Similar to teenagers leaving home, ants do something similar
Hot summer days, like today, signal an important time in the ant breeding calendar. During the summer months they begin 'nuptial flight' period of their reproductive cycle. Similar to teenagers leaving home and moving away to a new city, the ants do something similar.
To prevent cross-breeding they relocate to another territory and set up a new colony elsewhere. Variables like temperature, weather and localised conditions will dictate which species will choose which day to fly to a new colony. There is not one 'flying ant day' but rather many different ones throughout a period of few weeks during July and August. That's why on days like today you'll see hundreds, if not thousands of ants in flight - they are quite literally moving house ready to start a new life.
It is believed that the ants pick the day based on the temperature, humidity and day length. The warm and humid conditions make the day perfect for the start of their breeding period. Flight is eased by the heat of the air, the humidity makes the ground softer for mated queens to dig nests.
As with everything in ant colonies, the Queen is the leader. She is the one who will lead the males to a new nest by producing pheromones to attract them to a new territory. It's thought that she will then mate with several males during the nuptial period to fertalise millions of eggs.
Ants have a really important role in our ecosystem; they improve soil quality, help pollinate flowers and feed on other species that eat plants. If that weren't enough, they're also a vital source of food for other animals.