Meteorological measurements are measurements of the physical properties of the atmosphere. These measurements are made at all elevations in the troposphere and the stratosphere. For measurements made at elevations above ground or tower level, instruments can be carried aloft by balloons, rockets, or airplanes. Ground radar is used to detect the presence of water in the form of droplets or ice crystals at all elevations and the winds associated with them. Lidar (optical radar) of selected wavelengths is used to detect the presence and amount of aerosols and other constituents of the atmosphere and to determine cloud height. Instruments on satellites measure properties of the atmosphere at all elevations.