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Summer of 95

Why Isn't The Freezing Point Lower At Altitude?

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Hi folks, this might seem a really stupid question but it's got me puzzled and no amount of remembering A-level geography or GCSE chemistry can help:

I know that water boils at lower temperatures at higher altitudes due to the lower pressure. Why doesn't it also freeze at lower temperatures too?

According to this table http://www.hi-tm.com/Documents/Calib-boil.html, at 3000ft altitude water boils at 206.6F; 5.4F below sea level BP. So why does it still freeze at 32F near the top of Snowdon or Scafell?

I know about "supercooled water" clouds that remain as liquid at -20 or so until they find condensation nuclei, but how can snow still fall/settle and lakes freeze at the sea-level freezing point 3000ft up? And the boiling point at the 25in/850mb level is about 203F, so shouldn't the freezing point be about 23F (-5C)? Yet I've seen it snow at sea level with the 850mb temp around 28F (-2C).

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this may answer your basic query about why?

Its a simple answer and better explained than mine would be at this time of the night!

Water expands a lot when it boils: one kilogram of water is one litre of liquid water, but it becomes about 1700 litres of steam at atmospheric pressure. This means that even modest increases in altitude can measurably reduce the boiling temperature. Some people complain that this affects cooking and even the taste of tea at altitude.

It is also true that pressure changes the melting temperature. However, because the volume occupied by a kilogram of liquid is not much different from that occupied by a kilogram of solid, this effect is very small unless the pressures are very large. For most substances, the freezing point rises, though only very slightly, with increased pressure.

hope that helps

In effect changes in pressure, up or down, have a greater effect on the boiling point of water. That effect is very much smaller, something like 0.001C, I think is the figure, for the change by about 50000ft. Someone correct me if that value of altitude is wrong, but the change in freezing point is minute.

There is a very complex answer involving differential calculus and advanced physics!

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All I know is you can't pop popcorn on a mountain.

So I think John's explanation has the upper hand here.

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So in other words, the lower the pressure, the smaller the range of temperatures that produce a liquid state? Very interesting; I suppose then there must be a pressure below which ice just sublimates straight to steam.

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