Surface-layer stability is important in many processes, such as in the surface energy budget, atmospheric pollution, and boundary-layer parametrizations. Most previous studies on stability, however, conducted either theoretical or observational investigations at specific sites, thus leaving a gap with regard to the large-scale pattern. Here, wind speed and temperature observations at multiple heights from the wind-tower network of China are used to estimate the stability during the 2009–2016 period. A series of data-quality-control procedures are conducted and data from 170 wind towers with more than 2 years of valid observations are selected. The degree of stability is classified by the Obukhov length, which is derived from the wind speed and temperature at 10 m and 70 m above ground level, combined with information regarding the roughness length. Overall, the occurrence frequency of surface-layer instability exhibits significant temporal and spatial variability, being particularly larger in spring and summer than in autumn and winter. The maximum frequency of summertime instability occurs in the time period 1000–1200 local solar time, approximately 2 h earlier than in autumn. Geographically, the peak instability frequency occurs much earlier in the day in north-west China than in other regions, likely owing to the arid and semi-arid land cover. Also noteworthy is the steady increase in instability frequency observed during the period analyzed here, likely resulting from the reduction in the vertical gradient of wind speed. Our findings call for explicit consideration of stability variability in the wind-energy industry and in fundamental boundary-layer investigations in China.

Obukhov length,Roughness length,Surface-layer stability,Chinese wind-tower network