We had a summer rain shower yesterday. Afterwards I could see steam rising off the street as the water evaporated. I could walk barefoot on that street before the shower and while the water was evaporating. If the water was boiling away, surely I wouldn’t be able to walk across it without scalding my foot.
What’s going on here?
In: Planetary Science
>Afterwards I could see steam rising off the street as the water evaporated. I could walk barefoot on that street before the shower and while the water was evaporating. If the water was boiling away, surely I wouldn’t be able to walk across it without scalding my foot.
There are some solid explanations here as to how water evaporates below the boiling point; I need not duplicate them. I just want to add a quick note:
You didn’t see any steam at all. In fact, you never have and never will! Steam is the gaseous form of water, and just like air and most other gases, it’s invisible. What we call ‘steam’ in conversation is actually many tiny condensed water droplets- liquid water- the same as clouds or fog. So when you see ‘steam’ rising from a puddle after a rainstorm, what’s actually happening is this:
The water molecules with high enough energy in the puddle break free from the puddle as gaseous water and escape into the air (itself just a collection of other gas molecules). You cannot see these gas-phase water molecules). After bouncing around in the air, they tend to lose some of that energy (on average), to the point where they can no longer overcome their tendency to stick to each other. So a multitude of tiny, cool water droplets forms as they ‘stick’ to each other. This is the ‘steam’ you’re seeing.
The sun doesn’t need to boil away the entire puddle in one go. It just needs to boil the individual molecules at the surface. And when those evaporate, boil the new surface. And so on until the puddle is done.
That’s how ocean water rises into the air to become clouds.
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