Ramp-like and wave-like coherent structures are commonly found in the nighttime temperature and velocity time series measured within and just above plant canopies. Recent observations in forests show that these coherent structures are manifestations of Kelvin-Helmholtz instability, which is evidenced by the strikingly well-organized Kelvin-Helmholtz billows in the time-height temperature contour plots. The objective of this study is to report the common features of the billow structure and the associtated secondary instabilities. The billow events are frequently found near the treetops at clear night with strong radiative cooling and moderate wind. The primary Kelvin-Helmholtz billow trains are quite steady and can last for a few tens of minutes. The ratios of the billow wavelength to the amplitude agree well with both the predication from linear wave models and wind tunnel measurements. The billows are centered above treetops, extending from mid-canopy to about two tree heights. Statically unstable temperature structures are found within the billows. The secondary convective instability is manifested by the fine-scale fluctuations in the temperature traces. The mean temperature profiles are modified by the convective instability.