Monthly Archives: August 2015

Cloud Atlas

The definitive reference to cloud types and Cloud species is the ICAO Cloud Atlas which can be downloaded for free from this site:

WMO Cloud Atlas Vol 1    WMO Cloud Atlas Vol 2

There are a number of less detailed cloud type images on the internet.   For example

Clouds Online

The Australian Bureau of Meteorology have a good references to cloud observation

Australian Bureau Chapter 13

Cloud Types Graphic

Current ceilometers have a range out to about 25,000 ft,  and other models with larger telescopes built in,.  can reach up to about 40000 ft.   For use in aviation,  at airports a range of 12,00 ft is considered adequate.

Ceilometer Laser Safety.

Ceilometers must be eye safe and meet Class or Class 1m  Laser Safety standard under the international specification IEC 60825-1  or ANSI Z136 in the USA

The phrase “eye-safe” is used below.

Class 1:   This class is eye-safe under all operating conditions.

Class 1M:  This class is safe for viewing directly with the naked eye, but may be hazardous to view with the aid of optical instruments. In general, the use of magnifying glasses increases the hazard from a widely-diverging beam (eg LEDs and bare laser diodes), and binoculars or telescopes increase the hazard from a wide, collimated beam   Radiation in classes 1 and 1M can be visible, invisible or both.

The beam from a ceilometer has a very low divergence,  which is mainly determined  by the finite size of the laser source and the ceilometer lens/mirror focal length,  but can also be effected by spherical aberration and diffraction effects in the optical path in the instrument.

Wikipedia Entry: Laser Safety

Sky Condition, How is it assessed ?

A human observer looks at the sky and estimates the coverage in 8ths ,  0 being clear sky and 8 being overcast.   The human observer then estimates cloud height and applies these estimates of cover for each layer.    It is quite obvious that if there are no breaks in the sky,  any higher layers present cannot be estimated.    The human observer also suffers from the “packing” effect of an oblique line of sight , and usually tends to overestimate cover.

For each layer the human observer will give the condition FEW,  SCATTER,  BROKEN AND overcast.

A ceilometer can only “see” cloud above it,  so can only estimate the sky condition by analysing heights over a time period.

The Sky Condition Algorithm in the 8200-CHS  is based on that developed by the US National Weather Service and used in their automated surface observing system (ASOS) units and  guidelines published by the World Meteorological Organization.

A study by the Hughes STX Corp. found that when ceilings were under 5,000 feet, this algorithm agreed with the human observer 78% of the time. With fog, the comparability was 84%, with rain it was 69%, and when snowing 74%. During rain, the NWS Algorithm reported more changes than the human observer.

However at the transition between scattered and broken cloud coverage 4 oktas humans often report too much cloud coverage. This is attributed to the “packing effect;” a condition where an observer does not see the openings in the cloud decks near the horizon due to the viewing angle.   Pilots tend to overestimate the coverage even more than ground observers because of visual compression.

The 8200-CHS algorithm is not biased by the “packing effect” because it measures only the sky conditions passing over the sensor

Details of the 8200-CHS specifications can be found here 8200-CHS Page

Why tilt a ceilometer?

A slight tilting of the ceilometer gives  better performance in rain.

Rain drops tend to flatten as they fall.  See the NASA explanation for this 

Consequently, when aligned vertically the backscatter from raindrops may be sufficiently high to cause difficult in resolving the cloud base above the rain.

However when tilted, the backscatter of laser pulses by the raindrops is reduced .

In heavy rain even a tilted ceilometer cannot resolve the cloud base since the integrated backscatter quickly dominates and prevents further penetration up to the cloudbase,  while extinction in the return path also starts to extinguish the return signal..

Please refer to the screen below.


These tests were carried out during light rain.  The tilted unit shows resolvable cloud base while the untilted unit reverts to vertical visibility.

Cloud Types: Cumulo Nimbus and AltoStratus

Stratus clouds,  irrespective of altitude have a more or less constant and solid base ,  indicating stable layers in the atmosphere and lack of thermal activity.

By contrast the Cumulo Nimbus cloud betrays a high degree of vertical motion of the atmosphere and thermal activity.  Cumulus congestus and Towering cumulus display a moderate degree of vertical extent

In the curtain plot below,  the classic rapid change of cloud level characteristic of Cb cloud can be seen between 19.00 and 23.00 hours.  After that time the cloud base solidifies and rises to 6500 ft

Cb cloud gives way to As cloud

Cb cloud gives way to Ac cloud

Ceilometer Signal to Noise.

There are a great many factors effecting the performance of ceilometers,  but the key 2 issues for any given cloud volume backscatter coefficient  are:

  1. The eye safe limit of the ceilometer .  Infra Red Ceilometers must operate as Class 1M lasers  which limits the energy density of the beam.
  2. The noise level.      The inherent noise level of the ceilometer is the ultimate determinant of the signal to noise ratio which enables the ceilometer to discriminate  cloud boundary.

The 8200-CHS has an extremely low level of inherent noise,   which is tested for each ceilometer and is recorded as per the backscatter profile below.


The external source of noise is the shot noise of the scattered and or direct solar radiation within the spectral acceptance of the sensor .   The laser operates around 910 nm and the filtering can only limit the “out of  band”  component of the solar noise spectrum.  The ceilometer will thus t\detect cloud at higher altitudes at night.