In most forecasts in the United States, wind speed is described in miles per hour. (Outside this country, it would be expressed in kilometers per hour; most other scientists also prefer to use the metric system). However, the Federal Aviation Administration, National Weather Service, and other groups that work with air and ocean travel, will use knots (one knot equals 1.15 miles per hour, or 1.85 kilometers per hour). Internationally, wind is also commonly measured in meters per hour. For vertical wind speeds, meteorologists use microbars per second, which indicates pressure change with altitude over time, or centimeters per second.