Acre-foot (acre-ft). The volume of water required to cover 1 acre to a depth of 1 ft. It equals 43,560 ft3 (cubic feet), 325,851 gal (gallons), or 1,233 m3 (cubic meters). Contents. The volume of water in a reservoir or lake. Content is computed on the basis of a level pool or reservoir backwater profile and does not include bank storage. Convection cloud. A cloud which owes its vertical development, and possibly its origin, to convection. Convective cell. An organized, convective, fluid motion characterized by the presence of distinct upward motion in the center of the cell and sinking or downward flow in the outer regions. Cubic foot per second (ft3/s). A rate of discharge. One cubic foot per second is equal to the discharge of a stream of rectangular cross section 1 ft wide and 1 ft deep, flowing at an average velocity of 1 ft/s. It equals 28.32 L/s (liters per second) or 0.02832 m3/s (cubic meter per second). Cubic foot per second per square mile, (ft3/s)/mi2. The average number of cubic feet of water flowing per second from each square mile of area drained by a stream, assuming that the runoff is distributed uniformly in time and area. One ft3/s per square mile is equivalent to 0.0733 m3/s per square kilometer. Dew point (or dew-point temperature). The temperature to which a given parcel of air must be cooled at constant pressure and constant water-vapor content for saturation to occur. Drainage area of a stream at a specific location. An area, measured on a horizontal plane, bounded by topographic divides. Drainage area is given in square miles. One m2 is equivalent to 2.590 km2 (square kilometers). Exceedance probability. The probability, expressed as a percentage, that a given magnitude of flood discharge will be exceeded during any given year. The reciprocal of exceedance probability is recurrence interval. Thus a flood magnitude with an exceedance probability of 1 percent has a recurrence interval of 100 years. Flood. Any streamflow that overtops natural or artificial banks of a stream and inundates land not usually underwater. Front. The interface or transition zone between two airmasses of different densities. Gaging station. A particular site on a stream, canal, lake, or reservoir where systematic observations of gage height or discharge are made. K Index. A measure of the airmass moisture content and static stability given by: where T is temperature and TA is dewpoint, in degrees Celsius, and the subscripts denote pressure level in millibars. The larger the K index of the airmass, the more unstable it is. Low. Center of low barometric pressure. National Geodetic Vertical Datum of 1929 (NGVD of 1929). A geodetic datum derived from a general adjustment of the firstorder level nets of both the United States and Canada, formerly called "mean sea level." Millibar (mb). A unit of pressure equal to 1,000 dynes per square centimeter. Peak discharge. The maximum instantaneous discharge attained during a flood. Peak stage. The highest instantaneous stage attained during a flood. Precipitable water. The total ...