Laser Ceilometer

Principle of operation

Vaisala CT25K Ceilometer

CT25K Ceilometer

This section describes the Vaisala CT25K, however other units generally employ a similar operating function.

The Vaislala CT25K ceilometer transmits a single vertical or near vertical-pointing laser pulse from the unit’s transmitter. This pulse will be reflected or scattered when it encounters cloud. Any reflected signal back towards the ceilometer will be sensed by the unit’s receiver. The height of cloud base can be determined using the speed of light, and the time delay between the launch of the laser pulse and the detection of the reflected signal.

The CT25K produces raw data at 15 second intervals (four pulses per minute), with a measurement range up to 25,000ft.

The raw data collected over a 30 minute period is processed by a Sky Condition Algorithm (SCA) in the AWS to produce estimates of cloud amount and height for up to three cloud layers. The data in the most recent 10 minute period is given a double weighting to produce a better response time in situations when cloud cover is changing rapidly.

Limitations and Interpretation of output

The raw data provided by the ceilometer presents cloud height information to observers as the cloud passes over the unit. The Sky Condition Algorithm reports can provide guidance for estimations of both cloud height and amount.

Due to the limitations of the ceilometer’s sampling procedure, and the algorithm’s processing procedure, staff should be mindful of the following when interpreting the SCA output:

  • When more than one cloud layer is detected, the cloud amount for any higher layer(s) will include the amount(s) of the lower layer(s) because the SCA assumes that any higher cloud will be obscured by lower layers. This will lead to an overestimation of cloud amount at higher layers in some cases.

  • Upper cloud layers will not be reported at all if they are entirely obscured by lower cloud directly above the ceilometer during the half hour sampling period.

  • The SCA can give incorrect observations when cloud of scattered or broken proportions is stationary or is slow moving. If scattered or broken cloud remains directly above the ceilometer for a period of time, it can result in an incorrect overcast report. On the other hand, if the ceilometer is directly under a clear patch of sky for the sampling period, it can result in an incorrect report of a clear sky.

  • Because the reported cloud amount is a temporally-based measurement producing a weighted average amount, there is an effective time lag of approximately 10 minutes built into the system. Thus, in a situation of rapid onset of broken or overcast stratus, the algorithm can take 10 minutes before it produces an output of broken cloud. Similarly, episodes of short-lived (less than 10 minutes) broken cloud may not be reported by the SCA.

  • The ceilometer will function normally in light precipitation, shallow fog and blowing dust or snow. However as these weather phenomena increase in intensity, a point will be reached where the ceilometer can no longer unambiguously identify the cloud base. In these instances the ceilometer will report the vertical visibility and this is processed by the SCA as an effective cloud base.

  • When the sky is dominated by convective (cumuliform) cloud, the bulging sides or sides and tops of leaning cloud masses can be incorrectly interpreted as being a cloud base. This may lead to the SCA providing a report of multiple cloud layers that do not actually exist.

  • The conversion from oktas to the terms FEW, SCT, BKN and OVC when reported via the SCA differ from those derived via a manual cloud observation:

Manual observation: FEW = 1 to 2 oktas; SCT = 3 to 4 oktas; BKN = 5 to 7 oktas; OVC = 8 oktas.

Ceilometer: SCT = 1 to 3 oktas; BKN = 4 to 6 oktas; OVC = 7 to 8 oktas. Note: FEW is not used.

Ceilometers are also installed at non-staffed sites to provide data where there would otherwise be none available.

Most display consoles will allow users to view both the raw ceilometer output (the cloud base height for each individual laser pulse) as well as the 30 minute SCA output. Below is an example of a typical display:

Ceilometer Display

Typical Console Display

The four figures on the top line are the raw data for the most recent minute, with the SCA processed data on the second line.

The One Minute Data (OMD) message extract also displays ceilometer information. The ceilometer output for each individual ‘laser pulse’ is given following the ‘CL:’ group identifier. The SCA output is given following the ‘CL30:’ group identifier. In the message below, the CL30 data is decodes as: 2 oktas at 2800ft, 6 oktas at 3400ft, 8 oktas at 6000ft.

OMD YMML 20141027 235247 DATE:20141028 TIME:1052 CL:03650/03550/03550/03400/99999 CL30: (02,028/06,034/08,060) VI:15404 VI10:8888 SWV:A6B15 MSG:4916/139/999/509

The Sky Condition Algorithm output also appears in the remarks (RMK) section of the METARAWS/SPECIAWS message and in the main body of the report for a METAR/SPECI AUTO. When interpreting cloud information in an automated report, be sure to take into account the variation in oktas conversion as described above.

SPECIAWS YBLT 280035 26021/31KT //// 13.5/09.2 1013.1 RMK WDM10:261 WSM10:021KT MWG10:031KT RF00.0/000.0/000.0 CLD:SCT019 SCT026 BKN036 VIS:9999 QFF:10130 BV:13.5 IT:21.4 VER:2.4.2 SWV:3.2 OID:SYSTEM/CCF1 SNT:201410280035 SP30/10/10/10/5/2/99/00/1500/5000/wg/WAP:10 MSG:1378/286/000/000

SPECI YBLT 280035Z AUTO 26021G31KT 9999 // SCT019 SCT026 BKN036 14/09 Q1013 RMK RF00.0/000.0

The SPECIAWS and OMD messages below from Sydney airport show the variation of the oktas conversion when processed via the SCA compared with a manual observation.

SPECIAWS YSSY 160130 30006/10KT 240V350 5000 2000N -RA 2ST005 3ST010 5CU018 15.5/14.1 1014.2 RMK WDM10:301 WSM10:006KT MWG10:010KT RF01.4/004.2/004.8 CLD:SCT006 SCT010 OVC018 VIS:1900 QFF:10142 BV:13.5 IT:24.1 VIS REDUCED TO N VER:2.4.1 SWV:2.16.4 OID:JBLOGGS/STANDBY SNT:201309160132 SP30/10/10/10/5/2/99/00/1500/7000/UI/VI/WAP:10 MSG:3371/351/000/000

OMD YSSY 20130916 012855 DATE:20130916 TIME:1130 CL:01600/01550/01350/01000/99999 CL30: (01,006/02,010/07,018) VI:02679 VI10:1800 SWV:A6B15 MSG:4855/139/999/492


The CT25K ceilometer uses an invisible vertical-pointing laser beam to obtain raw data and is classified, according to US and European standards, as a Class 1 laser device. This implies that they present no known biological hazard unless viewed with magnifying optics.

Despite the Class 1 classification, the Bureau takes the view that there is potential danger and accordingly advises staff to always avoid looking into the ceilometer. It is especially important not to look into the ceilometer with any form of magnifying glass, binoculars, telescopes, etc. as this may cause irreparable damage to the viewing eye. Any personnel who inadvertently look into the beam, with or without magnifying optics, should seek immediate medical advice.

(Note: While this section refers to the Vaisala CT25K Ceilometer, the information is generally applicable to other laser ceilometer units).