It has been a few years, but we were there after labor day and the weather could not have been better. A little cool in the morning, but by afternoon it was warm.
Did not play in the water so can't comment there.
Will it be too cold to play with water during that time?
Whatever the temperature, the water is too cold all year to play in IMHO. (BTW, you're not allowed in the thermal features.) One place to swim is "An established spot is on the Gardner River two miles north of Mammoth on the North Entrance road; this spot is known as Boiling, or Hot, River. It is only open during daylight hours, and it temporarily closes during periods of high water." But Yellowstone Lake and most of the streams away from the geyser basins are pretty cold all year.
I guess it somewhat depends on your definition of cold. Some of us dont think its cold unless its below 30, some think its cold below 60.
Yellowstone's elevation ranges from 5000 to 11000 ft, with an avg around 8,000 ft, so you're in high country where the temps are naturally cooler, especially when the sun is down or behind clouds.
When I was in Yellowstone, mid Aug, it was very comfortable during the daytime sun, but down near mid 30's overnight. When I climbed the Grand Teton in mid July I got snowed on near the top, but a swim in Jenny Lake felt good on the way out.
The weathermen can't even guess it a day ahead of time.
At 5000 ft elevation our normal frost in Cody is about the second week of September. Last fall we didn't get our first frost until about the 1st week of October. Yellowstone is quite a bit higher in elevation.
The only swimming hole inside the park that I know of is on the Fire Hole Canyon Loop road ( it is a oneway from north to south) just south of Madison Junction, if I remember right it is the first right after Madison Junction.
Don't know if it will be too cold or not in late Aug. but have seen people swimming and playing in early Aug. there.
Hope this helps.
For all Yellowstone has to offer I would think "playing in the water" would be the least of my considerations. I think I could play in the water in a whole bunch of other places but there is only one Yellowstone. We had some sleet in the first part of Sept but we still considered everything PERFECT including the weather.... IMO..
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We were at Fishing Bridge during the first week of Aug 06 and it dropped below freezing on at least two nights. I warmed up nicely during the day.
What do you mean by "play with water"? With only a couple hot spring sections of exceptions on the Firehole River, The water is ALWAYS cold.
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I think you will be fine. The last few years summer has came later and lasted longer. As for swimming Fire hole is good, but is very crowded. Be carefull because when it is busy it is hard to get a camper through that loop. It can freeze any time of year in the park, as for nice weather Aug is a great month. You will miss some of the crowds because some school years start by then so not as many vacationing families.