L’heure Bleue~

My first encounter with the term “The Blue Hour” was when I saw a photo titled L’heure Bleue in a book or National Geographic decades ago. The image had captured a woman walking along a Paris street, her movements slightly blurred against the background. The woman and the entire streetscape were bathed in this beautiful blue light.

I thought the photo was taken by Henri Cartier-Bresson, the French master of street photography and “the decisive moment,” but I have been unable to verify this as fact. Google has unfortunately become almost 80% product placement and influencer crap, but that’s a topic for another day. I’m still keeping an eye out for the photo and author.

I’ve been fascinated by the phenomenon ever since I saw the image and found out this atmospheric occurrence has a name. The Blue Hour usually catches you by surprise while looking out your window, walking down a street or gazing out over a harbour or field. It grabs you and makes you pause. You know you are in the moment.

Above~ The Blue Hour occurring during the Snowmageddon of 2015 in New Brunswick.

The truth is L’heure Bleue never ends, it just keeps circling the planet and revealing itself ahead of sunrise and after sunset as the Earth rotates. That’s right–it is occurring simultaneously at two different locations. The duration varies by season and proximity to the equator. The closer you are to the equator, the shorter will be the duration, and the less difference between the shorter-in-winter/longer-in-summer variation. At the equator it lasts for exactly 8 minutes, summer and winter.

Here at the 45° latitude (same as Montreal, Canada) L’heure Bleue varies from 11 to 16 minutes, December 21 to June 21 respectively. In Cincinnati, Ohio (39° latitude) the seasonal difference is 11 to 13 minutes. In Cuba (23° latitude) it is 9 to 10 minutes. Jumping back up north to Norway (60° latitude) it lasts 21 to 55 minutes, December to June. [data provided by PhotoPills]

The point is it is always The Blue Hour somewhere, and wherever that somewhere is, it lasts almost the exact same length of time (within seconds) in the morning and evening occurrences.

So what is the Blue Hour, how do I know when it will occur, and how long does it last?

The atmospheric occurrence known as The Blue Hour is caused by red light in the spectrum being absorbed by ozone in the atmosphere (known as Chappuis absorption) that leaves more of the blue spectrum visible. The effect is noticeable when light has to travel a long path through the Earth’s atmosphere. This occurs during the morning and evening twilight when the centre of the Sun is between -4° and -6° below the horizon, although according to TimeandDate the bluish cast “is likely to occur” to -8°. The concept reminds me of the old darkroom days using subtractive filters in processing colour negatives.

The Blue Hour occurs on the cusp of Civil and Nautical twilight. Twilight is divided into three 6° diminishing phases of light–Civil, Nautical and Astronomical. It occurs right prior to Nautical twilight in the evening and after Nautical twilight in the morning. So if you know when Nautical twilight is going to start or end you have the timeframe. Or you can use an App, which I’ll mention later.

The Blue Hour is both a term and a definition. It is a “term” that applies loosely to the blue cast effect that can be visually stunning at times during twilight. The overall effect has also been called The Magic Hour. It is a “definition” because it is defined by astronomers as a specific time period that the centre of the Sun is -4° to -6° below the horizon.

Why does it last exactly 8 minutes at the equator? Because the earth rotates 360° in 24 hours roughly, which means if you do the math (360/24) that works out to 15° per hour, which is why our time zones are 15° apart. Since 15° per hour equals .25° per minute (15/60) that means it takes 4 minutes to move one degree at the equator. So 8 minutes at the equator is our starting point to move 2° from -4° to -6°.

First: Overall Concept~

Above~ Perhaps this excellent graphic from the Aaron D. Priest website will explain a lot.

Second: When is Nautical Twilight~

Above~ Screenshot of the Sun Module on the TimeandDate website. Simply set your location (or nearest city) and the above screen will come up. Since the Blue Hour precedes Nautical Twilight in the evening, if you live say north of the Ohio River to Montreal, Canada, subtract 10 – 15 minutes (depending on the season) from the start of Nautical Twilight time and you have when the Blue Hour starts. If morning is your thing, add it to the end.

I have seen information that the Blue Hour starts 1/2 hour after sunset or before sunrise. Well, this is true part of the time depending on where you live but it isn’t accurate everywhere all the time. In many instances it might begin 20 minutes after sunset. Why does it matter? Because if you are going someplace to capture it with your camera at half hour after sunset it may already be over. Or if you arrive 7 minutes into it you may have trouble focusing and composing the photo. The location and time of year matter.

Third: There’s an App for that~

Above~ Phone screenshots of the Sun Module of the PhotoPills App from a couple of different timeframes. Notice the Blue hour telling exactly when it will start and end and so on. PhotoPills is my goto app for this kind of thing, best $12.95 Cdn I ever spent, think it is $10.99 USD now. Excellent Lunar Module as well plus some really neat tools for planning and eliminating star trails. Like Stellarium, it will work offline, and has been called the “swiss army knife” of apps for photographers. [I have no affiliation]

Another very popular planning app is The Photographers Ephemeris which I haven’t used. It is free.

If all you want is the start/end time then the BlueHourSite has a free online calculator where you enter in your location to get the time. There are apps as well which I haven’t used such as Phototime (free) and Blue Golden Hour (IOS, $1.99 USD).

You will sometimes notice a 1 minute discrepancy between TimeandDate and different apps. All these websites and apps use algorithms that have to round off seconds in different ways and I’m ok with that.

The three phases of twilight~

Above~ from the twilight module of the TimeandDate website.

Twilight is one of the most magical times of the day. It is part of the endless variety of this amusement park we live on, with changing seasons, phases of the Moon, rhythm of the tides, parade of the planets and so on. So maybe it is worth knowing a thing or two about.

Twilight is divided into three phases of -6° each once the centre of the Sun is below or coming up to the horizon. They are Civil, Nautical, and Astronomical. Looking at it from an evening point of view:

Civil Twilight:

Starts at sunset. Enough natural sunlight is available that artificial light may not be required. Called civil because most countries have civil laws governing when street lights, headlights, aviation and nautical lights are required for safety. In the morning it ends at sunrise.

Nautical Twilight:

The stars are beginning to show but the horizon is still visible. You will need artificial light for some things. The term dates back to a time when sailors used the stars to navigate. Light starts dropping off rapidly by this point in the evening, and often the most dramatic Blue Hour effect is in the first 5 – 10 minutes of Nautical Twilight.

If you are Moongazing through a telescope or binoculars there is still some blue around the Moon. About 10 minutes into it (say 45 minutes after Sunset) is a good time to align a goto telescope because the brightest stars are revealing themselves yet you aren’t getting overpowered by the star field. Constellations are starting to come out. A truly wonderful time of day.

Astronomical Twilight:

Most of the stars are out now except for the faintest ones, but it is not totally dark so you may have trouble seeing the more subtle objects. In the summer it is starts about 1 1/2 hours after Sunset.

What atmospheric conditions create a spectacular Blue Hour?

Why are some Blue Hours barely even noticeable and others dramatic? Quite frankly, I wish I knew.

This is a project/puzzle I keep going back to, because if you can find a correlation between certain atmospheric conditions during the day and a better than normal blue hour occurrence during twilight, a person could set up sort of an alert system for oneself. In other words, if A exists and B exists then is C is likely to happen? It has to be repeatable to be valid. Here are some of my lines of inquiry:

• What effect does Transparency and Seeing as listed on the Cleardarksky website have on blue hour intensity? It would make sense. But the thing is, some of the most dramatic blue hours appear during a light snowfall, drizzle or sometimes even fog. Not much transparency and seeing in those circumstances.

• Does a dramatic Sunrise or Sunset indicate a more intense Blue Hour later on? TBD.

• Do white fluffy cumulus clouds against a beautiful blue sky during the day set up a good evening Blue Hour? Do they set up some kind of refraction effect of blue sky bouncing off the earth and back again against the clouds? Not necessarily from what I’ve seen, but still TBD.

• Do the types of clouds and ceiling height affect the intensity of the Blue Hour?

• What effect does ceiling height, barometric pressure, dew point, temperature and humidity have on the blue hour intensity? Any? It has to be something, but what is it?

This last one is what I’m working on now, and the only thing I can think of is keeping notes of duds and winners to see what is going on, if anything, and is there a correlation and repeatable pattern.

Photographing the Blue Hour~

Spontaneous photos~

The only way I’ve found that works most of the time is to have a camera already on a tripod that you can grab. Why? Because by the time you notice a particularly stunning Blue Hour it is already well under way and almost over. The light is changing so rapidly by this point that you will have difficulty even attaining focus, let alone putting your gear together. So if it is handy you are halfway there. 45 seconds is everything when the light is dropping off, or vice-versa in the morning.

It helps to have the camera in a “parked position” of Autofocus off, lens turned to Infinity, AutoISO off, preset to Aperture Priority or Manual Mode, cable/shutter release set to remote or whatever.

However, the important thing is to try to capture what you see happening in front of you. So if a handheld shot is all you can get using any kind of camera or cellphone then so be it. Take it, you will probably still wind up with a beautiful azure blue colour.

Planned photos~

The hints and guidance provided by the experienced hands at PhotoPills, Icelandic Tours, Aaron Priest and others (see links below) are worthwhile. Basically, their ideas work and their guidance is worth following. I’ll just mention some of them.

Assuming you already know or have an idea when the Blue Hour will start and have a location in mind:

Arrive ahead of time. Set up at least 10 – 15 minutes before the Blue Hour starts. Why? Because you need to be able to compose and focus your shot and turn autofocus off. The camera probably won’t autofocus once you’re halfway into the Blue Hour.
It is important to get something you’re happy with in the focus and composure department and then just leave it alone. Why? Because once you’re well into it, you won’t be looking at the pic on the screen at all, you will be looking at the histogram and making adjustments. You’ll be “flying by instruments” at that point.
For an evening session, start with ISO 100, lens stopped all the way down, Auto ISO off, Autofocus off. Try Aperture Priority or Manual Mode.
Once the Blue Hour is under way, take a pic at least once per minute or every 45 sec. At first, you may not notice much. Then things will start to change.
Once a shutter speed of 30 sec is no longer adequate, start opening the lens a bit and then determine when you want to raise the ISO. Just keep making adjustments.
A big drop off in light probably means you’ve just gone into Nautical Twilight (evening). Keep shooting, the blue cast will go from azure to almost navy.
Go into The Blue Zone. This is the time period in Nautical Twilight that matches The Blue Hour in length.

Go into the Blue Zone~

The Blue Zone is a term I’ve coined for myself that incorporates what the camera or human eye will actually see. The camera does not care about our human concepts, it simply records what is in front of the lens. Since the blue cast “is likely to occur” [see TimeandDate website] when the Sun is between -4° and -8° below the horizon, this goes well beyond the official Blue Hour timeframe of -4° to -6° and into Nautical Twilight. How far or how long does it last? Answer: the exact same length of time of The Blue Hour for that particular day to move another 2°. So if a website or app is telling you that The Blue Hour will last 12 minutes then double the time to get the full effect.