Peak dragonfly season and the start of the green darner's fall migration have squadrons of them swirling over Dolores Park, Lake Merritt and grassy meadows across the Bay Area. It's free, it's happening now, and here's when and where to catch it.

If the sky over your neighborhood park has looked busier than usual this month, you're not imagining it. Dragonflies are out in force across the Bay Area right now — hundreds at a time over the grassy hill near Fairyland at Oakland's Lake Merritt, squadrons wheeling above the crowds at Dolores Park in San Francisco, and clouds of them over lawns, ponds and meadows in between. It's free, it requires no ticket and no reservation, and the best viewing is happening this week and likely for weeks to come. All you have to do is look up.

The surge is real enough that "why are there so many dragonflies?" has become a recurring question on local Reddit boards, as SFGATE's Amanda Bartlett documented this month. The short answer, according to an entomologist she interviewed at the California Academy of Sciences, is that several conditions are lining up at once: a cool, breezy stretch of weather, favorable wind currents, and a well-stocked buffet of mosquitoes and other small prey. Put simply, we've hit the annual peak.

There's a calendar reason the timing feels dramatic, too. Mid-August is roughly when the fall migration of the common green darner — a big, green-thoraxed, blue-tailed species — gets going, a multi-generational journey that Science News has compared to the monarch butterfly's (external source, opens in a new tab). Migrating darners can cover dozens of miles a day and bunch up into the kind of loose, high-flying swarms people have been noticing overhead.

The Bay Area is good dragonfly country in general, with more than two dozen resident species, from the blue-eyed darner to the flame skimmer. They're worth watching for their own sake: the East Bay Regional Park District notes (external source, opens in a new tab) they can hit around 30 miles an hour, fly backward, hover in place, and eat hundreds of mosquitoes a day, which makes them nature's best argument against the bug at the bottom of the food chain.

Where and when to go. You don't need a destination — you need water and open grass. The reliable combination is a pond, lake or creek next to a sunny meadow, which is exactly what Lake Merritt (try the lawns near Fairyland and the Lakeside Park side) and Dolores Park deliver. In the East Bay, the park district points to lakes, ponds, creeks and meadows district-wide; Tilden's Jewel Lake, Lake Chabot and the Coyote Hills marshes are all fair game. Go on a warm, still afternoon — dragonflies are sun-powered and most active when it's warm and calm, so a foggy morning is the wrong call. Bright, breezeless mid-afternoon is the sweet spot.

If you've got two hours: pack a blanket, get to a lakeside lawn by early afternoon on a clear day, and just sit. The show comes to you — the darners patrol the same stretch of grass over and over, and you'll start picking out the greens from the blues without moving a muscle. Bring nothing you'd mind a dragonfly landing on.