Abstract: A rain gauge with automatic priming siphon which uses the principle of indirect measurement of the rainfall by determination of its weight, for which it has a base (1), on which rests on a load cell (3) bearing a container (2) which receives the rainwater via a mouth (4). The emptying of the container (2) is assured periodically by means of a siphon (7), with the container (2) presenting a variable section with a minimum at the height of the bend of the siphon (7). In this way the priming time of the siphon (7) is radically reduced without affecting its discharge time. In addition, the bottom of the container (2) presents a reduction in section, and attached to it is a damper (6) which directly receives the rainwater channelled via the mouth (4). In this way the precision and security of functioning of the siphon (7) are furthermore increased.