T V W  G-6 Static stability.  The initial tendency to return to a state of  equilibrium when disturbed from that state. Supercell thunderstorm.  A large, powerful type of  thunderstorm that forms in very unstable environments with  vertical and horizontal wind shear. These are almost always  associated with severe weather, strong surface winds, large  hail, and/or tornadoes. T-tail.  A type of glider with the horizontal stabilizer mounted  on the top of the vertical stabilizer forming a T. Thermal.  A buoyant plume or bubble of rising air. Thermal index (TI).  For any given level is the temperature  of the air parcel having risen at the dry adiabatic lapse  rate (DALR) subtracted from the ambient temperature.  Experience has shown that a TI should be –2 for thermals to  form and be sufficiently strong for soaring flight. Thermal wave.  Waves, often but not always marked by cloud  streets, that are excited by convection disturbing an overlying  stable layer. Also called convection waves. Thermodynamic diagram.  A chart presenting isopleths of  pressure, temperature, water vapor content, as well as dry and  saturated adiabats. Various forms exist, the most commonly  used in the United States being the Skew-T/Log-P. Thrust.  The forward force that propels a powered glider  through the air. Total drag.  The sum of parasite and induced drag. Towhook.  A mechanism allowing the attachment and release  of a towrope on the glider or towplane. On gliders, it is located  near the nose or directly ahead of the main wheel. Two types  oftowhooks commonly used in gliders are manufactured by  Tost and Schweizer. Trim devices.  Any device designed to reduce or eliminate  pressure on the control stick. When properly trimmed, the  glider should fly at the desired airspeed with no control  pressure from the pilot (i.e., hands off). Trim mechanisms  are either external tabs on the elevator (or stabilator) or a  simple spring-tension system connected to the control stick. True altitude.  The actual height of an object above mean  sea level. V-tail.  A type of glider with two tail surfaces mounted to  form a V. V-tails combine elevator and rudder movements. Variometer. Sensitive rate of climb or descent indicator  that measures static pressure between the static ports and  an external capacity. Variometers can be mechanical or  electrical and can be compensated to eliminate unrealistic  indications of lift and sink due to rapid speed changes. Vertical axis.  An imaginary straight line drawn through  the center of gravity and perpendicular to the lateral and  longitudinal axes. Yaw movement occurs around the vertical  axis and is controlled by the rudder. Visual meteorological conditions (VMC).  Meteorological  conditions expressed in terms of visibility, distance from  cloud, and ceiling equal to or better than a specified minimum.  VMC represents minimum conditions for safe flight using  visual reference for navigation and traffic separation.  Ceilings and visibility below VMC constitutes instrument  meteorological conditions (IMC). Washout.  Slight twist built in towards the wingtips, designed  to improve the stall characteristics of the wing. Water vapor.  Water present in the air while in its vapor form.  It is one of the most important of atmospheric constituents. Wave length.  The distance between two wave crests or  wave troughs. Wave window.  Special areas arranged by Letter of  Agreement (LOA) with the controlling ATC wherein gliders  may be allowed to fly under VFR in Class A Airspace at  certain times and to certain specified altitudes. Weight.  Acting vertically through the glider’s center of  gravity, weight opposes lift. Wind triangle.  Navigational calculation allowing  determination of true heading with a correction for  crosswinds on course.