Small-scale (less than 15 km horizontal wavelength) wavelike structures known as ripples are a common occurrence in OH airglow images. Recent case studies attribute their origin to the presence of either convective or dynamical instabilities. However, little is known about their frequency of occurrence and period. The Maui-MALT Observatory, located at Mt. Haleakala, is instrumented with a Na wind/temperature lidar, which allows the determination of whether the atmosphere is dynamically or convectively unstable, and a fast OH airglow camera which takes images every 3 s with a sensitivity high enough to see the ripples. This study reports on 2 months of observations in October/November 2003 and in August 2004, eight nights of which also included Na lidar measurements. The imager results suggest that instability features occur in the 85- to 90-km region of the atmosphere for around 20% of the time. The nominal observed period for the ripples is between 2 and 4 min. While there are clear night-to-night variations, the average observed period is similar for both the 2003 and 2004 observations. In addition, a few of the small-scale structures are not ripples caused by instabilities but rather have features consistent with their being short horizontal wavelength evanescent waves. Their fractional intensity fluctuations are as large or larger than those of the ripple instabilities. Unlike the instabilities, the origin of the evanescent waves is not determined.