The gods must have been listening when the cast of "Hair" sang "Aquarius/Let the Sunshine In" Wednesday night, because the sky cleared up during their brief performance outside Theory's Meatpacking District store. Tie-dye-clad ushers directed fans to spread out on Ikea mats across Gansevoort Square for the free showcase, presented by the Theory Icon Project. And though the weather held up, the stifling heat came as something of a shock to the suede-, polyester- and — of course — wig-clad cast, who have been rehearsing indoors, with air-conditioning, before their July 22 opening at the open-air Delacorte Theater. Over tacos and frozen margaritas at Los Dados, star Jonathan Groff said, "Today was a big wake-up call — it was the first time we were in our wigs and costumes outside and it looked like we had all just taken a bath together when we got offstage. We're all going to lose about 20 pounds in water weight every performance."