There are multiple ways to calculate the stability index, and HYSPLIT's method does not have a day/night dependence. HYSPLIT calculated stability index is based on comparing HYSPLIT calculated vertical mixing coefficient (Kz) with empirical ranges of Kz at the top of the surface layer based on Draxler, R.R., Estimating vertical diffusion from routine meteorological tower measurements, Atmospheric Environment, 13, 1559-1564 (1979): see https://doi.org/10.1016/0004-6981(79)90065-9.
READY/HYSPLIT stability classes based on vertical mixing coefficient Kz.
1. Strong insolation corresponds to sunny midday in midsummer in England; slight
insolation to similar conditions in midwinter.
2. Night refers to the period from 1 hour before sunset to 1 hour after sunrise.
3. The neutral category D should also be used, regardless of wind speed, for overcast
conditions during day or night and for any sky conditions during the hour preceding or
following night as defined above.
PG classes for fluctuations in wind direction and the vertical temperature gradient.