Today’s Question: I have recently moved from Houston to northern Michigan. I noticed that the twilight time is increased here. My question regards the blue hour. Does it also increase (more than the hour before sunrise or after sunset)? Also, is the blue hour altered when it is overcast?
Tim’s Quick Answer: The duration of “blue hour” does indeed vary depending on your position on the globe, with the duration being longer near the poles and shorter near the equator.
More Detail: One of the first things to understand about blue hour is that it isn’t really considered to be a full hour in duration. The term is really just a shorthand reference to that period a bit after sunset or before sunrise when the sky is illuminated with blue light and the landscape is still relatively dark.
The actual duration of blue hour (which doesn’t really have a strict definition) depends upon the relative speed of the sunset (or sunrise). The closer you are to the poles, the longer blue hour will last. The closer you are to the equator, the shorter blue hour will be.
For example, a recent calculation for blue hour using one particular app shows that in New York City the duration is 39 minutes, while in Miami, Floria, blue hour will only last 32 minutes.
The priority for planning a given photo should obviously be the specific location and subject matter available, of course. But it is worth keeping in mind that when you are farther north or south from the equator, blue hour will last a bit longer. And when you are closer to the equator, blue hour will be of a shorter duration.
It is also worth noting that the duration of the sunrise or sunset will also be affected by your position, with the sunrise or sunset taking longer the farther you are from the equator.
As for overcast conditions, that is something of a mixed issue. Generally speaking, you’ll get the best blue hour effect when the sky is as clear as possible. That said, if the conditions are just right, partial overcast clouds can provide an interesting effect. Those clouds can potentially reflect some of the color of blue hour, but perhaps more interesting is that low clouds can reflect some of the color of the lights of the scene you are photographing, assuming there are lights available to illuminate the clouds.