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=== angle of incidence ====== D layer absorption ===In the low altitude D layer, the density of atmospheric particles is much greater than the density in the higher E and F layers.The D layer absorbs HF frequencies below the 20-meter band. When the D layer dissipates after dark the lower frequencies are free to propagate by skywave or skip from the E and combined F layers. Because of this, ham radio operators use the higher HF bands of 20-meters and above during the daytime, and use the lower bands at night.While the 20-meter band and higher frequencies are not significantly attenuated by D layer energy absorption due to their smaller wavelengths, absorption at lower frequencies is usually too severe for DX. You can expect the 20-meter band to work to some extent both day and night.* Frequencies above 20-meter up to MUF propagate better at day.* Frequencies below 20-meter propagate better at night.=== angle of incidence ===Concerning the D layer and NVIS: Near Vertical Incidence Skywave (NVIS) technique causes lower HF signals to travel through the D layer in the shortest dimension, nearly at a right angle to the layer's spherical shell overhead. As a result, absorption of low frequency signals is minimized and NVIS regional communications is viable during daylight hours, particularly on the 40-meter band.Solar flares: increased ultraviolet and X-ray radiation from solar flares will affect radio communication in only 8 minutes of the solar event occurring.