When the weather observer carries out the hourly observation at a weather station, cloud is one of the more important elements to be reported. The information gives forecasters in Ireland, and in many other countries, valuable clues about the thermal structure of the atmosphere. It may also be possible to tell from the cloud report if a front is approaching the area, if thunderstorms are likely, or if frost or fog may be expected after dark. And much of the information to be gleaned from cloud comes through a knowledge of the height of the cloud above the ground.
The simplest way to assess the height of cloud, but one which is obviously a little imprecise, is just by estimation. A skilled observer can estimate the height of low cloud to within 50 ft, and that of higher cloud to 500 or 1000 ft. In the early days of meteorology that was the only possible methodology by day, but at night a cloud searchlight could be used.
The searchlight provides a very powerful beam of light, pointing vertically into the sky and situated about a hundred yards away from the observer; a bright spot appears where the beam hits the base of the lowest cloud. The observer used a simple device called an alidade - similar to the protractor you used at school and bearing a vague resemblance to a sextant - to measure the angle with the horizontal made by a direct line from him or her to the illuminated spot beneath the cloud. Given knowledge of this angle, and the horizontal distance to the searchlight, elementary trigonometry yielded the vertical distance from the ground to the illuminated cloud-spot. In practice, of course, tables were available, from which the height of the cloud could be read directly, once the measurements were made.
But modern technology provides cleverer ways of getting at this information: a ceilometer does the trick. This instrument sends a laser beam vertically upwards in very short pulses. This beam of light is scattered in all directions when it hits the cloud layer overhead, and part of it is deflected downwards again into the instrument. The time taken for the light to travel upwards to the cloud and then return to the ceilometer can be used to calculate the height of the cloud very precisely.
Like most instruments nowadays, the ceilometer incorporates a computer to do this calculation very rapidly, and the output from the computer can provide a trace on a chart, giving a continuous record of the cloud height for those who may wish to make some use of it.