Swarming is the way that a honey bee colony reproduces. This usually happens in early spring time when a colony is rapidly growing.
Before the honey bees swarm they feed on honey before departing on their journey to a site for the newly formed colony. When they are ready to go the bees leave the hive and can appear as a cloud in the air. They tend to be sighted when they are resting as a cluster protecting the queen. Sometimes this cloud is spotted in the air or you may hear the rumbling hum overhead.
The resting place for a swarm is usually located within a 100 yards of the parent colony. Although swarming honey bees have little interest in stinging people you should stay away and call a beekeeper immediately. They have sent out scout bees to find a safe place to set up their new colony, you want a beekeeper to come and relocate them before they move into your home.
Swarms can be harmed by high winds and heavy rainfall, so please do find a beekeeper as soon as possible if you see a swarm.