The E layer ( E-layer) is characterized by small, thin clouds of intense ionization, which can support reflection of radio waves, rarely up to 450 MHz. Sporadic-E events may last for just a few minutes to several hours. Sporadic E propagation makes VHF-operating radio amateurs very excited, as propagation paths that are generally unreachable can open up. There are multiple causes of sporadic-E that are still being pursued by researchers. This propagation occurs most frequently during the summer months when high signal levels may be reached. The skip distances are generally around . Distances for one hop propagation can be anywhere from to . Double-hop reception over is possible.