Charts can sometimes show neat results, and the WSPRnet database offers lots of possibilities. After collecting almost 5000 reports on 20m with my QCX-mini at 1W, this showed up in some scatter charts. First shows the distance of the various spots for the whole collection.

Could it be that the horizontal white bands in the chart are not only there because of the lack of receiving stations, but are caused by skip distance / hopping?

The second chart shows the SNR values for all spots, and a trend line. Interesting that there is about a 10dB reduction over the whole distance. Is this what I should expect?