Menu Sign In Contact FAQ
Welcome to our forums

Cloud base - how is it measured?

Here are a few dumb questions for @JulietBravo and others who know how ceilometers work, and what exactly (or rather, inexactly) they measure.

1. The first question is, for a typical large commercial airport like Heathrow or Stansted, how many ceilometers are used to provide data for the ATIS and METARs, and if more than one, how is the data processed and combined into a single height? When a civil engineer is trying to determine soil properties for a building foundation, we drill several boreholes and extrapolate, average (or guess) between them. Do big airport METARs do the same?

2. Next, what does a ceilometer measure, or rather, how is it calibrated? As we approach the base of cloud, we often experience gradual reduction of visibility starting several hundred feet below the point where we (arbitrarily?) say to ourselves “we’re in cloud”. Where, on that scale, does a ceilometer say “this is cloud” (albeit presumably in Finnish )

3. How is that data is processed into a METAR which reports, to +/- 50 ft, a height which is varying in three dimensions (laterally and in time) by one order of magnitude more than that?

4. How do experienced instrument pilots use ATIS and METAR reports to inform, say, a go/no-go decision? For en-route VFR, we are used to subtracting a “safety margin” of several hundred feet (or much more) from reported and predicted cloudbase to be sure that the flight can be completed in VMC. Is the corollary of that when planning an IFR flight that we may expect to see the surface from several hundred feet higher than the reported cloud base when executing a cloud break or approach?

Last Edited by Jacko at 22 Dec 12:34
Glenswinton, SW Scotland, United Kingdom


Thank you for your exam questions.

Q1: The simple answer is that I don’t know; I guess larger airports may have more than one ceilometer but they are expensive pieces of equipment so they will not want to duplicate these unnecessarily. You need to distinguish between AUTO METARs where there is no human intervention and the traditional METAR which is compiled by an ATCO with appropriate met training, or possibly still in some places (but not the UK) by a met observer. The traditional METAR will be compiled using visual observations supplemented by instrument data, which may include a ceilometer.

Q2: The principle of the ceilometer is based on the time needed for a short pulse of light to traverse the atmosphere from the ceilometer to a backscattering cloud base and back to the ceilometer receiver. Practical tests that I have made over the years suggest that ceilometers and humans detect cloud at about the same time.

Q3: My ceilometer has an algorithm which uses a time series of ceilometer data to calculate the cloud layer height and amount of clouds at different layers over a half hour period. Again comparable observations over a long period of time show the algorithm to be reasonably accurate, and particularly so in the context of determining the base of low cloud which is crucial for aviation ops. Inaccuracies are at the margin and usually unimportant; for example, FEW clouds may be spotted by a human observer but omitted in an AUTO METAR, but this will be reported as NCD or NSC – no cloud detected or no significant cloud. The main disadvantage of a ceilometer is that it will not on its own detect TCU or CB.

Q4: For an instrument approach, the go/no go decision is based on RVR, determined either instrumentally or by a factored visual estimation of meteorological visibility. My experience is that provided the RVR is greater than 800m on a runway with good approach lighting, then whatever the reported cloud base, you can usually expect to see the approach lights before your DA.

Happy Christmas.

EGNC, United Kingdom

Thanks, that’s interesting.

If I understand the answer to Q3 correctly and an AUTO METAR is based on the previous 30 minutes of data, there could be quite a bit of “hysteresis” – so that when cloud is lifting the reported cloud base may be significantly lower than the actual, and vice versa? Or is the algorithm cleverer than that, i.e. some sort of weighted average?

I only ask because I was pleasantly surprised yesterday afternoon by how pessimistic the various nearby Essex METARs turned out to be and I wonder if this is a likely “feature” of AUTO METARs reporting low cloud bases, or whether it was a matter of chance.

Glenswinton, SW Scotland, United Kingdom

Yes the algorithm is clever than that and based on a continuously updated average biased towards more recent observations.

EGNC, United Kingdom

This [ local copy ] might be interesting for measuring distances to clouds.

Shoreham EGKA, United Kingdom

There are different kind of ceilometer installations and different kinds of processing. At ZRH we use normal vertical ceilometers and we have 8 of them positioned around the airport on the extended centerlines of the approach runways. With that many measurements, you get a pretty good picture of what is going on.

Others use less and have algorythms programmed wich observe each ceilometer over a time and calculate with wind information what kind of celing you have.

Yet there are other methods in development which go away from point measurements to scan devices which can scan the whole sky. I have not seen any of these operational but in experimental they look great. LIDAR is one of them. We used one we had as an experimental device during the vulcanic ash crisis and it did show the ash very well over Switzerland.

LSZH, Switzerland

Mooney_Driver wrote:

it did show the ash very well over Switzerland

I’m surprised any measurable ash even made it to Switzerland. Can the concentration be quantified and how much was it really?

Last Edited by Rwy20 at 02 Jan 13:47
7 Posts
Sign in to add your message

Back to Top