What environmental factor can affect the accuracy of a plumb bob when used outside?

When using a plumb bob outdoors, the most significant environmental factor affecting its accuracy is wind. A plumb bob relies on gravity to provide a vertical reference line, and any disturbance from wind can make it swing or sway, leading to an inaccurate reading. Wind can create pressure on the string or cord that holds the plumb bob, causing it to deviate from its true vertical path.

While other factors like humidity, temperature, and rain can influence working conditions and material behaviors, they do not impact the actual suspension and positioning of the plumb bob as directly as wind does. Therefore, in the context of maintaining precision in measurements outdoors, wind stands out as the key factor that can undermine the accuracy of a plumb bob.