Pilot balloons are not designed to carry payloads and are usually tracked from the ground using a theodolite. A theodolite is a surveyor’s tool used to measure angles on horizontal and vertical planes. The elevation of the pilot balloon and its azimuth angles are factored into the observations to determine wind speed and direction at different heights in the atmosphere. Meteorologists can determine the height of the cloud ceiling during the day by measuring how long it takes the balloon to disappear into the clouds. For this reason, pilot balloons are also sometimes called ceiling balloons. Most pilot balloons are red, but sometimes the natural latex color is used for greater visibility on clear days. If the weather is overcast, a black balloon can be used as well.