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I'm trying to reduce a sauce I've got but I noticed something interesting.

When I raise the heat so the sauce is at a full rolling boil, there is barely any steam coming out so I presume there is little reduction happening.

However, when I reduce the heat to a slow gentle boil, there is quite a lot of steam coming off, so there is a lot more reduction happening.

Why is this the case?

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Physics to the rescue:

Contrary to popular belief you cannot see steam.

What you can see is tiny droplets of water that were steam (= gaseous form of water) before, but have condensed (= returned to liquid state) again on very small particles like dust motes. If the amount of droplets is big enough in a certain volume of air, they become visible. In other words, you have created a small cloud in your kitchen.

Now back to your pot:

  • If the pot is simmering, a lot of water will condense over or near the pot, especially if your kitchen is rather cool.
  • At a full boil, the steam disperses more, so when the condensation happens, the tiny droplets are spread out wider, making the "cloud" harder to see. Also, the air around the pot is likely to be a bit warmer, so that the gaseous H2O will likely condense at a greater distance from the source anyway.

This may lead to the assumption that a boiling pot emits less steam when actually the opposite is the case.

If you want to verify this, look for condensation away from the heat source, e.g. your cool kitchen windows. There, the different amounts should be very obvious.

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  • Nice answer, @Stephie! The last line sums it up beautifully!
    – Jolenealaska ♦
    Mar 3, 2015 at 14:05
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    The ambient humidity is also a factor. Dry air will "absorb" the steam quite easily, while very humid air will saturate quickly, forcing excess steam to condense.
    – Tristan
    Mar 3, 2015 at 15:46
  • Another easy experiment: take the pot to a full rolling boil then turn off the heat. The cloud of steam appears nearly immediately and remains visibly wafting from the pot after the water stops moving. When I saw that happen, my intuition shifted to conclude it's not the steam rising that's visible, but the steam cooling.
    – Dacio
    Mar 3, 2015 at 17:54
  • If you bring pilau rice to the boil with the gas on full with a lid on the pan, and a couple of steam holes in the lid, you'll see this effect. The steam is only visible when you reduce the gas. That's because there is less steam, which (confusingly) makes it more visible.
    – abligh
    Mar 3, 2015 at 20:36

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