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.