13/12/2017
The invention of the ceilometer

This artícle by Kristian Pagh Nielsen was first published Vejret 153, the Meteorological Magazine of the Danish Meteorological Society. The article below is a shortened version.

Who invented the ceilometer?

Before the invention of the laser, ceilometers utilized beams of light from projectors. These were directed against the cloud base to form a spot of light and the height of this could then be determined from trigonometry.

According to online sources the idea of how to make a ceilometer was first conceived by englishman Ty Beck in 1897. This is incorrect. As will be retold here, the principle was demonstrated as early as 1871 by the Danish meteorologist Poul la Cour, who is the likely inventor of the ceilometer.

Three detours to the ceilometer

Fig1: graphic by KP Nielsen

The roads to new inventions are rarely straight. Before Poul la Cour invented the ceilometer he took several detours. The first of these detours occurred on a journey to Jutland, more precisely the region of peninsulas east of Aarhus shown in Fig. 1. It was January and all of these areas with their characteristic coastlines were covered with snow. La Cour writes [1]:

“During a stay in Jutland in January 1871 ½ a Danish mile from Kattegat I noticed that a dark cloud had loomed in the sky over the sea. At times it loomed heigher and at other time it loomed lower in the sky; but its appearance and shape did not change. It was obvious to compare this cloud and the brighter part of the sky with the dark sea and snow-covered land from a purely optical point of view. If you imagine the cloud layer as a translucent object with an approximately even base, a large fraction of the light rays will be thrown back from the fields of snow and add illumination to the cloud layer above them, but only a few light rays will be thrown back from the dark ocean over which the cloud layer will remain darker. … This induced a hope in me that the angular height together with the known distance to the coast would enable a reasonably accurate measurement of the actual height of the cloud layer…”

La Cour performed several test measurements and estimated that he could determine the cloud base within an uncertainty of 10% by using a hand held device to measure the angular height of the darkened cloud parts.

Two other detours to the ceilometer were taken, which were both of similar nature to the first detour. In February 1871 la Cour measured dark shadows on the cloud base caused by the woods in Northern Zealand. The shadow from the woods were caused by the fact that the trees did not have snow on their crowns at the time and were darker than the surrounding snow-covered fields. Lastly, la Cour noticed that the nighttime illumination of central Copenhagen, which in 1871 was the only place in the region that was gaslit, illuminated the cloud base above the city so that the height of this could be measured.

The first ceilometer demonstration

After his detours la Cour completed his paper from 1871 [1], with a brief description of the actual invention of the ceilometer:

“To the statement above I wish to attach another message. Following a suggestion from Professor Holten, I have succeeded in using a concave mirror to throw an image of Drummond’s light upon the clouds at zenith. By observing this image from two places with some distance between them it is possible to calculate the height of the clouds; and the observation is thus possible regardless of time and place.”

To summarise there can be no doubt that Poul la Cour demonstrated the ceilometer before Ty Beck in 1897!

Who was Poul la Cour?

La Cour was the first Dane to be educated as a meteorologist. After his education he visited the Royal Dutch Meteorological Institute and learned the principles of weather forecasting of C. H. D. Buys-Ballot. In 1872 the Danish Meteorological Institute was initiated with a basis in these principles and led by Niels H. C. Hoffmeyer and Poul la Cour. La Cour was responsible for setting up and maintaining the national observation network. He also made inventions for telegraphy including a 12-fold telegraph and the phonic well. Later he invented a multispectral optical telgraph, the electricity producing wind mill, the cratostate, the wind tunnel, and the method of storing wind power produced electricity in hydrogen through the electrolysis of water.

[1] La Cour, P. (1871): Maaling af sammenhængende Skylags Høide (English translation: Measurements of the height of coherent cloud layers). Tidsskrift for Physik og Chemi, 10, 321-334.