Ever notice the reddish orange clouds at night that look something like this?

While most of us would associate the colour of the clouds with the setting of the sun, it’s time to think again. Research have shown that the redness in these clouds shows the interaction of clouds with sky glow from cities. With increased cloud cover, long-wave red light radiation from cities that are usually emitted into space during clear night sky are scattered back down on earth, resulting in the reddish glow on clouds and the night sky. 

In natural environments with low levels of light pollution, clouds aid in making the night sky darker by blocking out a large proportion of light from stars as well as the surrounding light from cities. However the opposite is true in urban areas, whereby the presence of clouds actually lead to significant brightening of urban night skies and intensify the extent of ecological light pollution (Kyba, 2011). 

To understand how clouds amplify light pollution, let us first take a look at the composition of clouds. According to Kyba (2011), clouds are essentially made out of a dense collection of aerosols that are generally non-absorbing at visible wavelengths. This means that clouds are highly reflective and therefore have the capacity to amplify sky glow. 

In this study that looks at the effects of cloud cover on sky glow in Berlin, results point to the fact that the sky becomes brighter when the sky is heavily overcast. 

In the figure above, in urban areas (black line), the sky is significantly brighter in areas which have denser cloud cover compared to that of clear skies (note that lower values of mag/arcsec² indicate brighter nights). 

Additionally, this figure shows the degree to which cloud cover exacerbates the impact of light pollution. From panel A, we see that as the sky becomes more and more overcast (indicated by higher oktas), sky brightness increases rapidly. Data from this study also states that the average observed sky brightness when night skies were fully overcast was 16.5 mag/arcsec², which is around 10600% brighter than that observed for natural dark skies at the dark-sky park in Hungary (Kyba, 2011). 

This added amplification that cloud cover on light pollution has its implications. As we have discussed previously in various of our blog posts, many of the behaviors of organisms are dependent on the presence of moonlight. Because of the relatively high frequency of overcast conditions, further exacerbated by other forms of pollution, this cloud amplification effect takes a strong toll on the ecosystem and organisms in urban areas. 

Through understanding the effects that cloud cover has in intensifying light pollution, light pollution should no longer be seen in isolation with other forms of pollution. As mentioned earlier, the formation of clouds is also highly dependent on the emission of aerosols, which is an example of a particulate air pollutant. 

 

Stay tuned for more!

Jean 

 

References: 

• Kyba, C.C.M., Ruhtz, T., Fischer, J.,& Ho ̈lker, F. (2011). Cloud Coverage Acts as an Amplifier for Ecological Light Pollution in Urban Ecosystems. PLoS ONE 6(3).