Layers of nocturnal insect migrants at high-altitude: the influence of atmospheric conditions on their formation

Radar studies of nocturnal insect migration have often found that the migrants tend to form well-defined horizontal layers at a particular altitude. In previous short-term studies, nocturnal layers were usually observed to occur at the same altitude as certain meteorological features, most notably at the altitudes of temperature inversion tops or nocturnal wind jets. Statistical analyses are presented of 4 years of data that compared the presence, sharpness and duration of nocturnal layer profiles, observed using continuously-operating entomological radar, with meteorological variables at typical layer altitudes over the U.K. Analysis of these large datasets demonstrated that temperature was the foremost meteorological factor that was persistently associated with the presence and formation of longer-lasting and sharper layers of migrating insects over southern U.K.