High Altitude clouds fall into 2 categories,
- Those with a low base and vertical development are Cumulonimbus and Towering Cumulus. At the base these are water clouds, so the ceilometer only “sees” a few hundred feet into the cloud.
- Those with a high base, including Cirrus, Cirrostratus and Cirrocumulus and Altostratus
The Cirrus cloud family are composed of Ice Crystals, and are very often “optically thin”and they have low backscatter coefficients, so are difficult to detect with ceilometers, because the laser pulse energy is limited to eye safe levels.
Altostratus may be composed of ice crystals. In some ice crystal altostratus, very thin, rapidly disappearing horizontal sheets of water droplets appear at random. The sizes of the ice crystals in the cloud tended to increase as altitude decreased. However, close to the bottom of the cloud, the particles decreased in size again
Altostratus cloud with a water phase may have a strong backscatter signal and can be picked up as in the case below
Alto stratus Cloud at 17,000 ft
Ref 1 : Wikipedia : Alto Stratus entry.
Ref 2. Wikipedia Cirrus Cloud entry.
All ceilometers which are set up for long range cloud height measurement are “far sighted”, having a blind region in front of the unit. This is shown in the diagram below , and the height of the blind spot Rio is heavily dependent on the axial separation d , the beam divergence and the telescope angle of acceptance. The signal is maximised at the full overlap distance Rovf as shown below.
Since most ceilometers are designed for the best acheivable signal to noise ratio, the telescope angle of acceptance is set to the limit of focal length, sensor active area and lens aberration.
The single lens designs, such as the CL51 and 8200-CHS feature a low value of d and thus a much reduced overlap height
Single lens overlap geometry
There are a number of different optical arrangements to enable the reduction of d to zero or to a small value to minimise the overlap height.
One form of “single lens” Ceilometer, using a “split lens ” approach (reference (Vande Hey, J. ; Coupland, J. ; Richards, J. ; Sandford, A. )
It is worth noting that earlier designs of dual lens ceilometers actually utilised the blind spot to reduce the required dynamic range to prevent overload of the return signal processing channel, and greatly reduce optical crosstalk in the instrument itself ( known as To crosstalk)
Later ceilometers using the single lens optics, such as the MTECH SYSTEMS 8200-CHS feature special techniques to minimise optical crosstalk and very high dynamic range analog to digital converters to enable detection of fog close to the ground without saturation of the signal
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
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.
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
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
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.
The Cloud Appreciation Society has some great cloud Photos and covers a lot of information about clouds from all over the world.
Cloud Appreciation Society
Here is one of the really good photos .
Cloud Appreciation Society : A classic Cumulonimbus with an arcus feature on the leading edge, spotted near Pipestone, Minnesota, US. © Edward Carberry