Wunderground Meteorologist Shaun Tanner

Last Updated: 5:55 PM GMT on November 19, 2009 — Last Comment: 6:42 AM GMT on November 22, 2009

	Lesson: Twilight

Posted by: shauntanner, 5:49 PM GMT on November 05, 2009

Today, I will once again tackle an atmospheric phenomena that nearly everybody sees everyday. It has been so ingrained in our psyche because we see it everyday that most of us do not even know what twilight is and how it comes about. Have no fear, let's walk through it.

I am not too interested in the technical definitions of twilight as scientifically defined. It is interesting to note, however, that some crimes carry stiffer penalties if they are committed at night so a technical definition of when nighttime actually occurs is necessary. Rather, I am just interested in how twilight comes about. As usual, pictures follow if you are just interested in pretty WunderPhotos.

Try this someday. Turn on a singular source of light in your home. Make sure the room where the light is has no windows or do this experiment in the dead of night. Now, turn off the light. Notice anything? Absent of any other sources of light in the room, the atmosphere of the room should be pitch black. Now, turn the light back on. For this next experiment, keep the light on but take the light source outside the room. Put it down the hall, in another room, just put it somewhere else outside the room but relatively close. Now go back into the room. Notice anything now? Is the room completely dark as it was before? Chances are that at least some amount of light has made its way back into the room. Why? Well, nearly all the surfaces in your home has some sort of reflective quality. These surfaces reflect the light from the source outside the place where it is at, and at least some of it finds its way into the experiment room.

Well, the atmosphere reacts the same way as the reflective surfaces in your home. Without an atmosphere, the surface and sky would become immediately dark once the top of the sun moves below the horizon. Conversely, the surface and sky would stay dark until the moment the top of the sun peaks above the horizon in the morning. Imagine, dark...dark...dark...dark...BRIGHT! It would be an interesting change to our lives.

Rather, the atmosphere acts to reflect and scatter sunlight back to the surface even once the sun has already set. For instance, when the sun sets, that means the surface no longer has a direct line of sight. But, the higher you move up in altitude, the more likely you are to have a direct line of slight to the sun. Thus, the higher altitudes of the atmosphere still receive solar radiation even when the surface does not. This is the same atmosphere that is capable of reflection sunlight (clouds) and scatter sunlight (Nitrogen and other gases) back down the surface. Thus, the surface still receives sunlight well after the sun actually sets. The reverse is true for morning twilight. The upper atmosphere will receive solar radiation first, reflecting and scattering it to the surface before the sun rises.

So, now you know that twilight is not just some tweeny movie series.

Fingers of Twilight

This photo was uploaded by: teach50

Twilight

This photo was uploaded by: shenandoah

Over the City Twilight

This photo was uploaded by: richie123098

City of Cardiff, Capital of Wales, U.K. at Twilight

Twilight before the rise

This photo was uploaded by: ftogrf

Sunrise for a change

Alaskan Twilight

This photo was uploaded by: katy99780

With days getting shorter and nights darker, the sun sets at an hour when we can appreciate the lingering beauty. The undersides of a few clouds turn beautiful colors as the sun sinks below them.

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1. presslord 6:26 PM GMT on November 05, 2009
! ! ! ! Thanks!

2. mobal 6:49 PM GMT on November 05, 2009
Looking forward to a lesson on Sun Dogs!

3. lawntonlookers 8:32 PM GMT on November 05, 2009
Thanks. The EVA from the ISS or Shuttle are interesting when the go from daylight to darkness with no twlight.

4. JFLORIDA 8:42 PM GMT on November 05, 2009
This is a another good blog Meteorologist Shaun Tanner. As the film and photo people like to say we are Guaranteed two "Magic Hour(s)" each day. Just as the sun sinks on the horizon and for a time after, that scattered light makes everything seem just a little more interesting.

5. zoomiami 12:22 AM GMT on November 07, 2009
Nice post - one new thing to learn today

6. Skyepony 3:29 AM GMT on November 08, 2009
Is there a name for that moment in twilight when the atmosphere & clouds especially glow the brightest?

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