Famous examples are the French and Latin American wave of 1954, and the American wave of 1973. Flaps - shorter duration regional peaks. These may last up to a few months. Flaps occasionally recur in the same geographic area over two or more years. Concentrations - short duration local peaks.
Concentrations can occur during or seperate from flaps and waves.
Some of the most interesting of these have been the September-November 1965 New Hampshire USA concentration and the two day Levelland concentration of November 1957. The Causes of Variation Though frequently debated, the causes of these variations remain unknown. Some have claimed a correlation with the distance of Earth from Mars or Venus. Others have claimed a variety of periods not related to outside causes. However, except for isolated instances, UFO concentrations, flaps, and waves have defied prediction.