At Dusk or Dawn, point your Antenna along the Grey Line

The grey line (US: gray line), also called twilight zone or terminator, is a contour band around our globe that separates the sunlit portion from nocturnal darkness. These zones of dusk and dawn are somwhat diffused regions since the Earth's atmosphere tends to scatter the light into the darker areas. Furthermore, there are some time delays involved with the ionisation and de-ionisation of the ionosphere by sunlight.

Propagation of the lower shortwave frequencies (upto about 10MHz) is more efficient along the grey line then at other times. The reason is that the D-layer, which absorbs HF signals below 10MHz, dissappears rapidly after sunset and needs time to build up after sunrise, while the other, refractive layers E, F1 and F2 are still, respectively already, ionised.

It is important to note that, except at the equinoxes (21 March & 21 September), the grey-line direction will be different at sunrise from sunset, meaning different areas of the world can be contacted in the evening from those in the morning. Over the span of a year, the grey-line direction will vary 23° either direction, reaching this maximum tilt on the 21 June and 21 December, respectively.

In order to deal with this complexity, a tool like the GlobeView java applet, designed by Chris Bruns, proves to be indispensable. Chris, a cartography expert, was even so kind to apply a few changes to his applet in order to render it even more useful to us radio-amateurs.