Worst day I ever saw was in NYC in 1975...on Staten Island. It was 114 and the air was so thick you could barely see the sun. I was working as a stage-hand (like a roadie) for the New York Shakespeare Festival's Mobile Theater and we were setting up the theater in a parking lot near the beach. Guys would work for 30 minutes then go sit under the boardwalk and try to hydrate and recover--and we had been doing this most of the summer.
There was nothing called a "Heat Index" then...it was just HOT!!!

Growing up in the NY area, and then later living in NC and then the DC area, heat and humidity ALWAYS came together. No such thing as "dry heat" wherever I lived so, when they say "It's 90 but with the heat index it feels like 97" I say, no, this is what 90 ALWAYS feels like to me...97 feels much, MUCH worse than this. Maybe in Arizona and New Mexico they can get that "dry heat" that doesn't feel so hot, but I've never really experienced that. So I never pay attention to heat index calculation.

(all I can think of is Robin Williams in "Good Morning, Viet Nam!" saying "It's Hot! It's always Hot!")

Meanwhile, the idea is to circulate your water at least twice a day. How long is kind of complicated because you have to figure flow rates of the pump, the filter, the ACTUAL flow rate of water through the system in gallons per minute, then how many minutes it takes to process all the water in your pool and how many hours that translates to (if your pump moves water at 30gpm, it takes 4 hours to do one cycle of 7000 gallons). So it's probably safe to figure two 6 hour cycles per day. If that's not enough, extend the daylight one to 8, or 10 (if you do 10 or 12 day hours that you can try cutting the night hours to 4). You'll learn over time what is "not enough" time.

Carl