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. Dry adiabat.  A line on a thermodynamic chart representing  a rate of temperature change at the dry adiabatic lapse rate. Drag.  The force that resists the movement of the glider  through the air. 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.  G-2