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 

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 

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). 

## W 

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. 

