Green Flash At Sunrise ©

Introduction

Beginning in late 2002, continuing throughout 2003, and into early 2004, some 87 instances of the Green Flash phenomenon were recorded during sunrise observations in Las Vegas, Nevada, averaging more than once every five days. Obviously, what is often thought to be a rare event seen at sundown in the ocean is fairly common place at sunrise under the proper viewing conditions. Furthermore, the often faint green luminescence seen at sundown in the ocean pales in comparison to the incandescent green seen at sunrise in Las Vegas. This is a summary of the solar observations that lead to this result.

Background

My personal interest in the Green Flash dates back many years to reports from colleagues who had spent time in the Pacific and had seen the Green Flash occasionally under proper viewing conditions during sunset in the ocean. Consequently, during cruises in later years many evenings were spent at the shipboard rail looking for the event and seeing it occasionally. During a Caribbean cruise in 2001, Clari, my bride of a few months, was both intrigued and challenged by the process of attempting to view that faint green light at sundown. In January 2002, shortly after the winter solstice, Clari and I moved to a new location in Las Vegas with latitude/longitude coordinates (36.2021, -115.3284) at 3000 feet elevation above mean sea level (this location is in Summerlin in northwest Las Vegas). The home faces west and the back porch and yard affords an elevated 180º view of the eastern horizon which is approximately 25 miles away across the Las Vegas valley. We enjoyed watching the northward travel of the sunrise across the eastern horizon and then its return following the 2002 summer solstice. As the sun moved back to the south, we became interested in noting the position of sunrise against the horizon. The jagged ridge line to the east forms a "fingerprint" of the horizon and allowed us to mark the point of sunrise on a panoramic photo rather precisely relative to the viewing location. As the 2002 winter solstice approached I became interested in the point of maximum southern travel and in late December 2002 I enlisted Clari's help in noting the exact point of sunrise. Thus, it was on December 22, 2002 that Clari made our first observation of the Green Flash. While my attention was diverted, she was watching the horizon and remarked, "There it is. You just missed it. And should it be GREEN!?" That visual observation started the process of observations throughout 2003 and is the basis of this paper.

Observation Methods and Techniques

The observation methods are simple. All that is required is visual observation, an accurate time piece, and a panoramic photograph of the horizon. (Later in the project a video camera with zoom capability was used to record the phenomenon.) The viewing requirements are likewise simple: 1. A clear view of the cloudless and haze-free eastern horizon 2. Be in place a few minutes before sunrise 3. Carefully watch the location where the sunrise is expected 4. DO NOT gaze at the disk of the sun after sunrise The first records were jotted notations on a horizon photo as shown in Figure 1, below. This photo is the southern half of the eastern horizon from our viewing point with the equinox on the left and the winter solstice on the right. The northern half of the sunrise travel is not shown. Also, the panoramic photo is pieced together from three different daytime photographs. The notations were made as they occurred and although are not easily readable in the photograph the figure is shown to indicate the simplicity of the recording process.

Figure 1. The southern half to the eastern horizon from the equinox to the winter solstice.

Figure 2, below, shows the early records of the first Green Flash observations. The first several days were jotted down as they occurred and then added to a computer record. Sunrise time recording was begun on January 18, 2003 and eventually all observations were entered into a computer record which is still being maintained. Note: the third column is a subjective description of the quality of the observation categorized as OK, Good, Great, or Excellent.

Theory - Why Does the Green Flash Occur?

There are many explanations for what causes the Green Flash. However, W. L. Humphreys (Physics of the Air, McGraw-Hill, 1940) provides an excellent description of the role of atmospheric refraction in producing the effect at both sundown and sunrise. Humphreys notes (on pages 466-467) that as the last star-like ray of the sun's disk begins to disappear below the horizon atmospheric refraction causes the ray to be dispersed spectrally with red bent the least followed by orange, yellow, green, blue and finally ultraviolet. Since ultraviolet is not seen and blue is preferentially absorbed by the sky the most noticeable visible color is green and it is that green luminescence that has been referred to as the "green flash". Humphreys also notes that the same effect should be observed at sunrise in the reverse order of colors. Finally, Humphreys points out that the visibility of any of the refracted colors is dependent upon having only a limited area of the disk visible. Since all points of the sun are refracted in the atmosphere neighboring points mix in complementary colors producing white. Thus as more of the disk is visible at sunrise (or sundown) the refracted colors are washed out to white. This is particularly apparent at sunrise as it is only the momentary appearance of the first rays of the rising sun that produce the green color followed by the rising disk that rapidly becomes incandescent white as more of the sun appears. Figure 3, below, is a schematic representation of how the first ray of the rising sun can produce the visible green portion of the spectrum at the viewing point for this project.

Figure 3. Schematic representation of viewing geometry

Finally, keeping in mind Humphreys' explanation above, it is interesting to consider what might happen if the sun's disk rises against a very jagged horizon as is the case in Las Vegas (recall Figure 2, above). In particular, if the sun were to produce neighboring but small pinpoints of light it should be possible to see multiple instances of the green flash if they are near simultaneous appearances. Figure 4, below, shows a schematic representation of four notional configurations and how they can lead to multiple events.

Figure 4. Notional representation of geometry effect

Summary of Observations to Date (January 30, 2004)

Since beginning the observations on December 22, 2002 a total of 87 instances of the green flash have been recorded. Figure 5, below, summarizes the observations to date in a bar graph format which sums the number of observations per month for each month. The bars are coded to show the number of observations per month according to the subjectively assigned value of the quality of the observation ranging from barely noticeably green (OK) to bright incandescent green (Excellent). Instances of multiple green flash observations are noted by the numbers above the bars. The first visual observation of a double green flash was on March 28, 2003. In that case, the sun arose behind a sharp peak as noted in Figure 4, above, and produced a near simultaneous double green flash. Double events were observed in August and November of 2003 as well. In an effort to try to photograph the phenomenon, a digital video camera with zoom capability was employed starting in December 2003. Since the dynamic range of most video cameras is much less than the dynamic range and sensitivity of the eye it is possible to record after the disk has risen beyond the point where it is no longer safe to look with the eyes. The fact that December 2003 and January 2004 have a greater number of multiple green flashes is attributed to the use of a video camera which allows reviewing the event on tape and counting instances of multiple flashes that would not be safely visible to the eye.

Figure 5. Summary of Green Flash observations from December 22, 2004 to January 30, 2004

Representative Still Photos Green Flash Observations

The following figures are representative frames from video recordings of green flash observations during the period of December 2003 to January 2004. Examples are shown of single, double, and triple green flashes that are representative of the 87 events observed to during the period. As mentioned above, using a camera to capture the event does not provide the dynamic range that is visible with the eye. Thus, the photos are considered successful if green is visible in the image. In most of the best cases the momentary incandescent ray seen with the eye is captured on film (or digitally, in this case) as scattered green light below the image.

Figure 6. A frame of a single Green Flash recorded on January 9, 2004. Note the green border at the lower part of the image and the scattered green light visible against the dark horizon. Also, the date, qualitative assessment, and time of sunrise are noted on the frame.

Figure 7. A single frame of a double Green Flash recorded on January 26, 2004. In this example the near simultaneity of the events made this observation clearly visible to the unaided eye.

Figure 8. A frame of a triple Green Flash recorded on January 18, 2004. Note that in this case the bright spot on the left is the first Green Flash which has already moved to white in color, the far right spot is just showing green and the middle spot is the last to show green. While not simultaneous events the sequential rise is still visible to the eye as a triple event.

The still photos shown in the figures above are examples of the green flash at peak color. The following movie shows several additional examples from early development to the point where the rising disk prevents further observaion.

Movies of Typical Green Flash Observations

The following movie was edited from video recordings of green flash observations during the period of December 2003 to January 2004. Twelve examples are shown including single, double, triple, quadruple, and quintuple green flashes that are representative of the 87 events observed to during the period. As mentioned above, using a camera (or video camera, in this case) to capture the event does not provide the dynamic range that is visible with the eye. Thus, the recordings are considered successful if green is visible in the image. In most of the best cases the momentary incandescent ray seen with the eye is captured on film (or digitally, in this case) as scattered green light below the image. To view the movie, click on the link below and adjust the media player parameters to suit your viewing preference.

GreenFlashMovieSilent.mpg

Clari’s Beads and Other Observations

A simple project like this can lead to many interesting observations. Among them are verifying the time asymmetry at the solstices and the latitude effect of the sun’s travel between the solstices along the horizon which is about 58º at the Las Vegas latitude.

However, the most interesting insight occurred while considering these observations in the context of eclipses. For a total eclipse, in which the moon obscures the sun, there is a well known effect called “Bailey’s Beads” that may be observed when rays of sunlight flash through the jagged profile of the moon’s rim. Such an eclipse is a relatively rare event in that it seldom offers a convenient platform for viewing. Of course, a total eclipse of a different kind occurs every day at sunrise and sundown and is largely ignored because it is so common. It would seem a name is needed for the multiple beads of (often green) light that shine through the sometimes jagged edge of the earth’s horizon at the ending of the total eclipse each morning when the sun emerges from behind the earth. Perhaps “Clari’s Beads” would be appropriate for this phenomenon since she was the first to observe the effect in Las Vegas, NV on December 22, 2002.

Conclusions

Typing in “Green Flash” on an internet search engine is an interesting exercise. There is an enormous amount of information about the Green Flash at sundown including theory, explanations, simulations, and hundreds of photographs. However, there is relatively little information about the Green Flash at sunrise. Certainly, there is nothing that would indicate or suggest the approximately 21.5% frequency of Green Flash observations that were recorded in Las Vegas from December 22, 2002 to January 30, 2004.

While our particular viewing location seems ideally suited to observe the phenomenon (i.e., high percentage of generally clear skies, low levels of haze and smog, and a long optical path to the eastern horizon) it is unlikely that the location is unique. Indeed, many other locations in Las Vegas offer the same or similar viewing conditions. Surely there are multiple locations across the United States and the world, for that matter, that would favor these observations.

If Green Flash observations are so “common” here why isn’t it better known? That remains a good question. It would indeed be remarkable that the answer is simply that no one ever bothered to look! In any case, there is no reason why you should not pick a clear day, get up early, be in a good viewing location before sunrise, and see for yourself. If you are lucky you may even see “Clari’s Beads”.