Isopleth.  A line connecting points of constant or equal value. Isotherm.  A contour line of equal temperature. Katabatic.  Used to describe any wind blowing down slope. Kinetic energy.  Energy due to motion, defined as one half  mass times velocity squared. Lapse rate.  The decrease with height of an atmospheric  variable, usually referring to temperature, but can also apply  to pressure or density. Lateral axis.  An imaginary straight line drawn perpendicularly  (laterally) across the fuselage and through the center of  gravity. Pitch movement occurs around the lateral axis, and  is controlled by the elevator. Lenticular cloud.  Smooth, lens-shaped clouds marking  mountain-wave crests. They may extend the entire length of  the mountain range producing the wave and are also called  wave clouds or lennies by glider pilots. Lift.  Produced by the dynamic effects of the airstream acting  on the wing, lift opposes the downward force of weight. Limit load.  The maximum load, expressed as multiples of  positive and negative G (force of gravity), that an aircraft  can sustain before structural damage becomes possible. The  load limit varies from aircraft to aircraft. Load factor.  The ratio of the load supported by the glider’s  wings to the actual weight of the aircraft and its contents. Longitudinal axis.  An imaginary straight line running  through the fuselage from nose to tail. Roll movement occurs  around the longitudinal axis, and is controlled by the ailerons. Mesoscale convection system (MCS).  A large cluster of  thunderstorms with horizontal dimensions on the order of  100 miles. MCSs are sometimes organized in a long line of  thunderstorms (e.g., a squall line) or as a random grouping  of thunderstorms. Individual thunderstorms within the MCS  may be severe. Microburst.  A small-sized downburst of 2.2 nautical mile  or less horizontal dimension. Precipitable water.  The amount of liquid precipitation that  would result if all water vapor were condensed. Placards.  Small statements or pictorial signs permanently  fixed in the cockpit and visible to the pilot. Placards are used  for operating limitations (e.g., weight or speeds) or to indicate  the position of an operating lever (e.g., landing gear retracted  or down and locked). Pitot-static system.  System that powers the airspeed altimeter  and variometer by relying on air pressure differences to  measure glider speed, altitude, and climb or sink rate. Pitch attitude.  The angle of the longitudinal axis relative to  the horizon. Pitch attitude serves as a visual reference for the  pilot to maintain or change airspeed. Pilot-induced oscillation (PIO).  Rapid oscillations caused  by the pilot’s overcontrolled motions. PIOs usually occur on  takeoff or landings with pitch-sensitive gliders and in severe  cases can lead to loss of control or damage. Pilotage.  Navigational technique based on flight by reference  to ground landmarks. Parasite drag.  Drag caused by any aircraft surface that  deflects or interferes with the smooth airflow around the  airplane. Olphin flight.  Straight flight following speed-to-fly theory.  Glides can often be extended and average cross-country  speeds increased by flying faster in sink and slower in lift  without stopping to circle. Multicell thunderstorm.  A group or cluster of individual  thunderstorm cells with varying stages of development.  These storms are often self propagating and may last for  several hours. Mixing ration.  The ratio of the mass of water vapor to the  mass of dry air. Minimum sink airspeed.  Airspeed, as determined by the  performance polar, at which the glider achieves the lowest  sink rate. That is, the glider loses the least amount of altitude  per unit of time at minimum sink airspeed. K L O P M  G-4