1. A method for synthetic three-dimensional representation of a terrain, said method being used in a piloting and navigation assistance system of an aircraft, said assistance system comprising at least a navigation system, a meteorological database comprising at least altitudes of a bottom and of a top of a cloud layer, a cartographic database, an electronic calculator, and a display system allowing superimposition of synthetic images on terrain perceptible through the display system, wherein said method comprises: calculation, for a determined position of the aircraft, of the terrain perceptible through the display system; selection of a hidden part of the perceptible terrain situated between the bottom and the top of the cloud layer; calculation of a three-dimensional cartographic representation of the selected part of the terrain; and display of the said cartographic representation by the display system.