Identify the primary external factor that affects a glider's performance.

The primary external factor that significantly affects a glider's performance is wind conditions. Wind influences both the lift generated by the glider and the overall aerodynamic efficiency during flight. Rising air currents, known as thermals, as well as updrafts and downdrafts created by terrain, can enhance a glider's ability to gain altitude. Conversely, strong headwinds can reduce groundspeed and increase drag, while tailwinds can improve speed over the ground but may require careful management for safe landings.

While air density, temperature inversions, and humidity levels also play a role in the performance of a glider, particularly in relation to lift and overall efficiency, wind conditions have the most direct and immediate impact during flight. Variability in wind patterns can present pilots with both challenges and opportunities, making it essential for glider pilots to understand and utilize wind conditions effectively to optimize their performance.