There are a number of factors that affect when ants flyout of their nests to mate - something which is known a nuptial flight.

The time of day that the ants set off is pre-set - ants from different species will fly at different times - but members of the same species will share the same flight time. In one study of town ants in Louisiana for example all ants took off between 3.30am and 4.15am - and the flights peaked at in the 5 minute period just before 4am. This co-ordination appears to be controlled by a circadian mechanism.

The other triggers for the flight are environmental - and again vary from species to species. Ants can be very picky about getting the conditions just right and can wait for several weeks within the nest for the right conditions. In the case  of the town ants the first condition was that at least 7mm of rain fell before they flew and the temperature had to be at least 16 degrees C. Again other species have different preferences some fly when rain is threatening, there is evidence that temperature, wind direction, humidity and moonlight can affect the decision to fly or not. In some situations, such a drought, the flight will be put off entirely until the following year.

Both male and female winged ants are produced. When they leave the nests they mate. The females go on to set up new colonies - using sperm stored from the mating to produce new ants. The males die. The preference for certain weather conditions is presumably so that conditions for mating and setting up a new nest are at their most optimal.