Ants are very small creatures, and they have no shortage of outside threats: predators, shoes, ant poison, you name it. Rain seems like it would be a big problem for these tiny insects, but surprisingly you won't see a colony swept away in a downpour. The crafty bugs are safe and dry in their subterranean home.
The ant fortresses you find on your lawn have a number of defenses. The first is the anthill that sits right on top of the nest. The mounds are usually made with special kinds of dirt or sand that absorbs water and dries quickly. Water hitting the convex dirt tends to bead and run off the side.
Some colonies, like mangrove ants, will send a soldier ant to plug up the entrance hole with its head. This living stopper prevents rain from entering but is only a temporary fix, as it blocks gas exchange with the surrounding area outside the anthill. [PDF]
When heavy rain leads to water entering the tunnel, it usually doesn’t go far. Ants tend to burrow at least a foot underground and have an intricate system of tunnels that work like storm drains. As long as the rainfall isn’t too heavy, the water will pass through the nest without pooling.
With some clever tunneling, ants can trap air in various chambers throughout the nest, and entrances to chambers come from below, preventing water from coming in.
Even if the tiny insects are caught in the rain outside, they still stand a chance. Ants are too light to break the surface tension and can walk on water. Fire ants are known for clinging together to create rafts when swept away in the rain. You can see in this video how buoyant they are: