Cloud Tops Calculation

CPL Assist
From all the reports-back I have had from people who take the Com. radio exam, I have never heard of one which does not include a cloud top calculation based on weather radar.

The basic principle of the exercise is that; if you tilt the weather (conical pencil) beam upwards until a cloud formation on the radar screen disappears, you can calculate the height, from your altitude to the top of the cloud, from the triangle formed by the angle of tilt and the distance from you to the cloud. The distance can be estimated by reading the distance rings on the radar screen, otherwise known as the plan position indicator (PPI).

Because the beam has width (it spreads from the dish like light from a torch), to cause the cloud to completely disappear, it has to be tilted up until the very bottom of the beam misses the cloud. This means that the centre of the beam will, at that time, be somewhat above the cloud. The amount by which the centre of the beam will be above the top of the cloud is half the beam width. The beam width is defined as the angle of inclusion between the top and the bottom of the beam; typically 5°. Half of a 5° beam is 2.5°.

The picture shows a typical example in which you can see that, if you want to calculate H, the line representing the angle to the top of H is the one that forms the bottom of the beam. The tilt mechanism on the instrument in the cockpit is manual and, when set, only indicates the angle of the centre line of the beam. If the indication was 12° and the beam width was 5°, to calculate the angle of the bottom of the beam; subtract half the beam width i.e. 2.5° from the 12° of tilt to get the angle of the triangle as 9.5°. From there, just use trig or the 1 in 60 rule to calculate H. Don’t forget to add the altitude of the aircraft to get the altitude of the cloud top.

Having shown you the simple form of the problem, it has to be said that the examiner can complicate matters by having the aircraft climbing at an angle with stabiliser on or off. I have even heard a report of a question where the beam was tilted down to find the cloud top.