Abstract

Clouds are not something you usually want during any kind of solar eclipse. But in the case of the October 14th, 2023, annular eclipse, viewed from Uvalde, Texas, the dramatic appearance of mid-thickness clouds actually enhanced the event by providing incredible photograph opportunities. In this article, we discuss the experience of the U.S. National Science Foundation's National Solar Observatory outreach trip to Uvalde, viewing the eclipse and interacting with crowds beneath partially cloudy skies. We also highlight the need for future safety studies and clear messaging regarding the observation of eclipses under mid-thickness clouds (where solar viewing glasses become redundant).

1. Safety Message

Looking directly at the Sun without certified solar eye protection can result in serious eye damage. With the exception of the totality phase of a total solar eclipse (where the Moon fully blocks the Sun), eye protection must be used for all partial eclipse phases — including during partial eclipses, annular eclipses, and the partial phases of a total eclipse. This article discusses how mid-thickness clouds can block enough sunlight to provide an opportunity for phones and cameras to capture unsaturated eclipse photos, which is not possible or safe with the unaided eye alone.

2. Visiting Uvalde

In preparation for the October 14th, 2023, annular solar eclipse, a team from the U.S. National Science Foundation's National Solar Observatory (NSO) traveled from offices in Boulder and Hawaii to Uvalde, Texas. This group of scientists, education and public outreach officers, and volunteers arrived in Uvalde two days prior to the event and visited schools in the district to educate children about the next day's eclipse. Uvalde is in a special location, sitting within both the October 14th, 2023, path of annularity and the April 8th, 2024, path of totality. We chose to visit this area in particular to build relationships within the community and maximize the impacts of both events.

A key component of our visit was to explain the difference between annular and total solar eclipses and the different methods to view them. The critical point of this communication was that solar glasses should be removed for the few minutes of totality in April 2024 (and kept on for the rest of the eclipse) but worn for the entire duration of the imminent annular eclipse in October 2023. These are the typical safety viewing instructions for viewing eclipses.

Throughout this day, as we visited from school to school, one thing was consistently on our mind — the weather. At that time, clouds were forecast for the following morning, and they were expected to break into partial clouds around midday (the time of annularity). The eclipse viewing event was organized at the baseball stadium of Sull Ross State University in Uvalde, and so we were not flexible on location. With a plethora of other activities planned for the event, we knew it would never be a complete dud. But, given that the eclipse was the main attraction, the clouds looked set to make the difference between a good and great experience for the visitors and Sull Ross volunteers.

3. The Day of the Event

The morning of the eclipse, part of our group sat in a local cafe for breakfast. The weather outlook appeared bleak, with a solid gray, overcast sky above us. We chatted to eclipse chasers one the table over, who were about to drive two hours west to clearer skies. With an event waiting for us down the road, we had no such luxury. As pessimism started to take hold, we began telling ourselves - ‘at least it's not a total eclipse’. With the overcast skies, we were not hopeful for a large attendance at our event either. Would the invited school kids, teachers, and barman from the night before come to our eclipse viewing event under the clouds? We would have to see. If not, at least the waitress, cook, and state troopers in the cafe were happy with the last-minute eclipse glasses we gave them.

As we left breakfast and arrived at the baseball field to set up activities, a trickle of visitors began to arrive. We optimistically set up solar telescopes and sunspotters, hoping for a break in the thick cloud. At the anticipated time of first contact, as the Moon started to graze the edge of the Sun, neither were anywhere to be seen.

Our first glimpse came about 20 minutes after first contact. It was a fleeting view between the clouds as the words ‘eclipse glasses on!’ echoed across the field. The optimism was brief, as the Sun soon after disappeared. ‘At least the visitors have seen something,’ we said to ourselves. The clouds even provided theatrical-level drama to the event as the crowd cheered and booed. The clouds parted and quickly returned to block our view.

But after another 20 minutes, something changed. The previously-thick clouds had thinned out a little. And so, despite no patches of blue sky in sight, pointing a simple iPhone camera toward the sky revealed it... the crescent Sun. The clouds had reached the perfect thickness. Thin enough that sunlight could pass through but thick enough that phones and cameras could easily resolve the eclipse clearly — with no extra filter. We whipped out our solar glasses to check the view through them, but we saw nothing; the clouds were too thick.

The photographs we began to capture were fantastic, since the clouds yielded a far more dynamic foreground than blue skies and solar filters could provide. Figure 1 shows an example iPhone image. From my perspective, at least, the photos through the clouds created a sort of optical illusion, giving the Sun a much larger apparent size than it typically does through solar glasses — much like how the sizes of the Moon and Sun are further exaggerated near the horizon, as the landscape provides scale for our perception. The clouds also provided an eerie feeling, darkening the sky more than the eclipse would by itself.

Lowering my iPhone, I then brought out my camera (fit with a 150mm zoom lens) and began experimenting with the exposure. After a bit of trial and error, I captured the first frame in Figure 2. Similarly to the iPhone photos, these images used no added solar filters (which are required to image a blue-sky eclipse, regardless of how low you set the ISO and exposure time). Upon closer reflection post-eclipse, I noticed that sunspots were even visible on the Sun in the image, analogous to how English monks and Chinese astronomers observed non-magnified sunspots through clouds centuries ago.

Just as the partially-eclipsed Sun was visible to cameras, it was also visible to the naked eye. The cloud was thin enough for the Sun to be visible, but thick enough for there to be no obvious straining to the human eye. However, as we know, human eyes are sensitive. Just because the Sun was visible without apparent straining, there was no guarantee that it was safe to look at. Non-optical wavelengths of light, sudden changes to cloud coverage, or other factors, could easily still make Solar observations through clouds unsafe. Although the safety requirements for blue-sky solar viewing are well known, to our knowledge, there is little literature on the impacts of viewing the Sun through mid-thickness clouds, where solar viewing glasses are useless.

Only seconds had passed between our first glimpse at the cloud-covered eclipse and initial photographs before we needed to make a decision. We had just spent days reinforcing the fact to the general public that eclipse glasses should be worn at all times during an annular eclipse. And yet, members of the public were beginning to notice for themselves that the Sun (invisible through eclipse glasses) was visible to their naked eye. Aware that this was potentially dangerous, this posed a rapid challenge for our team. Potential messaging for this scenario was not something we had discussed, and we expect this to be the case across the majority of eclipse viewing events. We quickly ran across the field to discuss between team members.

Due to the thicker cloud cover that morning, the crowd at our event was far smaller than it would have likely been with clear skies. The crowd of around 50 were mostly college students and adults, as families with younger children would have been less likely to make the trip out with low chances of seeing the event. Given this older, more sensible audience, it was easy to decide on our message and circulate it quickly. As the audience was beginning to notice that the eclipse was visible to their naked eye through the clouds, we announced the uncertainty in the safety of viewing it directly and encouraged the audience to instead capture this mid-thickness cloudy eclipse with their phone cameras — a much safer method than viewing it directly. We were confident in this decision, and we knew that looking at the digital display on their phones instead of directly to the sky was safe in this scenario.

The mid-thickness clouds were present all the way to annularity, providing a view of the annular eclipse to our phones for most of this period — with many capturing excellent photographs in the process. Fortuitously, we even got a glimpse of the eclipse against the blue sky (with eclipse glasses) towards the end of annularity. As the annularity ended, the clouds parted, and we viewed the rest of the eclipse through solar glasses in the typical fashion. The remaining panels of Figure 2 show photos of the ring of fire through the cloud. Many photos of the cloudy eclipse were captured by the public, which would not have been as clear during a cloudless eclipse. In this case, the clouds likely contributed significantly to the quality and quantity of photos shared across social media by the event attendees.

4. Conclusions

During the October 14th, 2023, annular eclipse, the presence of mid-thickness clouds resulted in unique photo opportunities of the event across the Uvalde region. We later spoke to organizers of another viewing party an hour away, who shared a similar experience. In the end, the clouds added to the experience, amplifying the drama and atmosphere of an otherwise blue-sky eclipse. To our knowledge, the influence of cloud-covered solar viewing on eye safety is an under-studied area. Presumably, one might expect there to be some thickness of clouds where this is safe — but it is difficult to determine in the field whether or not these conditions are met. It was for this reason that we recommended the use of phone cameras to make the most of this opportunity. We recommend that the eclipse education community consider this scenario for future eclipses. Both in the form of interdisciplinary studies on the influence of different cloud thicknesses on solar viewing eye safety and the need for messaging preparedness in the case of audience members noticing that the eclipse is visible to their naked eye through the cloud.

Acknowledgments

We thank the Brinson Prize Fellowship for supporting both Ryan’s scientific research and outreach activities for the 2023 and 2024 eclipses. We acknowledge the support of NSF CAREER SPVKK1RC2MZ3. We also thank Sull Ross State University and Southwest Texas Junior College for their support during the eclipse. NSO is thankful to NSF for all contributions to its eclipse efforts.