Strangely bright morning twilight over Switzerland

Yesterday morning (04.11.2014) I woke up at 6:40 as usual and noticed that it was relatively bright out. I had a better look around 7 and concluded that it was definitely quite a bit brighter than it was the day before at that time. There was also a strange, yellow tint to the light.

I looked up when the sun comes up in Basel (where I live) and found out it's at 7:18. Later I did some googling to see if anyone else had noticed this and came across this article about something similar happening in 2008. It mentions polar stratospheric clouds as a likely cause. The following evening however, I didn't notice anything remarkable. And as I write this the next morning, The sun has now been up for an hour but due to clouds it looks darker than it did an hour and a half earlier in the morning yesterday.

So I'm wondering, what is the cause of this unusually bright morning twilight? And can I expect to see it again in the coming days/weeks/months?

Something that might relate to this is the siberian winter that seems to await Europe.

PS: I also wouldn't be averse to other people around Europe confirming that they also noticed this.

1 Answer

If these were polar stratospheric clouds, that could explain a brighter than normal sky either before sunrise or after sunset. The higher the clouds are, the more lead/lag time you will have to observe the phenomena before the sun comes into view. The reason for this is simply that the clouds are in direct line-of-sight with both you and the sun, but you and the sun are not in line-of-sight. The sun illuminates these clouds and being cold icy clouds they reflect much of this light to your eyes. 100% of the light you see is scattered giving the light a diffuse glow.

Unfortunately, I'm not aware of any stratospheric forecast products for these clouds. The closest thing is an analysis of the 450 K $\theta$-surface showing area within the polar vortex with temperature at or below -78 C. These depict the area within the polar vortex that are favorable for polar stratospheric clouds. However, these do not show where the favorable area is, nor give an indication if these clouds will actually form. You can view the northern hemisphere plots here. The other stratospheric products deal with the size of the polar vortex, UV-B penetration and ozone analysis which do not aid in the forecast of these clouds.