There are many ways to determine how high above ground the clouds are. Here are just a few of them--can you think of any more?
- you can time a helium-filled balloon from the second you let it go until it disappears into the cloud--it will rise at a known rate if you always put the same amount of helium in it;
- at airports, pilots taking off can tell you how high the plane was when they entered the cloud; or
- you can use a piece of sophisticated equipment known as a ceilometer.
Several different types of ceilometers are used to measure cloud heights. They all shoot a laser beam up into the sky through a hole in the top of the cover. If there is cloud above the ceilometer, the cloud particles will reflect some of the beam, bouncing it back to the ceilometer. The length of time it takes for the signal to return will tell us how high the cloud is. The laser ceilometer can measure clouds as high as 3 kilometers above the ground.
At some locations, a special shield is installed on top of the ceilometer. Can you guess what it's for? It looks like a ring of metal spikes running around the edge of the opening but without blocking the laser beam. To find out its purpose, move your mouse over the picture.
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