Glossary A B C 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. Centering.  Adjusting circles while thermalling to provide  the greatest average climb. G-1 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. 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. 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. Barograph.  Instrument for recording pressure as a function  of time. Used by glider pilots to verify flight performance  for badge or record flights. 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. 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.