Ground effect.  A reduction in induced drag for the same  amount of lift produced. Within one wingspan above the  ground, the decrease in induced drag enables the glider to fly  at a lower airspeed. In ground effect, a lower angle of attack  is required to produce the same amount of lift. Dust devil.  A small vigorous circulation that can pick up dust  or other debris near the surface to form a column hundreds  or even thousands of feet deep. At the ground, winds can be  strong enough to flip an unattended glider over on its back.  Dust devils mark the location where a thermal is leaving  the ground. Dynamic stability.  A glider’s motion and time required for  a response to static stability. Elevator.  Attached to the back of the horizontal stabilizer,  if controls movement around the lateral axis. Height band.  The altitude range in which the thermals are  strongest on any given day. Remaining with the height band  on a cross-country flight should allow the fastest average  speed. House thermal.  A thermal that forms frequently in the same  or similar location. Human factors.  The study of how people interact with their  environments. In the case of general aviation, it is the study  of how pilot performance is influenced by such issues as the  design of cockpits, the function of the organs of the body, the  effects of emotions, and the interaction and communication  with the other participants of the aviation community, such  as other crewmembers and air traffic control personnel. Empennage.  The tail group of the aircraft usually supporting  the vertical stabilizer and rudder, as well as the horizontal  stabilizer and elevator, or on some aircraft, the V-Tail. Flaps.  Hinged portion of the trailing edge between the  ailerons and fuselage. In some gliders, ailerons and flaps are  interconnected to produce full-span flaperons. In either case,  flap change the lift and drag on the wing. Flutter.  Resonant condition leading to rapid, unstable  oscillations of part of the glider structure (e.g., the wing) or  a control surface (e.g., elevator or aileron). Flutter usually  occurs at high speeds and can quickly lead to structural  failure. Induced drag.  Drag that is the consequence of developing  lift with a finite-span wing. It can be represented by a vector  that results from the difference between total and vertical lift. Inertia.  The tendency of a mass at rest to remain at rest or,  if in motion, to remain in motion unless acted upon by some  external force. Forward slip.  A slide used to dissipate altitude without  increasing the glider’s speed, particularly in gliders without  flaps or with inoperative spoilers. Instrument meteorological conditions (IMC).  Meteorological conditions expressed in terms of visibility,  distance from cloud, and ceiling less than the minimum  specified for visual meteorological conditions (VMC).  Gliders rarely fly in IMC due to instrumentation and air traffic  control requirements. Glider.  A heavier-than-air aircraft that is supported in flight  by the dynamic reaction of the air against its lifting surfaces,  and whose free flight does not depend on an engine. Graupel.  Also called soft hail or snow pellets, these are  white, round or conical ice particles  1 ⁄8  to  1 ⁄4 inch in diameter.  They often form as a thunderstorm matures and indicate the  likelihood of lightning.  Inversion.  Usually refers to an increase in temperature with  height, but may also be used for other atmospheric variables. Isohumes.  Lines of equal relative humidity. I G F E H G-3