Ceilometer

A ceilometer is a device used to measure the height of the cloud ceiling or cloud base. Some ceilometers are alternatively used to determine the concentration of aerosols in the atmosphere. These devices work by shining an intense beam of light that is modulated at an audio frequency toward overhead clouds. A photocell in the ceilometer’s receiver detects light reflected back from the cloud base to give the observer an estimate of the cloud ceiling height. Different ceilometers use either infrared or ultraviolet light transmitters or lasers to measure the cloud ceiling. There are also scanning receiver and rotating transmitter ceilometers. Regardless of how they work, ceilometers are valuable devices that serve many important practical purposes, especially in the aviation field. Historically, the creation of the ceilometer is credited to nineteenth-century Danish scientist and inventor Poul la Cour, who is best known for being a pioneer in telegraphy and the study of aerodynamics.

Background

The height of clouds can vary based on factors like cloud type, atmospheric conditions, and the level at which condensation occurs. The term cloud height can refer to either cloud depth or cloud ceiling. Cloud depth is a description of the distance between the base and top of a cloud. Cloud ceiling is a description of how high the base of a cloud is above Earth’s surface. As it relates to the concept of cloud ceilings, there are three cloud categories: low, middle, and high clouds. Low clouds are those that form anywhere from the surface to 6,500 feet (2,000 meters). Middle clouds form at altitudes of 6,500 feet to 13,000 feet (4,000 meters) near the poles; 6,500 to 23,000 feet (7,000 meters) at mid-latitudes; and 6,500 to 25,000 feet (7,620 meters) at the tropics. The cloud bases of high clouds are typically measured at 10,000 feet (3,000 meters) to 25,000 feet at the poles; 16,500 (5,000 meters) to 40,000 feet (12,200 meters) in temperate areas; and 20,000 (6,100 meters) to 60,000 feet (18,300 meters) in the tropics.

Although scientists had long studied clouds and their positions in the sky, it was not until Paul la Cour invented the ceilometer in 1871 that there existed a reliable way to measure the height of clouds above the surface. Born near Ebeltoft, Denmark, in 1846, la Cour studied at the University of Copenhagen and became the first Danish meteorologist. Early in his professional career, he was a keen observer of clouds. While conducting research in Jutland in 1871, la Cour noticed a dark cloud that appeared higher or lower in the sky at different times even though its size and shape did not change. This led him to wonder whether there might be way to accurately determine the distance of a cloud from the surface. La Cour subsequently constructed a simple handheld lighting device he could use to measure the angular height of darkened cloud parts and thereby determine the cloud ceiling. He later described how he used a concave mirror in the device to shine light on darkened clouds to calculate the height of said clouds. The device la Cour created was the first ceilometer.

Overview

The modern ceilometer is a meteorological instrument commonly used to determine the height of the cloud ceiling. It works by shining a strong light beam that has been modulated at an audio frequency at clouds in the sky. This light is usually produced by either an infrared or ultraviolet transmitter or a laser. Light reflected from the cloud base is detected by a photocell in the ceilometer’s receiver. The data collected allows the device to accurately measure how far the clouds are from the surface.

There are two main types of ceilometers, including the scanning-receiver ceilometer and the rotating transmitter ceilometer. The light transmitter of a scanning-receiver ceilometer is specifically fixed so that it always directs the light beam vertically. When the ceilometer is in use, the receiver’s parabolic collector scans continuously up and down the vertical beam produced by the transmitter. The device then searches for the point at which the light intersects with the cloud base. Once a reflection occurs, the ceilometer measures the vertical angle to the spot so that the cloud ceiling can be calculated. In a rotating-transmitter ceilometer, the receiver is fixed so as to exclusively direct reflections from directly overhead as the transmitter sweeps across the sky. Light is reflected downward when the beam intersects a cloud base directly above the receiver. Laser ceilometers are composed of a vertically aligned laser and a light detection and ranging (LIDAR) receiver in the same unit. The time it takes for reflected light to return to the LIDAR receiver allows the ceilometer to determine the cloud ceiling. Laser ceilometers are especially useful in some applications because analysis of backscatter profiles from laser light can detect precipitation or the presence of certain gases.

Ceilometers serve many practical purposes. The most common, and perhaps most important, of these purposes is providing critical data for aviation safety. Cloud ceiling data is one of several key pieces of atmospheric and meteorological information that pilots and air traffic controllers need to safely operate aircraft. Ceilometers are also used to measure aerosol concentration in the atmosphere. This application helps scientists gather important information about air pollution and other atmospheric concerns. Using ceilometers to measure aerosol concentration also has other benefits. Some are specifically used to monitor for aerosols and other pollutants that originate from wildfires, as well as to predict whether these potentially dangerous materials will reach the boundary layer where they can become a public health threat.

Potential future uses for ceilometers exist as well. One possibility is that ceilometers could be used to evaluate the presence of industrial aerosols as part of local air quality studies. Ceilometers could also potentially be used to predict the likelihood of ground blizzards—hazardous winter weather events that occur when fallen snow is blown up off the ground by wind—may take place.

Bibliography

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