ARCTIC NIGHT AND DARKNESS
Syrjävaara Dark Sky Center, April 2024
Light pollution has gradually increased so much that arctic winter nights have largely disappeared unnoticed. Measurements of Night sky brightness taken from the ground prove the disappearance of night i.e., darkness, and how far light pollution is spreading in the arctic region.
Syrjävaara Dark Sky Center February 2, 2024. Sony a7s II, 10000 ISO, 8s, Samyang 2.8 / 14mm
Background
Syrjävaara’s Dark Sky center maintains online remote controlled night sky brightness measurement stations in three regions of Eastern Finland namely Juuka, Kaavi and Tuusniemi (Figure 1). The purpose of the measurements is to collect information about the night, darkness, and light pollution in the area.
The Sky Quality Meters (SQMs) with sky cameras are located in sites with no poorly directed artificial light sources. The sources of artificial light at night are from further away settlements. The closest settlement from the northern meter (SQM 1) is the center of Juuka municipality located about 18 km away. The closest from Syrjävaara meter (SQM 2) is Outokumpu town located 15 km away. The closest settlement to the southern meter (SQM 3) is 11 km from the city center of Tuusniemi municipality.
Figure 1: Locations of sky quality measurement sites on Light Pollution Map¹.
Sky quality measurements are taken on starry nights, when there are no auroras and the Moon is below the horizon. This measures the night sky brightness against light pollution produced by Artificial Lighting at Night (ALAN). The more stray light there is in the night sky, ALAN pollution inflicted by human use of ALAN, the more light-polluted the night sky is.
In this article, night and darkness refer to the time of day when the sun is at least 18 degrees below the horizon. In Finland, there are no dark nights in summer. Dark nights period runs from autumn to spring.
The Syrjävaara measuring station was established in the fall of the year 2021. The other two stations in the measuring chain were set up in March 2023; data from these measurements form the basis of this article. During the summer 2023, the stations were off because there was no night darkness to measure. Measurements commenced in the fall. The measurement period in this article covers the entire year. Based on the measurements, new information has been obtained about darkness and the atmospheric phenomena that affect it in the Arctic region.
The measurements were made in a geographically small area. However, the phenomena examined in the article are general characteristics of the atmospheric conditions in northern latitudes, they are not only specific to the location from which the measurements were made. Based on this, it is reasonable to assume that the same phenomena affecting the spread of light pollution is similar throughout the Arctic region.
The measurement period discussed here only covers one year. Therefore, annual variation and evaluation of effects requires a significantly longer observation period. There could be huge differences between measurement years.
Estimating and measuring darkness
The darker the night sky, the better the astronomical observability of celestial objects. The darkness of the night sky can be estimated visually and measured with a Sky Quality Meter. The less light pollution there is, the brighter and clearer the stars and the Milky Way appear. Light pollution reduces the visibility of the stars. If there is too much artificial light – sky glow – coming from the ground, the stars won’t be visible at all. The limit for darkness is that the Milky Way is generally visible in the night sky. If its hazy clouds of stars and dust are clearly visible, darkness is guaranteed. However, if only constellations or the brightest stars are visible from the Milky Way, then light pollution has taken dominance over the night sky.
When measured with a Sky Quality Meter, the numerical limit of the visibility of the Milky Way is 21.2 magnitudes per arcsecond squared. If the value is lower, the night sky is clearly light polluted. Correspondingly, magnitude values (mag.) higher than 21.2 indicate darkness. Pitch black environment, where you can’t see anything with the naked eye, is achieved only in full cloudy weather in a place where there are no light sources. For this the light meter shows values of mag. 22 or over. The highest values measured in the measurement site are above mag. 24.
Darkness and snow
In Finland and the Arctic region, the Earth’s rotation around the Sun creates a special condition for the dark season. The night is part of the year and night darkness is affected by phenomena, not all of which occur in middle latitudes. The winter season when Finland experiences Arctic darkness, depending on the latitude, starts at the beginning of September and lasts until the beginning of April (Figure 2). In southern Finland, the dark period is a couple of weeks longer and in the north correspondingly shorter. The night darkness gets longer as Finland approaches the winter solstice and Christmas.
Figure 2: Graphic Almanac for Joensuu². The dark period is marked in black.
Even though one might assume that night darkness deepens as night lengthens, this is not so, the nights in all three measurement locations in this article were brighter in midwinter than in autumn or spring. When the air temperature drops, the nights become more and more bright, and in the coldest months of the year, December-February, it is often so that no proper darkness is reached.
It has been suggested on various forums that the reflectivity of snow increases the brightness of the night sky in winter, because snow reflects up to 90 percent of the light back up. Estimates of this increase in brightness range between magnitudes 0.5 and 1.0. Roughly speaking, this means a one-and-a-half to two-and-a-half-fold increase in the amount of light. This is undoubtedly true in areas with ALAN sources, but it is not so in the unlit countryside. Measurements made during new snow in optimal conditions clearly show that (Figure 3). Compared to the values given by the light pollution map¹, the difference is only about 0.1 magnitude. In an area with no lighting or stray light, the effect of snow is very minimal, snow does not explain the brightness of winter nights.
Figure 3: Measurement at the time of new snow, when the snow is at its most reflective. The LPM value at measurement location is 21.84.
The effect of the northern lights
The effect of the northern lights is greater than the effect of snow. At the height of the measurement area on the horizon, often northern light present, the weak Lapland aurora borealis, is a normal phenomenon. Even the slightly increased values of the geomagnetic activity forecast3 already affect the darkness. Although faint aurora borealis is relatively invisible to the naked eye of a person unaccustomed to it, they are visible in the light pollution measurement and increase the brightness without noticing it, this is by about 0.5 magnitude. Even the very faint northern lights draw a beautiful cloud-like curve. Aurorae peaks appear as reduced values in the curve (Figure 4).
Figure 4: Very weak aurora borealis and peaks at 00.25 and 01.50. The maximum increase in brightness of about 0.45 magnitude compared to the previous (Fig. 3) curve from the same measurement location three days earlier.
Stronger aurora borealis naturally illuminate more, but they are usually only detected with the naked eye when they reach a height of at least 20 degrees. A large part of the aurora borealis affecting the measurements is ignored if the northern sky is cloudy or not visible at all. Aurora borealis are hundreds of kilometers high, so they light up the sky from the aurora oval far to central Finland.
Aurora coronas / crowns are however in a completely different category. They illuminate the sky by more than three magnitudes. In darkness measurement sites, the effect of the aurora borealis on light pollution measurements is clear. For this reason, one must always have a sky quality measurement meter and a sky camera as a pair of gauges, from which the situation of the aurora can be checked.
Darkness, atmospheric temperature and humidity
The measurement values of darkness from the measurement sites seem to follow the changes in the average monthly temperature of the year. The warmer it was, the darker the sky was, and colder it was, the brighter the sky was (Figure 5). The distance between the Syrjävaara measurement point and Joensuu airport measurement point is 38 km, hence the monthly average temperatures of the places are rather similar (Figure 6).
Figure 5: The starry nights of Syrjävaara – results from the measurement site.
Figure 6: Joensuu airport’s monthly average temperature curve4.
The most significant difference between the average temperature curve and the darkness values is in spring. The average temperatures in April and September are almost similar, but April is remarkably dark and dry compared to rainy September (Figure 6). The effect of low spring rainfall and dryness on the measured darkness is clear.
Figure 7: Average monthly rainfall at Joensuu’s Linnunlahti, nine kilometers southeast of the airport5.
The difference in humidity is also visible in the measurement curves. There is more rainfall starting from the month of July until autumn, and in winter, than is in spring (Figure 7). When precipitation is directly linked to air humidity, which scatters light and whose effects, together with the falling air temperature, appear in the measurement curves giving signs of instability (Figure 8).
Figure 8: Effect of humidity on darkness measurements.
Humidity causes the air to appear somewhat hazy even on the clearest nights. The Milky Way is certainly present in the sky, but it is not really clearly visible. The highest measurement score is reached between midnight and three in the morning (Figure 8). Low-level and middle clouds cause substantial variability.
Deviation of measurements
The darkness values of all measurement locations decreased after autumn (Figure 9). It is interesting that the deviation increased as the winter progressed. In autumn, the values are still quite close to each other, but in mid-winter the situation is undoubtedly different.
Figure 9: Rather strong cloud cover is evident before midnight in the SQM 1 (blue) and SQM 3 (yellow).
Strong downward spikes in the SQM 2 (red) tell about the passes of the forestry machine that was working at night (Figure 9). After midnight, the sky has cleared in all measurement sites. The clear sky measurements indicate dispersion of about 0.2 magnitude.
Figure 10: Sites measurement results at mid winter. Dispersion at about 0.4 magnitude.
In winter, the peak values of the measurement sites have decreased and differences in brightness are evident across sites (Figure 10). When the brightness of the different sites are compared to the best values achieved in spring, it is noticeable that the winter brightness of the northern site SQM 1 is at about 0.4 magnitude, SQM 2 at about 0.8 magnitude and the southern SQM 3 is about 0.6 magnitude less. When these changes are analyzed against the distance of the meters from nearest settlements (municipality/ city) with the largest ALAN source, the situation is as follows (Figure 11).
Figure 11: Brightness changes relation to source of ALAN.
Brightness does not decrease evenly in relation to the distance, instead, it has even doubled in some places (mag. 0.8). This suggests that there is something in the sky that reflects light. The peak of the reflection was 15 kilometers away as per the conditions of last winter (Figure 11). It can therefore be concluded that instead of the stray light forming one bright focus area of light that gradually weakens with distance, the reflection causes a light ring to form around the light source, which in suitable conditions expands the light pollution area by 15-20 kilometers (Figure 12). In the worst scenario, this can mean a tenfold increase in the area of the light pollution area. When this happens even around the slightest less significant light sources, the effect is clear.
Figure 12: Light Ring. Illustration made on the top of Light Pollution Map (ref: Light Pollution Map¹).
Troposphere and ice crystal clouds
The reason behind the Light Ring and the connection between the results of the darkness measurements and the monthly average temperature need further investigation. Dark measurements are made on clear, moonless nights. This means that there is then no cloudiness in the sky at the measurement location that can be seen with the naked eye. Reflections caused at the lowest part of the atmosphere, i.e., the troposphere’s low-level or middle clouds cannot be the main cause. In practice, as the cloudiness increases, the instability of the measurements also increases until the clouds completely cover the sky. The highest darkness values are reached when the sky is completely covered with thick enough clouds. Clouds cover the light.
It is certain that the amount of light coming from the starry sky does not vary. It is also very unlikely that during the coldest winter period, the impact of ALAN pollution would almost double. It can be assumed to be stable. The alternative remains that some factor above the visible clouds is condensing the stray light from ALAN pollution and spreading it.
The root cause of the phenomenon is probably the properties of the troposphere. At mid-latitudes and at the equator, the troposphere is about 20 kilometers thick, but at the poles it is only about 8 km thick (Figure 13). In addition, air temperature affects the thickness of the troposphere. It is thicker in warm weather and thinner in cold weather6.
Figure 13: Thickness of the troposphere7.
Figure 14: Atmospheric structure and composition8.
Near the earth’s surface, the clouds in the atmosphere are water vapor, but the higher up it goes, the atmosphere freezes (Figure 14). In the upper troposphere, the clouds are ice crystal clouds. They may seem thin and transparent, but the thickness of the clouds can be up to two kilometers9. Based on the measurement results, it seems that they also strongly reflect light.
Apparently, as the tropopause descends in the cooler weather, the ice crystal clouds move closer to the earth’s surface. Clouds may also increase and become thicker. They behave like a semi-permeable mirror or water surface, preventing light from freely spreading into the atmosphere, condensing it and reflecting it back. As a result, light pollution spreads more vigorously during the coldest time of winter.
Figure 15: The spread of ALAN caused by reflection in midwinter in eastern Finland. An illustration made on the top of the Light Pollution Map based on the measurements. (ref. Light Pollution Map¹).
As spring approaches, the average monthly temperatures start to rise. The troposphere thickens and the reflections caused by ice crystal clouds decrease. The night sky darkens again. The actual threshold is reached in March-May, when cloudiness and humidity begin to decrease. The air clears and visibility improves. Very quickly, the dark measurement curves become completely even. The darkest and starry nights of the year have arrived. The sky is deeply dark, the Milky Way and its details stand out perfectly. This phase lasts from right after the vernal equinox until the end of astronomical darkness. After that, the brighter nights begin.
Arctic night and light pollution
Though almost unnoticed, an increase in light pollution of barely one magnitude is enough to largely blur from view the starry sky; the Milky Way. At the same time, a third of the aurora borealis, the faintest aurora borealis, remain invisible. Stronger auroras dim by the same magnitude. Between the cycles of aurora maxima, which occur every eleven years, there are many years of lesser and fainter aurora borealis.
The atmosphere is not to blame for the brightening arctic nights, rather man-made light pollution sourcing from ALAN. Due to the complexity of the characteristics of the arctic atmosphere, light pollution spreads much more easily in the area than in the more southern regions of the globe. This means that considerably more attention must be paid to reducing light pollution in the Arctic region. The arctic night is unique but also particularly sensitive.
Arctic night in a nutshell
The darkest and most starry nights of the year are reached at the beginning of April. After that, bright summer nights begin and continue until the autumn equinox.
The darkest nights of autumn are achieved at the time of the autumnal equinox.
In September – October, the nights are dark but the increasing humidity blurs the starry sky.
As the winter progresses, the cloudiness increases such that there are very few starry nights between November-February. During this period of full cloudy (8/8) weather, pitch darkness is often reached at magnitudes between 22-24.
During the winter, ice crystal clouds reflect light pollution strongly, such that in December, when the sky is cloudless, a dark sky is still not reached in many places. It is especially bright near cities and towns. The brightness is between one and a half to two times. The dark places are considerably far from settlements.
When the temperature turns warm again at the end of February, the darker nights return. As the weather warms and the humidity decreases, the sky darkens and clears out towards March and the darkest nights of the year.
Sources
1 Light Pollution Map 2024. Interactive world light pollution map. https//www.lightpollutionmap.info/
2 Karttunen, H. 2024. Graphic Almanac for Joensuu. https://www.astro.utu.fi/zubi/ga/joensuu.htm
3 Auroras Now 2024. Aurora Alerts. https://aurorasnow.fmi.fi/public_service/
4 Finnish Meteorological Institute 2024. Weather data. https://www.ilmatieteenlaitos.fi/havaintojen-lataus
5 Finnish Meteorological Institute 2024. Weather data. https://www.ilmatieteenlaitos.fi/havaintojen-lataus
6 Karttunen, H. 2024. The Troposphere. https://www.astro.utu.fi/zubi/atmosph/tropo.htm
7 Timmermans, R. 2014. Studies of Atmospheric Dynamics from Space. ISBN 90-386-2191-4.
8 Karttunen, H. 2024. Atmospheric structure and composition. https://www.astro.utu.fi/zubi/atmosph/struct.htm
9 Haapanala, P. 2017. On the solar radiative effects of atmospheric ice and dust. Report series in Aerosol Science N:o 198, p 21.