AERODROME METEOROLOGICAL OBSERVATION AND FORECAST
STUDY GROUP (AMOFSG)
Montréal, 9 to 12 September 2008
DISCUSSION ABOUT THE OBSERVATION OF CLOUD LAYERS AND THE
CATION OF CEILOMETER(s)
(Presented by Michael Leroy)
This paper deals with the reporting of the cloud layers, the positioning of the
ceilometer(s) and the content of ATIS.
For precision approach runways, instrumental cloud b
ase measurements should be carried
out close to the location, where a middle marker is, or should be situated (Annex 3
Service for International Air Navigation , App. 3, Par. 4.5.1)1, i.e. "at a distance of 900
1200 m (3000
to4000 ft) fro
m the landing threshold at the approach end of the runway". SN/13 recalls why.
ICAO Annex 3
Meteorological Service for International Air Navigation,
Cloud observations for local routine and special reports
representative of the approach area.
Cloud observations for METAR and SPECI should be
representative of the aerodrome and its vicinity.
Air Traffic Services
, Chapter 4 (flight information service) refers
only to cl
ouds to be transmitted to the pilot, with the following words:
cloud below 1 500 m (5 000 ft) or below the highest minimum sector al
cumulonimbus; if the sky is obscured, vertical visibility, when available;
When available, huma
n observation is primarily used for the cloud layers, the
ceilometer(s) being used by the human observer to localize the bases of the cloud layers.
Based on the French experience, it is doubtful that a human observer reports both the
s representative of the aerodrome (and its vicinity) and the cloud layers representative of the
approach area. In fact, the cloud observation of a human observer is based on what he or she sees : the
central point is the observer location. The observer bei
ng located within the aerodrome, it is quite valid to
consider that he or she observes cloud layers representative of the aerodrome (and its vicinity), as
recommended for the METAR and SPECI. The measurements of one or several ceilometers are used to
he best evaluation of the height of the cloud layers.
With a human observer, it is probably quite frequent that the cloud layers transmitted in a
local report are in fact the same that the cloud layers transmitted in a METAR. In this case, the exact
on of the ceilometer, at
a distance of 900
1200 m from the landing threshold or at this landing
threshold, doesn’t really change the cloud layers reported.
It is different if the cloud layers representative of the approach area are calculated from
asurements of a ceilometer located as it is recommended, because these cloud layers are derived
from the measurements at the ceilometer’s vertical.
The recommended ceilometer’s location is the most important when the height of the
cloud base is low, close
to the decision’s height. In such a case, the added value of a human observer is
quite low. If this observer is located 2 km from the approach end of the runway (where it is recommended
to locate the ceilometer) and if the height of the cloud base is close
to the decision height (300 ft to 700 ft,
depending on the runway’s category and the type of plane), the angle of view of the cloud base from the
observer location is about 3 to 6°. With a so low angle of view, it is really quite difficult for a human
erver to describe the cloud layers specifically for the approach area !
Therefore, the importance of the location of the ceilometer is much more for the height of
cloud base of the lowest layer than for the complete description of the cloud layers. The hei
ght of cloud
base is operationally critic when it is close to the decision height. In such a case, the cloud layer is very
generally wide spread and the difference of height of cloud base from one point and another point at a
distance of 1200
1500 m is qu
ite low. So, the exact location is no so critic. It is what Météo
found in a study inter
comparing the height of cloud bases between the 4 ceilometers of CDG airport
(LFPG). But the results of such a study may depend on the site.
On a airport wit
h several runways equipped with a ceilometer, the local reports shall
include the cloud layers for each runway (Annex 3, appendix 3, 188.8.131.52.b). But the content of the
meteorological information of the flight information service (Annex 11) defines only “cl
possibility of having several cloud layers for several approach areas is not identified. Which one to
use ? In France, there is no problem of choice for the cloud layers in the ATIS, as the cloud layers
transmitted and used are in fact the cloud
layers representative of the aerodrome (the same cloud layers
transmitted in the METAR and SPECI). And in this case, the exact location of the ceilometer is not so
The recommended location of a ceilometer is really meaningful when th
e cloud layers
transmitted in the local reports are really representative of the approach area. It is the case in automatic
mode, when the ceilometer is located as recommended (
at a distance of 900
1200 m (3000 to4000 ft)
from the landing threshold at the
approach end of the runway)
. It is not so true when the cloud layers are
reported by a human observer.
When there is more than one runway in use, the local reports should contain cloud layers
for the multiple approach area. But the ATIS considers only one
set of cloud layers. As for multiple
visibility measurements, there is an inconstancy between the requirements of Annex 3 and Annex 11.
In this context, when the cost to install a ceilometer “at a distance of 900
1200 m (3000
4000 ft) from the landing
threshold at the approach end of the runway” is high, when it is an isolate
place, it may not be very efficient to follow the Annex 3’ recommendation for the ceilometer’s location, if
the advantage of such a location is not fully used.
ACTION BY THE GROUP
The AMOFSG is invited to :
ote the discussion presented in this paper; and
ake it’s content into account for further discussions about the ceilometer location
(SN/13) and the use of cloud layers in ATIS (SN/6).