I visited Calkins Nature Center in Iowa Falls, Iowa yesterday working on an IFHS Class of 1969 Project. I used a Neutral Density filter, really for the first time, since I was taking photographs in the middle of the day in harsh sunlight. It softened the light a bit and kept a blue sky blue.
I didn't go inside the TeePee as it was 95 degrees outside with a humidity of at least that... I have no idea what the heat index was but it had to be 1,000 (I exaggerate for effect).