When is propagation on the 80-metre band generally the LEAST effective?

The 80-metre band (3.5 MHz) is a lower frequency HF band that relies on skywave propagation for longer distances. During the daytime, solar radiation strongly ionizes the lowest layer of the ionosphere, the D layer. The D layer acts as a sponge, absorbing lower frequency radio waves like 80 metres rather than refracting them. In the summer, daylight hours are longer and solar radiation is more intense, making D-layer absorption much stronger and longer-lasting. Furthermore, summer brings higher levels of atmospheric noise (QRN) from frequent thunderstorms, which severely degrades weak signals. Therefore, the combination of intense D-layer absorption and heavy atmospheric static makes daytime in summer the absolute worst condition for 80-metre propagation.