The aim of this work is to document cirrus characteristics using ground-based measurements. A climatology of sub-tropical cirrus clouds is presented from the analysis of the Rayleigh-Mie lidar data collected at the Observatoire de Physique de l'Atmosphere de la Reunion (OPAR) over the period 1996--2001. The lidar laser operates at 532 nm. This climatology is based on the analysis of upward laser beam over 533 nights corresponding to 1643 hours of lidar probing. In this sub-tropical zone, two main seasons prevail for cirrus clouds occurrence: an austral winter, from May to October, and an austral summer, from November to April. As expected, cirrus are present more frequently during the austral summer, 13% of measurements, than during the austral winter, 1%. Subvisible cirrus clouds, characterised by an optical thickness below 0.03, compose a significant fraction, 65%, of the total cirrus observations.