A sprayer may drift onto the lawn and other plants.
Use a watering can.
Keen gardeners have a special red can for weedkiller.
You only need a few dry hours, but a rain-free day is better. Morning is best as sunlight helps the plant transport the chemical from the leaves to the roots, and morning sun is not strong enough to evaporate the droplets before they are absorbed.
If you use Pathclear instead, it also leaves a film that prevents new seeds germinating for several months, unless disturbed. Spring is better than Autumn for that.