Meteorologists measure cloud cover in oktas, units of measure that divide the sky into eighths. Readings run from 0 oktas, a clear sky, up to 8 oktas, a completely overcast sky. Special all-sky cameras, which photograph the sky images onto polished metal spheres, take the measurements.
In addition to measuring the percentage of cloud cover, meteorologists use all-sky cameras to pinpoint thunderstorm fronts, detect lightning strikes and sight mammatus clouds associated with tornadic activity. Meteorologist can also tell if the cloud cover is producing snow or if a sudden change in the type of cloud indicates seasonal changes.
Meteorologists also use ceilometers to measure the height and thickness of the cloud cover. They shoot laser pulses up into the cloud and analyze the amount of light reflected back, known as back-scatter. These beams travel upwards at the speed of light and measure the altitude of the cloud base and top. Sometimes the cloud cover is composed of multi-layers of clouds.
The information gathered from the all-sky cameras and the ceilometers allows meteorologists to forecast the weather more accurately. For example, if the ceilometers pick up cumulonimbus clouds, which are often shaped like anvils, a thunderstorm with hail and strong wind is likely. Cirrus clouds, which produce a thin, airy cloud cover, foretell good weather. If those same clouds thicken and grow dark, that's an indication of rain or snow.