Daylight Hours and the High Latitudes

There is no better time than the passing of the midsummer solstice to get to grips with daylight hours in the higher latitudes. Living in Norway it’s a subject that I’m asked time and time again about from those residing at lower latitudes.

If you want to compare daylight hours between two cities then the U. S. Naval Observatory website has a neat little page where you can input latitude and longitude and output the sun rise and set times for the year. I have chosen Bradford, England (W001 45, N53 47) which is the city of my birth, Bergen, Norway (E005 20, N60 20) which is currently my home town and Barcelona, Spain (E002 09, N 41 23) for comparison and because it also starts with a B - see map below that shows the different cities within their time zone. Purple is GMT and yellow is GMT + 1.

Plotted in Graph 1 are the sunrise and sunset times for Bergen, Bradford and Barcelona, the sudden vertical translation of the curves depict day light saving. The graph shows that for us in Bergen in the depths of winter the sun rises at 09.45 and sets at 15.29 giving us only 5 ¾ hours of daylight. In the height of summer however the sun rises at 04.10 and sets at 23.10 giving us 19 hours of daylight. As expected, the graph shows that at the higher latitudes (Bergen) the daylight hours are shorter in the winter time and longer in the summer time. It also shows there is less variation in daylight hours as you move closer to the equator.

Graph 1
However a couple things are odd in the graph. I would have expected the curves to lie equal proportion between each other throughout the year and to cross only at the equinox. This is the case for the Barcelona and Bergen curves, but not for Bradford. In the summer the sun rises in Bradford around the same time as in Bergen but it sets around the same time as in Barcelona. The inverse relationship holds for the wintertime. In addition, the switch in the length of daylight hours should occur on the equinox all over the world but in Bradford this date is staggered either side of the equinox in comparison to the other cities.

The reason for this behaviour can be explained by plotting the sunrise and sunset times normalised to the true local time, that being noon when the sun is directly overhead or the same as moving the cities to the same longitude (GMT) (Graph 2). It then becomes clear that the reason that the daylight hours in Bradford do not behave as expected in Graph 1 is due to the time zones and where a city lies within that zone. Bergen and Barcelona lie in the west of their time zone but Bradford lies in the east of the previous time zone. The map shows that there isn't a huge distance between Bradford and Barcelona laterally but there is one hour time difference.

Graph 2 - note daylight saving is not applied.
Now the curves behave as I would expect. Also plotted are the number of daylight hours that each city receives throughout the year.

What is interesting to observe is that on the equinox all locations on earth receive the same amount of daylight and if you calculate the amount of daylight each city receives throughout the year then each city receives exactly the same.













Twilight

The first graph (no 1) shows that the sun sets in Bergen at 23:10 at midsummer but I know from experience of living in Norway that it is light way past this time The graph shows daylight hours which is when the sun is above the horizon and it doesn't include the effects of twilight. Due to the earth’s shape, tilt and orbit around the sun, the duration of twilight varies greatly with the time of year and with latitude from a few minutes in equatorial regions to hours in polar regions. It is that which makes the difference for those residing at higher latitudes in the summer.

Twilight is the period of time when the sun is below the horizon but the earth is not completely lit nor completely dark. It can be divided up into 3 distinct intervals: civil twilight, nautical twilight and astronomical twilight based on the position of the sun in degrees below the horizon. The definitions of these intervals are shown in the Table below. The observations on the ground can be obscured due to light pollution and atmospheric conditions.

The most important interval for us is civil twilight which is the time that the sun is less than 6 degrees below the horizon, but more importantly it is the time that the horizon can still be seen clearly visible and terrestrial objects are easily perceptible without artificial light.

At high latitudes during the summer time the sun never goes below 6 degrees below the horizon and a continuous civil twilight exists. At the poles, twilight can be as long as two weeks at the equinox (white nights), while at the equator, it can go from day to night in as little as twenty minutes. At temperate-zone latitudes, twilight is shortest around both equinoxes, slightly longer around the time of the winter solstice and more longer during late spring and early summer.

Graph 3- note daylight saving is not applied.

Graph 3 illustrates that at midsummer in Bergen there are 2 hours from civil dawn to sun rise and from sun set to civil dusk. If we were to apply daylight saving to these times then in there would only be 1 hour of Nautical Twilight between 12.30am and 01.30am. Although we do not technically have 24 hour daylight this explains why it feels like we do.

It is wonderful having so much daylight in the summer. It means that after work you can go for a hike in the mountains, go climbing or out in the kayaks and still get home before dark. To live close to the equator would mean much less variation in daylight length throughout the year, so although you wouldn’t have to live through the long dark winters that we get at higher latitudes it would also mean you wouldn’t get the fantastic long summers either. In contrast, to live further north than Bergen would be simply a waste of daylight as we still need to go to bed and any dalylight past 11pm would be better rationed out during the winter.

For me I think Bradford and Bergen are close to being the perfect location in terms of daylight hours through the year. Perhaps somewhere inbetween - close to a latitude of N60 would be spot on.. where could that be?