Ceilometer CL31

for cloud height detection

Model: CL31
Manufacturer: Vaisala – Finland


  • Measurement range from 0 to 7.6 km (from 0 to 25 000 feet)
  • Modular design for easy installation and maintenance
  • Second-generation, advanced single-lens optics provides excellent performance also at low altitudes
  • Fast measurement enables detection of thin cloud layers below a solid cloud base
  • Reliable operation in all weather; unsurpassed performance in vertical visibility and cloud detection during precipitation
  • Latest technology from the worldleading manufacturer – based on the experience from more than 5000 installed Vaisala ceilometers worldwide
  • Extensive self-diagnostics with fault analysis

Vaisala Ceilometer CL31 is a compact and lightweight instrument for cloud base height and vertical visibility measurements. It detects 3 cloud layers simultaneously. CL31 uses a pulsed diode laser LIDAR (light detection and ranging) technology. CL31 is ideal for aviation and meteorological applications.

Measurement starts from ground level

The enhanced single-lens technology applied in CL31 ensures excellent performance starting at a height of virtually zero. This is due to the strong and stable signal over the whole measurement range. The single-lens technology provides unsurpassed reliability during precipitation, low clouds and ground based obscurations, which are the most critical phenomena in aviation safety.

Fast measurement

Fast measurement helps to detect thin cloud patches below a solid cloud base. CL31 provides a full backscatter profile for data visualization and research purpose.

The CL31 beam can be directed either vertically or tilted. The tilting option together with the novel optics design provides enhanced performance during precipitation by improving the protection given by the shield. In the measurement unit, a tilt angle sensor automatically

corrects the measured cloud distance reading to vertical cloud base height.

Extensive self-diagnostics

CL31 is fully automatic. In addition to cloud height data, the messages contain instrument status information based on comprehensive self-diagnostic routines.

In case of malfunction, the diagnostics help users to identify the failed module. CL31 features practical modularity and its easy-access door ensures fast servicing and high data availability.

Easy installation and maintenance

CL31 is easy to install. It has a radiation shield that protects the unit during precipitation and against excessive heat or cooling in extreme temperatures. The automatic window blower with heater improves performance by keeping the window clean and dry. In cold conditions heating prevents frost generation on the window.

Data messages

  • Cloud hits (up to 3 layers) and status information
  • Cloud hits, status, and backscatter profile
  • Emulation of CT12K, CT25K, LD-25/40
  • Sky condition (optional)

Operating environment


Operating temperature −55 … +60 °C (−67 … +140 °F),
Operating humidity 0 … 100 %RH
Wind tolerance Up to 55 m/s (123 mph)
Vibration Lloyds Register / IEC 60068‑2‑6 

5 … 13.2 Hz, ±1.0 mm 

13.2 … 100 Hz, ±0.79 g 

9.1 … 150 Hz, ±0.5 g

Pollution degree 2
Maximum operating altitude 3000 m (approx. 9800 ft)
IP rating IP65 (measurement unit) 

IPX6 (with radiation shield)


Inputs and outputs


Operating voltage 115/230 V AC 

100 – 127 V AC, 207 – 253 V AC

Power consumption
Power consumption (typical) 310W
Measurement unit 15W
Internal heater 100W
Window conditioner heater 175W
Window blower  20W
Frequency (min/max) 50/60 Hz 
No-break power supply 12 V sealed lead-acid battery, 2 Ah
Backup battery 12 V internal, 2 Ah
Overvoltage protection Low-pass filter, VDR
Overvoltage category II
Data • RS‑232 

• RS‑485, multidrop, 2‑wire 

• DXL421 modem module 

• LAN (Ethernet) interface option

Maintenance RS‑232
Baud rate
RS‑232 / RS‑485 300 … 57 600 bps
Modem V.21, V.22 300 … 1200 bps 


Mechanical specifications


Tilt positions  Vertical or 12° tilted
Measurement unit 620 × 235 × 200 mm 

(24.41 × 9.25 × 7.87 in)

Height with radiation shield 1190 mm (47 in)
Total 1190 × 335 × 324 mm 

(46.85 × 13.19 × 12.76 in)

Measurement unit 12 kg (26.5 lb)
Radiation shield and blower 19 kg (41.8 lb)
Total 31 kg (68 lb)
Plywood transport container
Container size 1400 × 490 × 450 mm 

(55 × 19.30 × 17.72 in)

Container weight 47 kg (103.6 lb)


Measurement performance


Measurement range  0 … 7.6 km (0 … 25 000 ft)
Measurement resolution 10 m (33 ft) or 5 m (16 ft), selectable
Reporting interval  Programmable 2 … 120 s, or polling
Measurement interval 2 s default
Distance measurement accuracy against a hard target  Greater of ±1 % or ±5 m (16 ft)


Spare parts and accessories


Cable termination box with extra transient protection Termbox-1200
PC maintenance cable QMZ101
Shock absorbing mounting pad for ship installations CT35022
Modem DXL421
Attachment mechanics for radio modem antenna  CLRADIOKIT
Graphical User Interface for Ceilometers CL-VIEW
Boundary Layer View Software for Ceilometers BL-VIEW
Bird deterrent CL31BIRDKIT
Air Quality Plug and Play Package for Ceilometer with laptop and preinstalled BL‑View CLAQPACKAGE




EU directives and regulations LVD, EMC, RoHS
EMC immunity IEC/EN 61326‑1, industrial environment 
EMC emissions CISPR 32 / EN 55032, Class B EN 61000-3-2 EN 61000-3-3
Electrical safety IEC/EN 60950
Environmental IEC/EN 63000
Eye safety IEC/EN 60825-1:2014
Compliance marks CE, RCM, China RoHS, UKCA

Ceilometer CL31


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