We were walking down the sidewalk in our suburban neighborhood, minding our own business, when we were surrounded by a dense swarm of these. At 1st I thought they must be mayflies. A few landed on us for portraits (convenient); I ended up with 1 in my mouth (less convenient -- though supposedly termites are a delicacy. But I don't think you're supposed to eat the wings!). Coincidentally, we had just heard that a house ~50 m away has extensive termite damage, so maybe they came from there. But there is a riverbank patch of deciduous woods close by too.

Update: found a swarm of them gathering on the ground right at the edge of those woods same time of year (May 15) in 2020, so I think that's where they're coming from.