It’s a night coolness variable which takes into account the mean minimum night temperatures during the month when ripening usually occurs beyond the ripening period (Tonietto & Carbonneau, 1999).
The Cool Night Index (CNI) for grape-growing regions ranges from CI+2 [very cool] to CI-2 [very warm] night temperatures:
Very cool CI+2 ≤ 12 °C (night temperature)
Cool nights CI+1 CI+1 > 12 ≤ 14 °C
Temperate nights CI-1 > 14 ≤ 18 °C
Warm nights CI-2 > 18 °C