Sky Brightness 3

As we saw in part 1 and part 2 of this series the typical measurements of sky brightness in Providence are between about 4.1 - 4.3 nelm (naked eye limiting magnitude) on clear nights. Here is a graph that shows a typical hazy summer night. The readings were taken on the night of July 1st into the morning of July 2nd of 2014 and are in the range that we commonly see. The dashed horizontal line is a somewhat arbitrary divider between typical and darker nights. When the sky brightness is below about 4.3 the observing is much better.

Looking at a graph of the sky brightness doesn't give an intuitive idea of what the sky actually looked like for observing. We can see this by looking at the wide angle views of the sky using the camera mounted on the roof. Here is a time lapse movie from the same night as the above graph.

Sky Brightness 2

In my previous post I began to analyze the data from the sky brightness meter at Ladd Observatory. Now we'll take a closer look at the broader trends. Here is a scatter plot showing the data from the summer and fall of 2013. The plot is a little busy but we're really only interested in the "bottom line" where the data points are at the lowest values. All of the nights are superimposed on one another with the x axis showing hours UTC. This graph summarizes how the sky brightness changes during the course of the night. The many values between 3.7 and 4.3 are due to nights that are more or less hazy. There moisture in the atmosphere scatters light from the city back down to us and causes the overall sky to look brighter.

If we follow the lowest readings there is a definite trend where the clearest nights start off at about 4.2 at the end of twilight and slowly, steadily, decrease to about 4.45 at 4 hours UTC. There is then a small but rather sudden drop to 4.55 after which the slow decrease continues until we are at about 4.6 in the early morning. I'm not sure what is causing the drop at 4 hours but it may be due to city lights that are on a timer. The takeaway here is that the sky is slightly, but significantly, brighter in the early evening. The best time to observe is after midnight local time through the early morning.

Sky Brightness

"The sky above the port was the color of television, tuned to a dead channel."
- Neuromancer, William Gibson, 1984.

At the Ladd Observatory we operate a weather station and a number of other rooftop instruments to monitor the environment. One of them is a sky brightness meter. On a regular basis we use the live data to judge the quality of the sky for observing. It is also used to document long term changes such as the increase in light pollution.

Sky brightness meter
and camera on the roof.

The meter is contained in a weather proof housing next to a wide field sky camera. The camera takes a low resolution image of nearly the entire sky every 10 seconds and these images can then be compared to the brightness readings. I can then verify what the sky looked like when a measurement was taken. When the sky is very cloudy it scatters light from the city and the readings are very bright. Haze or high humidity can also cause elevated readings.

The sensor is too sensitive to take a measurement during the daytime. It starts collecting data shortly after sunset when the sky begins to darken and stops during morning twilight just before sunrise. Last summer I calibrated the meter and we've now collected 300,000 data points in about one year. I thought this would be a good time to analyze what we have so far.

"The Red Skies" of 1883

"It is impossible not to conjecture a connection with the volcanic eruption in the Sunda Straits, by which, on Aug. 26, the island of Krakatoa disappeared wholly from the face of the earth."

"The terrible nature of this outburst can hardly be realized: the sky was darkened for several days, the noise was heard two thousand miles, magnetic disturbances were noted, the tidal wave was distinctly felt at San Francisco, and the atmospheric disturbance was sufficient to cause marked barometric fluctuations, which were noted by the barographs on the continent, in England and America, for several succeeding days."

- W. Upton, "The Red Skies." Science, 11 January 1884

During the fall of 1883 there was a remarkable atmospheric phenomenon which "attracted great attention not only from the general public, but from scientific men, who have endeavored to give a satisfactory explanation of it." At the time that he wrote those words Winslow Upton had just accepted the position of Professor of Astronomy at Brown University. Prior to this he had been Assistant Professor of Meteorology in the U.S. Signal Service from 1881. The phenomena that he endeavored to explain were the "recent fiery sunsets" seen throughout the world.

The Scream (1893) by Edvar Munch
(National Gallery, Oslo, Norway)

There were three different hypotheses as to the cause of the "blaze of brilliant red light" seen at sunset. One possibility was refraction through water vapor in the atmosphere. Another suggestion was that the Earth was passing through a cloud of meteoric dust. But the most likely explanation, as improbable as it sounded at the time, was that a large amount of dust from the eruption of the volcanic island of Krakatoa had been thrown up to such a height that it slowly spread around the globe. Microscopic examinations of residue from snow in Madrid and a rain-storm in Holland seemed to confirm the volcanic hypothesis by revealing the presence of particles that were similar in composition to the ash from Indonesia.

The sight of the blood red sky seen at sunset may even have inspired the Norwegian artist Edvar Munch who "felt a great, unending scream piercing through nature."

Note: I originally published this on the Ladd Observatory Weather Underground blog in 2011.

The summit station at El Misti, Peru (19,200 feet)

"The night is passed at the hut, and the final ascent to the summit made on the second morning. This occupies several hours, as the animal stops to rest every fifteen or twenty feet at this altitude. On two occasions I was obliged to walk a short distance to cross snow which had drifted across the path, and realized the extreme difficulty of breathing during the exertion required."

"The effect of the altitude upon me was chiefly to cause headache, sleeplessness and partial loss of appetite. On one occasion while at the summit I experienced a decided feeling of faintness for a short time."

- Winslow Upton, Physiological Effect of Diminished Air Pressure, Science, 27 December 1901

Misti summit station, Jan. 5, 1894.
Shut in by cloud [and] snow looking N. E.

During the academic year of 1896-97 Prof. Winslow Upton took sabbatical from his work as Director of Brown University's Ladd Observatory. He spent ten months at the new southern station of the Harvard College Observatory (elevation 8,050 feet) in Arequipa, Peru. His primary goal was to measure the geographical position of the station before astronomical observations could commence.

During this time he also made four ascents to the summit of the dormant volcano El Misti, which was the site of recording instruments (pictured above) maintained by Harvard. At the time it was the highest meteorological station in the world at an elevation of 19,200 feet.