What environmental factor can impact the velocity of air over an aircraft surface?

Altitude is a significant environmental factor that can impact the velocity of air over an aircraft surface. As an aircraft climbs in altitude, the density of the air decreases, which affects the flow of air over the wings and body of the aircraft. At higher altitudes, the thinner air can lead to alterations in lift performance and drag, which all relate directly to airspeed.

Additionally, the effect of altitude on the velocity of air can be observed through the principle of air density and the corresponding relationship with aircraft performance. As altitude increases, the aircraft may need to adjust its indicated airspeed to maintain the same true airspeed, which is vital for achieving the desired aerodynamic performance.

While temperature, humidity, and wind direction also play roles in overall atmospheric conditions, their effects on air velocity are not as direct or fundamental as altitude. Temperature changes can influence air density, but it is altitude that primarily defines the characteristics of the airflow over the aircraft surface.