High intensity halogen bulb is used to direct a beam of light, which projects a "spot" on the base of any cloud overhead. The observer can triangulate to determine cloud base height. The angle at the light and ground is 90°, the distance the observer stands from the light is known, and the angle from the point of view of the observer is known by using a theodolite (shown in photo below). Subtract from the cloud base height the height of the observer's eyes above ground level, and an accurate measurement is achieved.

The photo to the right shows the ceiling light (or ceilometer). It is used to measure cloud-base height. To the left of the ceiling light is a timer switch which activates the light. The observer goes a measured distance from the light and uses the theodolite (pictured to the left) to get an angle measurement.

The angle at between the ceiling light and the ground is 90° (more or less.) The distance from the ceiling light is known, and the angle measured by the theodolite is recorded. With simple trigonometry, the observer is able to get the distance between the cloud base and the ceilometer (cloud-base height).

A simple example would be if the observer measures an angle of 45-degrees from his position to the illuminated spot on the cloud's base. A right triangle with an angle of 45-degrees has, by definition, two 45-degree angles, therefore two of its legs are the exact same length. The result: the cloud height is exactly the same as the observer's distance from the ceiling light.

The photo to the right is the theodolite. This instrument is used to triangulate cloud base height. By focusing on the light spot on the cloud's base from the ceiling light, the observer is able to determine the angle from his/her position to the ceiling light and cloud's light spot. A dial on the theodolite records this angle. The theodolite is set up on a 5 foot tripod and leveled.