Also, why are white trails sometimes seen coming from the wingtips on takeoff and landing? What causes this phenomenon and why is it just at these times?
They are caused by water vapour in the atmosphere condensing out in the low pressure air in the wingtip vortex. You only see them on days when there is high humidity. The lower pressure air cannot hold as much moisture so it condenses out into visible vapour. Sometimes the whole upper wing surface can be covered in vapour on really damp days.