens to the energy the Earth receives from the Sun:

Source: NASA

That shows an average over the entire Earth. As you approach the poles, the percentage "reflected by clouds and atmosphere" goes up while total energy "absorbed by surface" per unit area goes down. And since ice has a high albedo, this is reinforced by the percentage "reflected by surface".

• So does this mean that if I were to place a ball outside in the sun, that the equator line would be significantly warmer than the polar regions? Surely this would never happen. Commented Apr 11, 2017 at 20:10
• @bmende Depends on the size of the ball and its internal energy budget. The heat we are talking about in this context is transferred by radiation and everything is well explained in the answer and follows the most basic geometric rules. Commented Apr 14, 2017 at 8:20
• @bmende: If you placed the ball in a vacuum, so that it wasn't warmed by convection, and made it of an insulating material, it would be obvious. Why not try it? Or just compare common things - say a lawn chair with the back laid flat, vs with the back tilted perpendicular to the sun. Commented Apr 14, 2017 at 18:24
• @bmende: No, I'm saying that if you do the experiment in a vacuum, and make the ball out of a good thermal insulator, you will eliminate the effects of convection & conduction, and so see just the radiation effects. As for your claim that Earth doesn't curve, sorry, but I've been high enough to see it for myself. Commented Apr 15, 2017 at 19:10
• @user7733 There is an obvious link to NASA's website under the image. If you don't like it, take it up with them. Commented Apr 17, 2017 at 14:12

The major reason isn't Earth's distance to the sun. The major reason is that earth's axis is tilted. The Arctic circle, for example, sees 24 hours of darkness in the winter, and 24 hours of sunlight in the summer.

If you go to the other side of the equator, you'll note that the seasons seem switched- the southern hemisphere is warmer in the winter than in the summer.

Take a look at how much sunlight each day has- you'll note that in the summer, the northern hemisphere has more hours of sunlight than the winter. The opposite is true for the southern hemisphere.

• Actually, if Earth's axis weren't tilted, polar regions would still be cold and equatorial regions would still be hotter -in fact, the difference would be greater because there'd be less mixing. Commented Apr 8, 2017 at 1:54
• @Bmende there is midnight sun in Antarctica as well, during the southern summer. Also the days in e.g. Australia and South Africa are longer in December than in June. Commented Apr 8, 2017 at 2:39
• @bmende -- youtube.com/watch?v=BgZa9oZDN5g Commented Apr 8, 2017 at 13:56
• @bmende - Those are crepuscular rays. Go outside sometime; you can see those anywhere. I am getting the distinct impression that you are indeed a flat earther. If so, that's kinda sad. Commented Apr 8, 2017 at 18:04
• @bmende It really depends on where you pause it. You can pause it between the legs, you can clearly see the sun. The solid angle of the sun is rather small, so the fact that the sun was eclipsed by the camera legs do not discredit the video. Commented Apr 8, 2017 at 19:03