Center of pressure. The point along the wing chord line where lift is considered to be concentrated. 

Centrifugal force. The apparent force occurring in curvilinear motion acting to deflect objects outward from the axis of rotation. For instance, when pulling out of a dive, it is the force pushing you down in your seat. 

Centripetal force. The force in curvilinear motion acting toward the axis of rotation. For instance, when pulling out of a dive, it is the force that the seat exerts on the pilot to offset the centrifugal force. 

Chord line. An imaginary straight line drawn from the leading edge of an airfoil to the trailing edge. 

Cloud streets. Parallel rows of cumulus clouds. Each row can be as short as 10 miles or as long as a 100 miles or more. 

Cold soaked. Condition of a self-launch or sustainer engine making it difficult or impossible to start in flight due to long- time exposure to cold temperatures. Usually occurs after a long soaring flight at altitudes with cold temperatures (e.g., a wave flight). 

Convection. Transport and mixing of an atmospheric variable due to vertical mass motions (e.g., updrafts). 

Convective condensation level (CCL). The level at which cumulus forms from surface-based convection. Under this level, the air is dry adiabatic and the mixing ratio is constant. 

Conventional tail. A glider design with the horizontal stabilizer mounted at the bottom of the vertical stabilizer. 

Convergence. A net increase in the mass of air over a specified area due to horizontal wind speed and/or direction changes. When convergence occurs in lower levels, it is usually associated with upward air motions. 

Convergence zone. An area of convergence, sometimes several miles wide, at other times very narrow. These zones often provide organized lift for many miles (e.g., a sea-breeze front). 

Critical angle of attack. Angle of attack, typically around 18°, beyond which a stall occurs. The critical angle of attack can be exceeded at any airspeed and at any nose attitude. 

Cross-country. In soaring, any flight out of gliding range of the takeoff airfield. Note that this is different from the definitions in the 14 CFR for meeting the experience requirements for various pilot certificates and/or ratings. 

Cumulus congestus. A cumulus cloud of significant vertical extent and usually displaying sharp edges. In warm climates, these sometimes produce precipitation. Also called towering cumulus, these clouds indicate that thunderstorm activity may soon occur. 

Cumulonimbus (CB). Also called thunderclouds, these are deep convective clouds with a cirrus anvil and may contain any of the characteristics of a thunderstorm: thunder, lightning, heavy rain, hail, strong winds, turbulence, and even tornadoes. 

## D 

Dead reckoning. Navigation by computing a heading from true airspeed and wind, then estimating time needed to fly to a destination. 

Density altitude. Pressure altitude corrected for nonstandard temperature variations. Performance charts for many older gliders are based on this value. 

Dewpoint (or dewpoint temperature). The temperature to which a sample of air must be cooled, while the amount of water vapor and barometric pressure remain constant, in order to attain saturation with respect to water. 

Dihedral. The angle at which the wings are slanted upward from the root to the tip. 

Diurnal effects. A variation (may be in temperature, moisture, wind, cloud cover, etc.) that recurs every 24 hours. 

Downburst. A strong, concentrated downdraft, often associated with a thunderstorm. When these reach the ground, they spread out, leading to strong and even damaging surface winds. 

Drag. The force that resists the movement of the glider through the air. 

Dry adiabat. A line on a thermodynamic chart representing a rate of temperature change at the dry adiabatic lapse rate. 

Dry adiabatic lapse rate (DALR). The rate of decrease of temperature with height of unsaturated air lifted adiabatically (not heat exchange). Numerically the value is 3 °C or 5.4 °F per 1,000 feet. 

