Climate Watch
As we approach the shortest days of the year, people sometimes notice that the amount of possible sunshine at winter solstice does not match the amount of “darkness” at summer solstice. And it’s not like there’s just a minute or two difference: we’re talking an hour or more. For example, on winter solstice in Nome the sun will be above the horizon for three hours and 55 minutes. However, on summer solstice in June, the sun is below the horizon only for two hours and 32 minutes. It’s even more dramatic closer to the Arctic Circle: Shishmaref is eight miles south of the Arctic Circle, but around summer solstice there are nearly four weeks when the sun does not set. However, the shortest day of the year has over two and half hours of possible sunshine.
Why is there this summer vs. winter difference?
It turns out this is caused by the atmospheric bending of light rays due to uneven variations in the atmosphere of temperature, pressure or moisture. When a celestial object like the sun or moon is nearly overhead, the light is passing through less than 50 miles of air (that’s how thin earth’s atmosphere is). However, when the sun or moon is very close to the horizon, the light is passing through many hundreds of miles of atmosphere. This results in the light rays being bent upward in a process called refraction, and so we can see the sun (and moon) even when it is really below the horizon and so would not be visible if there were no atmosphere (such as on the moon). Refraction through the atmosphere occurs everywhere on earth. Here at high latitudes, the sun spends a lot of time close to the horizon —much more so than at lower latitudes —and the refraction effect is amplified. The result is this dramatic seasonal difference we see. And in case you’re wondering, yes, refraction is taken into account in modern calculations of sunrise and sunset times.