The Met Office operated 2 Vaisala CT25k units at their Cardington field site before its closure in 2024, designated units A and B.

This instrument has a unique single-lens design which ensures excellent performance at low altitudes and is able to measure from nearly ground level to cloud base. The Vaisala CT25K Laser Ceilometer measures cloud height-or vertical visibility if the cloud base is obscured. The single-lens design also ensures reliable measurements in fog, rain, snow and haze. No field adjustments needed.

The CT25K can be tilted on its base allowing the beam to be directed manually in any direction between -15 to+90°. The system is fully automatic, transmitting messages of cloud height and instrument status to the controller. Its software includes an extensive set of self-diagnostic routines to ensure reliable operation. The CT25K can be used stand-alone or as an integrated sensor in a weather observation system.

This product is no longer in production and Vaisala, being superceeded by Vaisala's CL31 ceilometer.