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. 

## E 

Elevator. Attached to the back of the horizontal stabilizer, if controls movement around the lateral axis. 

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. 

## F 

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. 

Forward slip. A slide used to dissipate altitude without increasing the glider’s speed, particularly in gliders without flaps or with inoperative spoilers. 

## G 

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. 

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. 

## H 

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. 

## I 

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. 

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. 

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. 

