Do you have experience swimming outdoors? About how warm does the air have to be for swimming to be enjoyable?

It is often the water temperature that matters. 69 degrees feels pretty chilly and 72 degrees fairly comfortable. If you have a robe and towels waiting when you emerge, you don’t need the air temperature to be tropical.

The warmer the better, of course. Hgh seventies is a safe starting place.

Good point about water temp. I’m thinking of my timing for traveling to an area where all I can tell in advance is the seasonal air temps. But this would be a small lake that gets full sun, so probably would heat up well on a sunny day.

We have a river here that is pretty much just snow that melted a few hours ago. It can be 100º outside and when I jump in my body freaks out and instantly tries to get to shore.

If the water isn’t moving much you will probably be alright.

Depth of the lake and number of spring-fed areas also matters.

Lake Placid is deep and, as @johnpowell says, cold no matter how warm the air is. Plus, you can start swimming in a area that is moderately comfortable and arrive at one of the undeground springs that is connected to the permafrost. That is always a shock.

Water has a high heat capacity – it’s slow to warm up or cool down. First, the ground heats, followed gradually by the air. Water lags far behind.

That’s why the ocean is miserably cold in May, even if the days are very warm, yet lovely in September, long after the ground and air temperatures have become chilly. It’s also why hurricane season begins in late-summer or early-autumn; hurricanes are fueled by very warm water, and it takes a while for that heat to build up.

I love swimming in the ocean, but I usually wait until early-July. By then, the water’s very pleasant.

To me it depends. For comfort the air temp should be around 80 or warmer and water temp below 76 starts to feel cold to me. Even 78 degrees takes a bit of swimming around to feel comfortable. My favorite swims are in the late summer early fall where the air is colder than the water and swimming in the water feels real nice. To me the best swimming was in the Bahamas where the air was 82 degrees and the water was 82 degrees….that was niiiiiiice!

In Northern NY if the air is mid 80’s or up in the early Summer, because that water is cold. Later it warms up. One warning, when the wind is blowing watch out for the chop. You have to breathe on the side opposite the wind. The chop (waves) can be higher than your head.

@johnpowell I used to love swimming in that part of the country when it was super hot and those cold, cold rivers and swimming holes. When you get out your body just feels thrilled. There’s no way to explain it and I’ve never experienced it anywhere but there.

I also find myself enjoying a lot to swim on those river springs up in mountains valleys where the water is really cold. If I would take a guess, probably minimum air temperature about 7ºC (Celsius degrees), but I’m not sure about the exact water temperature. Because I try to concentrate more on raising my own body temperature to better mentally deal with the cold, I believe that the answer could be psychological. Curiously, a thing that also seems to make a difference is if there are other people swimming there as well or not.

I prefer the water a bit chilly. A really warm lake feels too much like a bathtub—not refreshing for me at all. I don’t like hot tubs for that reason.

I can handle the outside air being at least 50, as long as there is no breeze at all, but after 30 minutes I’d need to get out and dry off. To be completely comfortable, I like the weather to be 68 or warmer.

In terms of a lake heating up on a sunny day – only count on that if it’s a shallow lake. As others have said, a large body of water takes a long time to gain and lose heat.

You haven’t said much about your own preferences… have you swum outside before? Does it have to be warm for you to enjoy it? Personally, I really enjoy cold swimming..

Yes, I have swum a lot outside and I love warmer water—don’t really enjoy cold water. If it’s helpful, the place I’m thinking about is a big lake (looks big from pictures) in a northeast U.S. town. It’s clearly a good swimming lake in the middle of summer but I’m trying to figure out how it might be in mid-September. Average highs in the town across all of September are mid-70s.

I grew up in Wisconsin and frequently swam in Lake Superior and other rivers and lakes. I don’t mind cold water. I’ve swum in rivers and streams at 72N here in Norway. Family thought I was nuts, but it was the only way I could feel clean camping. My local beach is pretty nice and I swim a bit there, but cold (I’m at 63N) I like the sun warming my skin afterwards, but a sauna or warm blankets will do. Swimming and the water makes me feel good and I would rather have cold water than tepid bathwater feeling.

@marmoset Well, the good thing about lake swimming in mid-September is that the lake should still be warm from the summer. Usually, you worry about how soon you can swim in the spring/early summer, rather than how long you can swim into the late summer/early fall. This is the trade-off with water’s resistance to temperature changes.