The Global Ionospheric World Map indicates the frequency in MHz at which a signal transmitted more or less straight up (about 70 – 90 degrees) will no longer bounce back to Earth, but rather escape out into space.
In short, this picture shows the Maximum Usable Frequency (MUF) for Near-Vertical Incident Skywave (NVIS) propagation.
NVIS is the mechanism for reliable HF communications within a region (500 mile radius). Without NVIS, there is a skip zone or dead zone where neither groundwave propagation nor skywave propagation are effective. This skip zone is often a band around the transmission site extending from between 20 miles (where groundwave leaves off) and a few hundred miles (where skywave begins to take effect).
