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That photo of steam coming from the manhole cover in NY is pretty goofy. NY has 100s of miles of super-hot steam pipes underground. You'll see that same steam come up from manhole covers after a good rain in August. I'm pretty sure it's not from wastewater pipes.

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Most cities in the world don't have steam infrastructure like that and still have steaming manholes in the winter from hot wastewater.

Doesn't even have to be hot, it just has to be hotter than the air above ground, and relatively humid (how much depends on the temperature difference). You can also see rivers steam when it gets very cold, and they're often only a few degrees above freezing at that point.

Is all the steam from hot wastewater? With all the things going on underground in cities, I've assumed there must be heat (i.e., from basic thermodynmics), and of course it needs a place to escape.

Some of it is, and the picture editor could be excused for sticking with a generic "steam manhole NYC" stock image for illustration imo.

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