Nearly a week has passed since campaign kick-off and the first hundreds of iSPEX measurements have been made. iSPEX-EU started out with blue skies in Italy and Greece and, after merely rainy days with overcast in the northwestern part of Europe, first sunny hours now allow for iSPEX activity also in the streets of London, Manchester, Copenhagen, Berlin, and in the Netherlands.
Why exactly do we ask to measure only on sunny cloud-free days?
The iSPEX method is based upon measuring the intensity and polarization (optical properties) of the scattered sunlight. Tiny atmospheric particles, so-called aerosols, scatter and absorb sunlight as it passes our atmosphere, leaving an imprint on the sunlight intensity and polarization that can be measured at the Earth surface. Moreover, the sunlight that you can observe during a sunny day without aerosols, a clear sky, is at its most intense and it is polarized, whereas on a very hazy day with a large amount of aerosols the sunlight is far less intense and polarized.
What happens when there are clouds? Cloud droplets interact with sunlight in a similar way. Therefore, and in special in case of total overcast, we can’t any longer distinguish between the effects of aerosols on the sunlight that we measure with iSPEX and those of cloud droplets. So, this is why we ask you to measure under cloud-free conditions, or, stated differently, no clouds should be in that part of the sky that is scanned with iSPEX (see also our measurement guidelines).
Tip: For your next measurements, check out the latest weather forecast!