Sunday, June 7, 2015

Alaska Brightness

** It's been a while since I posed here but I thought the Deep Cold readers might like this. **

Did you know that interior Alaska actually receives more sun and twilight than any other place in the U.S.

Alaska is known for long summer days and long winter nights. However, if you average all 365 days together, everyone ends up with 12 hours of daylight and 12 hours of darkness throughout the course of the year no matter where in the world you are, right? Actually, that is not correct. 

Looking at the chart that accompanies this post (see Figure 1), the red line at the bottom of the chart shows that daylight at the equator averages about 12 hours and 15 minutes per day over the course of the year. Remember that the sun is a circle (not a point) and we receive daylight from the top of the sun's disc before the middle of the disk reaches the horizon. The same is true at sunset. This makes an average day over 12 hours everywhere.

Figure 1. Hours of daylight and daylight plus Civil Twilight by latitude. All data obtained from the U.S. Naval Observatory's Astronomical Observations Department.

At the equator, the sun moves nearly straight up and down with respect to the horizon. This means the sun rises quickly and sets quickly. As we move to higher latitudes, the path of the sun is more oblique; i.e., the sun moves more and more diagonally with respect to the horizon. It therefore takes longer for the entire disk of the sun to make is across the horizon at both sunrise and sunset. 

If the sun were a point and not a circle, the average length of a day would be 12 hours everywhere. Since the sun is a 2-dimensional circle from our perspective, the relative speed of the rising and setting dramatically changes the amount of light we receive. This effect is greatest at the Arctic and Antarctic Circles due to the effective ground speed of the sub-solar point near the solstices. The difference in cumulative day lengths between the equator and the Arctic Circle (for all 365 days) is 225 hours per year – or 37 minutes per day on average. 

Figure 2 and 3 show the length of daylight (Figure 2) and the combined length of daylight and Civil Twilight on the summer solstice.

 Figure 2. Length of daylight on the summer solstice.

Figure 3. Combined length of daylight and Civil Twilight on the summer solstice.

If we include Civil Twilight, which is when the entire sun's disk is below the horizon but by no more than 6°, the extra light for Alaska dramatically increases. At 69°N latitude, the 365-day average for daylight plus Civil Twilight is 15 hours and 6 minutes. At the equator, the 365-day average is only 12 hours and 52 minutes. The average difference in light (daylight plus Civil Twilight) is a shocking 2 hours and 16 minutes per day. Places just north of the Brooks Range (e.g., Umiat)  therefore receive the most usable light of any place in the U.S. Table 1 shows the cumulative length of daylight and Civil Twilight for Fairbanks, Barrow, Anchorage, and Juneau measured in hours.

Table 1. Annual hours of daylight and Civil Twilight for Fairbanks, Barrow, Anchorage, and Juneau.

The final two maps (Figure 4 and 5) show the length of daylight plus Civil Twilight for Alaska and the Lower 48. Again, note how much more light Alaska receives than the Lower 48 over the course of the year.

Figure 4. Average annual length of daylight plus Civil Twilight. The average is for all 365 days of the year. The map perspective is Alaska-centric.

Figure 5. Average annual length of daylight plus Civil Twilight. The average is for all 365 days of the year. The map perspective is Lower 48–centric.

The next time you hear someone complain about how dark it gets in Alaska during the winter, just remind them that interior Alaska is the light champion on the U.S.


  1. Hi Brian good post. So this is why we put aluminum foil on bedroom windows in Summer? After 50 years here the daylight at night doesn't bother me.

    Now it's time for the Midnight Zombies to mow their lawns and walk about looking for dark.


  2. Thanks Gary. I should add that since the earth is farther from the sun in the northern hemisphere summer, the revolution speed is slower. The reverse is true in the southern hemisphere. Therefore, the cumulative daylight/twilight is longer in the norther hemisphere summer than the southern hemisphere. We also see this when noting that Barrow has more 24-hour daylight days in summer than 0-hour daylight days in winter. I'll add a chart to demonstrate this tomorrow.

  3. Very nice, Brian. I've experienced the profound difference in sunrise/sunset rates in the tropics and in the Arctic, but didn't realize that it added up to such a large average difference in daylight. Good information.

    In a somewhat related matter, I've always pondered the effect of solar angle on hours of sunshine, owing to cloud shading effects. When the sun is low in the sky, it is much more likely that even partly cloudy conditions will block the sun (owing to non-zero cloud depth). So even though northern Alaska gets the most daylight, it will get fewer hours of sunshine than all but the cloudiest places in the tropics. I suppose one could do the calculations for some hypothetical cloud fractions and depths.

  4. Brian, congratulations on your many likes with the identical post on Facebook. Probably better that people like daylight amounts than the tin foil hats.

    Are you going to update this post to reflect the small calculation correction you found as announced on Facebook?

    1. Thanks Eric. For everyone else's benefit, the error you refer to was my formula added 24 hours of daylight on April 31st and June 31st. Of course those are fictitious dates. The net result was an extra 48 hours of daylight and Civil Twilight for every location on earth. Therefore, the error changed the y-intercept of the lines but not the relative magnitude. As soon as I discovered the error I made new maps and charts and updated them on this blog post and fixed the text description immediately. Unfortunately on FB, you cannot swap out images. Therefore, a follow-up post is in order on that site.

      As a side note, the popularity of the FB posts caught me quite off guard. A typical post on my page generates 100-200 views, a handful of Likes, about 1 Comment, and an occasional Share. The post showing the number of hours of daylight on the summer solstice is up to 337,536 views, 8,109 Likes, 1,226 Comments, and 3,917 Shares. It really was lightning in a bottle. Interestingly, the discussion of summer daylight is near and dear to the heart of Alaskans. On my Twitter feed, which is mainly followed by people in other states, a typical post gets about 100 views, a few Favorites, and a couple of Retweets. My Twitter post on the hours of daylight on the summer solstice received 82 views, no Favorites, and no Retweets. It really is an Alaska thing.

    2. I think it's safe to say that your Twitter feed recipients are of the more professional type. I think that most "common folk" don't use Twitter. And there is quite a few Outsiders who commented on your Facebook post. But I agree that it's an Alaskan thing. When I lived in the Lower 48, the lack of darkness was never an issue; I always got enough sleep and the stars always came out.

      It would be interesting to see how many views previous posts got from people looking around the Page. And how many people will stick around and follow along now.

      Perhaps you should do posts like "Is the 4th of July going to be rainy?"