Radio propagation that rely on reflection (actually refraction) of radio waves from the ionosphere. The ionosphere is a region of the atmosphere from about that contains layers of charged particles (ions) which can refract a radio wave back toward the Earth. A radio wave directed at an angle into the sky can be reflected back to Earth beyond the horizon by these layers, allowing long distance radio transmission. The F2 layer is the most important ionospheric layer for long-distance, multiple-hop HF propagation, though F1, E, and D-layers also play significant roles. The D-layer, when present during sunlight periods, causes significant amount of
Atmospheric Circulation Reconstructions over the Earth Atmospheric Circulation Reconstructions over the Earth The Atmospheric Circulation Reconstructions over the Earth, ACRE, is an international science project, began in 2008, that recovers historical weather observations to reconstruct past global and local weather patterns and so support meteorological reanalysis. The project aims to collect weather data from the past 250 years by linking international meteorological organisations to support data recovery projects and the imaging and digitisation of historical meteorological observations made at, for example, inland stations, lighthouses, or by ships at sea or in ports. The project aims to create historical datasets that are spatially and temporally complete, so