Observations of clouds fall into four categories namely estimation of the amount of cloud, recognition of the forms of clouds present, measurement or estimation of the height of the cloud base and determination of the direction from which the clouds are moving. Observations in each of these categories are part of the information to be included in routine reports.
The height of the cloud base at any land observing station is measured (estimated) from the ground in the vicinity of the observer. When the visibility is poor because of haze or mist, the cloud base is often ill defined and no rigid criterion can be laid down as to where the haze ends and the cloud begins. The base of the cloud is defined as the lowest zone which may be determined as follows:
(a) By estimation.
(b) By aircraft reports.
(c) By using ceiling balloons.
(d) Cloud Searchlights and Laser Range Finder (LRF)
(a) By estimation
Though instrumental methods of measuring cloud height should be employed whenever possible. Owing to one reason or other the use of instrument for this purpose is restricted to a minimum. More useful information can be provided by skilled visual observation. The ability to estimate cloud height satisfactorily can be acquired by long experience. The estimation of cloud base is done by comparing the base of the cloud with heights of known obstruction like hills, mast, tall buildings etc. So the observer must try to estimate the cloud height and check them by comparing with observation of air craft report, ceiling balloon or LRF.
(b) By Aircraft Reports
Information regarding cloud types and height of bases and tops at various levels can be accurately reported by aircraft using altimeter. Any aircraft, which are on scheduled or non-scheduled flight, can be requested through Air Traffic Control for this information.
(c) By Ceiling Balloons
The ascent of any balloon for a meteorological purpose can be used for obtaining cloud base height by noting the time that elapses between the release of the balloon and its entry into the cloud. In this method a balloon weighing about 15 gms is filled with fixed amount of Hydrogen to have a rate of ascent (ROA 9.0 kmph). This is let off and followed with naked eye binoculars or by Theodolite till it enters the base of the cloud. The time taken by the balloon is used to determine the height of base of cloud. During night a light (torch or candle) can be attached to the balloon.
(d) Laser Range Finder
Height of cloud base can be found out with greater accuracy with the help of Laser Range Finder (LRF), which is commonly used in Met Flight.
Observation of Clouds
The observation of cloud should begin with the identification of all the clouds present. This should be followed by an estimation of cloud amount and height. For the correct identification of cloud, it is necessary to maintain an almost continuous watch on the sky as a whole and to follow its evolution. A relative movement may reveal clouds previously hidden, there by enabling the observer to obtain some idea of their amount and height.
At night sky should be examined from a dark place, well away from lights, especially when the atmosphere is hazy. The observer should not attempt to make cloud observations immediately after coming out from bright lit place; time must be allowed for the eyes to become adapted to darkness.