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Way up on the gantry high above the bridge of the Tangaroa a Ceilometer, which is a type of Lidar or laser radar, sends pulses of infrared radiation into the atmosphere and this bounces back off cloud droplets and ice crystals in the clouds. If clouds are thick then we can only see the cloud base, if they are thin then we can see right through them. This graph is an output from the Ceilometer and shows that the cloud base is at about 1km in this case. The bottom axis of the graph shows time, the side axis shows altitude.