Solar observing is gravely hampered by the turbulent fluctuations of temperature and, therefore, refractive index, which occur in the lower atmosphere. However, above a height comparable with Obukhov's scale L (which by conventional definition is negative in daytime conditions), ascending thermal convection plumes are interspersed with sinking environmental air which is devoid of temperature fluctuations. This air affords intermittent intervals of excellent seeing lasting from a few seconds up to a minute or more, whenever |L| is less than the telescope height and the wind direction is favorable. |L| is typically several meters or tens of meters, and can be estimated in terms of the meteorological conditions. The main dependence is on wind speed, with light winds giving small |L|.
E. K. Webb, "Daytime Thermal Fluctuations in the Lower Atmosphere," Appl. Opt. 3, 1329-1336 (1964)