Yes I think it's probably due to a combination of factors. The temp. of the water; the temp. and vapour pressure of the 'outside' air and the movement, or lack of, air in the chambers. In sum, any factors that could affect the humidity within the chambers. I wouldn't have thought the temp. of the stream would vary all that much but then a slight variation could be all that it needs. Does the temp. in the chambers vary much?

Temperature underground doesn't vary much and you tend to get water vapour inside when the weather is warmer and after its's been raining.

Presumably this is because: (a) you get condensation in cool chambers when warm humid air is drawn in; and (b) when it's rained, you tend to get more water falling inside the mine and some of this will remain as vapour.

I do recall seeing a genuine cloud in the chamber at the start of the Croesor-Rhosydd through trip once - a lovely little misty thing hovering just under the roof...

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I do recall seeing a genuine cloud in the chamber at the start of the Croesor-Rhosydd through trip once - a lovely little misty thing hovering just under the roof...

IIRC it was the only thing holding the roof up 😉

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Temperature underground doesn't vary much and you tend to get water vapour inside when the weather is warmer and after its's been raining.

Presumably this is because: (a) you get condensation in cool chambers when warm humid air is drawn in; and (b) when it's rained, you tend to get more water falling inside the mine and some of this will remain as vapour.

I do recall seeing a genuine cloud in the chamber at the start of the Croesor-Rhosydd through trip once - a lovely little misty thing hovering just under the roof...

I didn't think the temperature would vary a great deal. Yes I agree with the point you are making. The micro climate of the chamber will alter. The warm moist air drawn in will cool (the cooler the air the less water vapour it can hold meaning the humidity of a particular 'parcel' of air will increase) and if the humidty in the chamber is already high saturation point will be reached and mist or even fog (the small cloud?) could well form. As for the condensation, I imagine the sides of the chamber would be very wet. That poses another slight problem. Why doesn't all the excess water vapour take this route? Presumably because of the size of the chamber.
The more water of course will increase the humidty through evaporation. If you had stagnant water lying I would expect maybe a mist to form on top. You've no doubt seen this above ground on ponds and lakes.


[tweak]errant bracket put back![/tweak]

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