Maternal grandparents house in the Highlandtown neighborhood was overwhelmingly asphalt and concrete, but a city park was two stone throws away. Very few houses had window fans, let alone A/C. On summer nights starting around 9pm, a procession would pass Nana's house, some armed with fold away aluminum loungers but most with blankets and pillows. Patterson Park had excellent public toilet facilities and the buildings could shelter some in the event of a T-storm, but not too many.. This practice of sleeping in the park probably violated a city ordinance but the police didn't bother anyone. The people came to sleep, not party or vandalize. Next morning, they'd rise, go home, and begin their day. I guess the practice ended in the late '50s and I don't think it existed in any of the other city parks. Could stand corrected on that. No one in my parents' neighborhood mentioned anything about sleeping in our nearest park. Kids used to jump off an outcropping and swim in the water at Herring Run despite signs warning of typhoid but no evidence or indication of snoozing.
When we moved to Northern Virginia in '63 we didn't have a/c. No need to go to the park...I would drag a blanket and a pillow, crawl out the window, and sleep on the flat roof that overhung the front porch. I'll bet Baltimore was even hotter, being more concrete than the burbs in Fairfax County were.