Meteorological instruments are used to measure various atmospheric conditions. Key instruments include thermometers for temperature (degrees Celsius/Fahrenheit), barometers for pressure (hPa, mb), anemometers for wind speed (m/s, km/h), rain gauges for precipitation (mm), and hygrometers for humidity (percent relative humidity). Other instruments such as ceilometers, disdrometers, and weather balloons equipped with radiosondes are used to profile clouds, precipitation, and wind conditions at varying altitudes above ground level. Measurement units vary depending on the parameter and instrument but generally relate to the International System of Units.