Design of a Network of Skywave Over-the-Horizon Radars Figure 3. Example of daytime (left column) and nighttime (right column) HF propagation conditions during winter, low solar activity (top row); winter, medium solar activity (middle row); and summer, medium solar activity (bottom row). Note the lack of propagation support during winter at low solar activity levels. The time standard for these plots is local solar time, the local time adjusted to true solar midday. 22 IEEE A&E SYSTEMS MAGAZINE DECEMBER 2017