For aviation purposes, a ceiling is the lowest layer of clouds reported as being broken (BKN) or overcast (OVC), or the vertical visibility in an obscuration like fog or haze. Clouds are reported as broken when five-eighths to seven-eighths of the sky is covered with clouds. If clouds are present but cannot be distinguished because of obscuring phenomena, the ceiling is reported as obscured (X). A sky condition reported as partially obscured (-X) indicates that some portion of the sky or cloud layer is visible through the obscuration. The maximum altitude above sea level at which an aircraft can maintain level flight under standard air conditions is called the absolute ceiling. The service ceiling is the height at which the rate of climb of any plane has dropped to 100 ft per minute. Current ceiling information is reported by the aviation routine weather report (METAR) – an automated weather station of various types.