Everyone knows the poles are cold. But think about it for a second: in summer, the Sun doesn't set there for months. If the Sun is shining around the clock, why doesn't it ever warm up?
The main reason is the angle of the sunlight. Because the poles are the farthest points from the equator, the Sun always sits low in their sky, so its rays arrive at a slant and are spread thin. On top of that, bright snow and ice reflect most of the sunlight that does arrive, and in winter the Sun disappears entirely for months.
A Sun That Never Climbs High
Earth is tilted about 23.5 degrees. That tilt gives us our seasons, but it isn't enough to lift the Sun high in the polar sky, even in summer. At the North Pole, the Sun rises around the March equinox and climbs for three months, reaching only about 23.5 degrees above the horizon at its highest.
Low-angle light is weak light. Think of a flashlight shining straight down versus at a steep slant. The slanted beam spreads its energy over a much bigger area, so each patch of ground gets less warmth.
Here's a surprise, though. Because the summer Sun shines 24 hours a day at the South Pole, more total solar energy reaches the surface there during summer than at the equator.
Snow Throws the Sunlight Away
So where does all that summer sunshine go? Much of it bounces right back into space. Fresh snow reflects about 90 to 95 percent of visible light. Over Antarctica, the snow reflects a little more than 80 percent on average.
This creates a feedback loop. Snow keeps the area cold, and the cold keeps the snow from melting. When ice does melt and expose darker water or ground, more sunlight gets absorbed, which can lead to more melting.
Months of Darkness
Then comes winter. At the poles, the Sun sets for about half the year. At the South Pole, polar night lasts about 186 days, from the March equinox to the September equinox. With no sunlight at all, heat steadily escapes into space. At the North Pole it's a bit shorter, about 179 days, because Earth's orbit around the Sun is slightly oval.
The Amundsen–Scott research station sits right at the South Pole, and its winter crew is completely cut off from mid-February to late October.
Why the South Is Colder Than the North
The two poles aren't equally cold. The North Pole sits at sea level in the middle of the Arctic Ocean, on shifting sea ice. The ocean acts as a reservoir of heat, so the North Pole is substantially warmer than the South Pole. Its record high is about 55 degrees Fahrenheit, while the South Pole has never risen above about 10 degrees.
The South Pole sits on a continent, and Antarctica has the highest average elevation of any continent. Higher ground is colder, which is also why East Antarctica is colder than West Antarctica. The lowest air temperature ever reliably measured, minus 128.6 degrees Fahrenheit, was recorded at Antarctica's Vostok Station in 1983.
So the poles stay frozen thanks to a team effort: a low, weak Sun, dazzling snow that reflects it away, and months of darkness. It's no wonder penguins wear such sensible coats.
