## Glossary 

## A 

Advection. The transport of an atmospheric variable due to mass motion by the wind. Usually the term as used in meteorology refers only to horizontal transport. 

Ailerons. The hinged portion of the trailing edge of the outer wing used to bank or roll around the longitudinal axis. 

Air density. The mass of air per unit volume. 

Airfoil. The surfaces on a glider that produce lift. 

Air mass. A widespread mass of air having similar characteristics (e.g., temperature), that usually helps to identify the source region of the air. Fronts are distinct boundaries between air masses. 

Amplitude. In wave motion, one half the distance between the wave crest and the wave trough. 

Angle of attack. The angle formed between the relative wind and the chord line of the wing. 

Angle of incidence. The angle between the chord line of the wing and the longitudinal axis of the glider. The angle of incidence is built into the glider by the manufacturer and cannot be adjusted by the pilot’s movements of the controls. 

Aspect ration. The ratio between the wing span and the mean chord of the wing. 

Asymmetrical airfoil. One in which the upper camber differs from the lower camber. 

Atmospheric sounding. A measure of atmospheric variables aloft, usually pressure, temperature, humidity, and wind. 

Atmospheric stability. Describes a state in which an air parcel resists vertical displacement or, once displaced (for instance by flow over a hill), tends to return to its original level. 

## B 

Bailout bottle. Small oxygen cylinder connected to the oxygen mask supplying several minutes of oxygen. It can be used in case of primary oxygen system failure or if an emergency bailout at high altitude became necessary. 

Ballast. Term used to describe any system that adds weight to the glider. Performance ballast employed in some gliders increases wing loading using releasable water in the wings (via integral tanks or water bags). This allows faster average cross-country speeds. Trim ballast is used to adjust the flying CG, often necessary for light-weight pilots. Some gliders also have a small water ballast tank in the tail for optimizing flying CG. 

Barograph. Instrument for recording pressure as a function of time. Used by glider pilots to verify flight performance for badge or record flights. 

Best glide speed (best L/D speed). The airspeed that results in the least amount of altitude loss over a given distance. This speed is determined from the performance polar. The manufacturer publishes the best glide (L/D) airspeed for specified weights and the resulting glide ratio. For example, a glide ratio of 36:1 means that the glider will lose 1 foot of altitude for every 36 feet of forward movement in still air at this airspeed. 

## C 

Camber. The curvature of a wing when looking at a cross section. A wing has upper camber on its top surface and lower camber on its bottom surface. 

Cap cloud. Also called a foehn cloud. These are clouds forming on mountain or ridge tops by cooling of moist air rising on the upwind side followed by warming and drying by downdrafts on the lee side. 

Centering. Adjusting circles while thermalling to provide the greatest average climb. 

