* When below 3,000 ft amsl or 1,000 ft above terrain, whichever is higher:
- Clear of cloud and in sight of the ground/water;
- Flight visibility 5 km.
* Special VFR: visibility of 1.5km and aircraft need to remain clear of clouds and some other requirements depending of CTR class etc. ( 3km for europe i think?)
- as tangoalphad said, visible moisture in icing conditions (most aircrafts with no anti/de-icing equipment prohibits flying into expected or known icing conditions, read your POH regarding that
- take into consideration the intensity of rain and type of droplets