Pressure altitude. The height above the standard pressure level of 29.92 "Hg. It is obtained by setting 29.92 in the barometric pressure window and reading the altimeter. 

## R 

Radiant energy. Energy due to any form of electromagnetic radiation (e.g., from the sun). 

Radius of turn. The horizontal distance an aircraft uses to complete a turn. 

Rate of turn. The amount of time it takes for a glider to turn a specified number of degrees. 

Relative wind. The airflow caused by the motion of the aircraft through the air. Relative wind, also called relative airflow, is opposite and parallel to the direction of flight. 

Rotor. A turbulent circulation under mountain-wave crests, to the lee side and parallel to the mountains creating the wave. Glider pilots use the term rotor to describe any low-level turbulent flow associated with mountain waves. 

Rotor streaming. A phenomenon that occurs when the air flow at mountain levels may be sufficient for wave formation, but begins to decrease with altitude above the mountain. In this case, the air downstream of the mountain breaks up and becomes turbulent, similar to rotor, with no lee waves above. 

Rudder. Attached to the back of the vertical stabilizer, the rudder controls movement about the vertical axis. 

## S 

Sailplane. A glider used for traveling long distances and remaining aloft for extended periods of time. 

Saturated Adiabatic Lapse Rate (SALR). The rate of temperature decrease with height of saturated air. Unlike the dry adiabatic lapse rate (DALR), the SALR is not a constant numerical value but varies with temperature. 

Self-launching glider. A glider equipped with an engine, allowing it to be launched under its own power. When the engine is shut down, a self-launching glider displays the same characteristics as a non-powered glider. 

Side slip. A slip in which the glider’s longitudinal axis remains parallel to the original flightpath but in which the flightpath changes direction according to the steepness of the bank. 

Slip. A descent with one wing lowered and the glider’s longitudinal axis at an angle to the flightpath. A slip is used to steepen the approach path without increasing the airspeed, or to make the glider move sideways through the air, counteracting the drift resulting from a crosswind. 

Speed to fly. Optimum speed through the (sinking or rising) air mass to achieve either the furthest glide or fastest average cross-country speed depending on the objectives during a flight. 

Spin. An aggravated stall that results in the glider descending in a helical, or corkscrew, path. 

Spoilers. Devices on the tops of wings to disturb (spoil) part of the airflow over the wing. The resulting decrease in lift creates a higher sink rate and allows for a steeper approach. 

Squall line. A line of thunderstorms often located along or ahead of a vigorous cold front. Squall lines may contain severe thunderstorms. The term is also used to describe a line of heavy precipitation with an abrupt wind shift but no thunderstorms, as sometimes occurs in association with fronts. 

Stabilator. A one-piece horizontal stabilizer used in lieu of an elevator. 

Stability. The glider’s ability to maintain a uniform flight condition and return to that condition after being disturbed. 

Stall. Condition that occurs when the critical angle of attack is reached and exceeded. Airflow begins to separate from the top of the wing, leading to a loss of lift. A stall can occur at any pitch attitude or airspeed. 

Standard atmosphere. A theoretical vertical distribution of pressure, temperature and density agreed upon by international convention. It is the standard used, for instance, for aircraft performance calculations. At sea level, the standard atmosphere consists of a barometric pressure of 29.92 inches of mercury ("Hg) or 1013.2 millibars, and a temperature of 15 °C (59 °F). Pressure and temperature normally decrease as altitude increases. The standard lapse rate in the lower atmosphere for each 1,000 feet of altitude is approximately 1 "Hg. and 2 °C (3.5 °F). For example, the standard pressure and temperature at 3,000 feet mean sea level (MSL) is 26.92 "Hg. (29.92 – 3) and 9 °C (15 °C – 6 °C). 

