Wake vortices and turbulence
Wake vortices are generated by aircraft including helicopters operating at high angle of attack, eg on takeoff and landing. Large GA aircraft can produce significant wake vortices. The heavier an aircraft, and the slower it is flying, the stronger its vortex and the greater the risk to following aircraft. The lighter the aircraft you are flying, the more vulnerable it is.
The vortices are generally invisible, last longer in calm or light wind conditions and are therefore at their most hazardous then, and are most dangerous close to the ground.