During summer daytime, which bands are the most difficult for communications beyond ground wave?

During the summer daytime, solar radiation is at its peak, heavily ionizing the D region of the ionosphere. This lowest layer of the ionosphere acts like a sponge for lower-frequency radio waves, strongly absorbing signals on the 160-metre and 80-metre bands. Because these low-frequency signals are absorbed before they can reach the higher E and F layers to be refracted back to Earth, sky-wave (long-distance) propagation on these bands becomes nearly impossible. As a result, daytime communication on 160m and 80m is typically limited to just the short ground-wave distance. Higher frequency bands like 20m and 30m easily pass through the D region with minimal absorption and bounce off the F region, making them the preferred choices for daytime long-distance contacts.