Clouds are an important basis for weather forecasting and a parameter that aviation safety needs to know.
Measuring the altitude of clouds is common using two methods: One is the hot air balloon method. Drop a balloon with a fixed flight speed; based on the drop to the bottom of the cloud and the high flight speed, and we will calculate the cloud bottom’s height. The other method is to use headlights, which emit a column of light that shines vertically on the cloud’s bottom, through the angle formed from the point of view under the cloud and the horizontal distance from the point of view to the headlight. Then, based on the triangle principle, we can calculate the bottom of the cloud’s height.
These two methods have certain limitations. For example, the hot air balloon method must first charge air, wait for the balloon from the ground to fly up to the bottom of the cloud; it takes a while; the longer the cloud is, the more the balloon sometimes penetrates the gap. It is not possible to measure the height accurately among clouds. The headlight method can only be used at night. Moreover, the light of the headlights is limited, and can only measure relatively low clouds.
To overcome this limitation, scientists have successfully built a cloud gauge called a “cloud measuring arc machine.” It takes advantage of a light-emitting device of three colors: purple, green, and blue, shining on the bottom of the cloud in the sky. When the light source shines on the bottom of the cloud, it is reflected, received by the machine on the ground. As we know, the speed of light traveling through the air is 30,000 km / s. Based on the time from when the device emits light to the time of recording, the cloud’s height will be calculated. “Cloud-measuring arc machine” has many advantages such as day, night, it is usable, using a receiver instead of the eye, so the results are accurate and timely. Because the light of the arc is still weak, the longer the lighting distance, the weaker the optical wave decreases very quickly. So if you want to measure several kilometers of the clouds, the reflected light receiver is so weak that it can not have a sensor. Besides, the light source of this machine has a rather large volume, difficult to move, and can only measure the height of the clouds located just above the device’s location. So, although this machine was much superior to airships and headlight, it was still limited.
In recent years, with laser technology development, scientists have successfully created “laser cloud gauge.”
The laser cloud gauge’s working principle is the same as an arc cloud gauge, but the laser power is large, the light emits parallel, the energy is highly concentrated so that it can be measured at the height of ten kilometers. The signal remains intact, making the receiver still able to absorb the response waves. The laser cloud meter volume is small and can also be made into a flexible light source; now, it is quite an ideal cloud measuring device.