80 and 40 meter WSPR NVIS Study

Charts of 80 meter and 40 meter WSPR Near Vertical Incidence Skywave (NVIS) spots SNR vs. distance
Clusters of spots in red were received during daylight hours. Those in blue during hours of darkness.
The antenna used was a thin wire 40 meter full wave horizontal loop at 6 meters above ground.
Near Vertical Incidence Skywave (NVIS) is an ionospheric propagation mode for HF communications under 500 km distance, generally on frequencies below 10 MHz. These charts show WSPR spot signal strengths of stations within 500 km of my station. They demonstrate that signal strengths are highest on 7 MHz during hours of sunlight and on 3.5 MHz during hours of darkness.
The WSPR spots vs. distance and time data are posted at https://picasaweb.google.com/114032640816757126398/NVISWSPRSpotData
For more on NVIS see http://en.wikipedia.org/wiki/Near_vertical_incidence_skywave