For the bored or interested, officially, they are contained in Appendix 9 to the Radio Regulations Annex to the International Telecommunications Convention (Atlantic City) 1947, for blocks QRA to QUZ inclusive, and in ICAO publications Dec 6100-COM/504/1 for blocks QAA to QNZ inclusive.
Radio hams could probably tell you a lot more about them, but there are a few interesting ones, like "QUQ - Shall I train my searchlight nearly vertical on a cloud, occulting if possible and, if your aircraft is seen, deflect the beam upwind and on the water (or land) to facilitate your landing?"
