Why do windows steam up from the bottom? In our bedroom there's a window with four fairly equally sized square panes, in a 2x2 square layout. Now it's gone cold they're getting condensation on them every morning, but it's at the bottom (and, to an extent, the sides) of each of the four panels - why is not all over?
I would guess that heat rising means the top parts are warmer. Water condenses from warm and wet air hitting a cold surface.
I would guess that heat rising means the top parts are warmer. Water condenses from warm and wet air hitting a cold surface.
I thought that, but I have two pairs of panes one of above the other, and it's the same pattern on both the top row and the bottom row. If it was 'hot air rising' then the bottom row would be covered and the top row would be clear, right?
My pet theory is that each pane has its own little convection current. Each window surface is much colder than the air inside the house and so you'll get warm air hitting it and cooling. Thus each pane will have a pocket of cooler air at the bottom, which is where condensation forms. That's all a bit of a guess though.
As the air hits the glass (top or bottom panes, both of which are cold) it cools. As it does so it cools and sinks due to being denser. As it does so it continues to cool. At some point during its cooling down fall the air reaches a temperature where the water vapour condenses. This starts at the bottom of the pane (where air is coldest as it sinks) and will continue growing upwards as far as the moisture/temperature balance allows.
This can be compounded in older windows where the inner space has leaked air into it, meaning you can get a temperature differential on the pane itself, with it being hotter at the top than the bottom.
I guess....
God just likes the aesthetics - makes it look like a christmasy
This could be nonsense but...
Glass is a super-cool liquid, it's not a solid, it is constantly moving, just very slowly. Therefore both panes are thicker at the bottom and that makes them colder and therefore more condensation-y.
This could be nonsense but…
Glass is a super-cool liquid, it’s not a solid, it is constantly moving, just very slowly. Therefore both panes are thicker at the bottom and that makes them colder and therefore more condensation-y.
I am very sure it has nothing to do with the tendency for old glass to become thick at the bottom due to "flow". If that were the case you could take an old pain of glass and fit it upside down and see the opposite effect. The effect would also be much more pronounced on very old glass than new glass - but its still seen on freshly fitted modern glass which has will be as close to even thickness as you can get.
I think stevious/benpinnicks explanations are much closer to the reality - basically some fairly complex fluid dynamics which are affected by the shape of the frame as well as convection etc.
That thing about mediaeval glass thickness is not true.
Water condenses from warm and wet air hitting a cold surface.
which is why it always baffles me when we get condensation on the inside of car windows.
yes houses are warmer, so when that warm, moist air hits a cold window then we get condensation, but surely the temperature inside a car is the same as outside.
benpinnick +1
With a side of gravity pulling down any droplets so even if condensation were even across the pane, when you observed it in the morning it's mostly at the lower edge.
yes houses are warmer, so when that warm, moist air hits a cold window then we get condensation, but surely the temperature inside a car is the same as outside.
Overnight the car will be colder than the air above it as it loses heat through radiation, cold air sinks, warm air rises so you get a very cold boundary layer over the ground/car. Moisture in that layer condenses.
You don't get condensation inside the car because:a
a) the temperature inside the car is slightly above the outside because it's somewhat insulated.
b) even if the temp is exactly the same, the enthalpy isn't, to get condensation you have to remove all the latent heat of vaporization from that moisture, which required heat to be conducted, and as above there's some insulation, and if the temp inside and outside is the same then there's no energy transfer.
c) the inside of the car is dry. You is there with the heat on ~21C (or more), drying out the 0C air from outside, the relative humidity is then incredibly low. So all the upholstery is also bone dry. So any residual moisture in the air has a lot of other material it can absorb into before it reaches the dew point of condensing on the glass.
And the exception that proves the rule, anyone with an old leaky car, or who's stored wet kit in it will know you absolutely can get condensation and ice on the inside of the glass!
which is why it always baffles me when we get condensation on the inside of car windows.
yes houses are warmer, so when that warm, moist air hits a cold window then we get condensation, but surely the temperature inside a car is the same as outside.
Air outside the car cools down much faster than the air trapped inside the car so the air in the car can be warmer than external.
And the exception that proves the rule, anyone with an old leaky car, or who’s stored wet kit in it will know you absolutely can get condensation and ice on the inside of the glass!
Or a Kia...we've had 3 and all of them suffer/ed with condensation and ice on the inside of the windscreen 😂
The fun bit is if you live somewhere flat and prone to really cold, still mornings (e.g. the Thames valley, Fens, etc.) then the car is colder than the air when you set off, and you get rapidly forming condensation and ice on the outside. So you need the wipers on even if it's not raining until the car's warmed up enough to prevent it.
yes houses are warmer, so when that warm, moist air hits a cold window then we get condensation, but surely the temperature inside a car is the same as outside.
The moisture is still there whether the air is warm or not and the glass is still the coldest surface... that you can see. In a van you'd have condensation all over the ceiling too but most of the internal surfaces in a car are lined - the glass is the only material one layer removed from the outside - and are also often surfaces you wouldn't see condensation on - how would you notice if moisture had condensed on the headliner or the carpet?
This could be nonsense but…
Glass is a super-cool liquid, it’s not a solid, it is constantly moving, just very slowly.
It is nonsense, I believe.
Old glass isn't uniform due to an old manufacturing process where it was rolled rather than, uh, whatever they do these days.
whatever they do these days.
It's poured onto one end of a long bath of liquid tin IIRC which keeps it liquid long enough to flow out evenly then it's drawn off as a continuous sheet at the far end.
We've got triple glazing and under certain conditions we get condensation on the outside of the glass (because the outer pane is well enough insulated that the warmth of the house doesn't raise it above condensation point). On the west facing windows it can be like that until the sun reaches it after midday.
The frames conduct more than the glass so you get a 5cm clear band all the way around the outside.
I'm guessing with double glazing the gas/air in the gap is warmer at the top than the bottom, and maybe the air gap isn't large enough for much of a convection current to form
but surely the temperature inside a car is the same as outside.
But when you were driving it the temp inside was higher, so could hold more moisture. When you leave the car to cool off the air inside cools and can no longer hold all that vapour, so it condenses out.
We’ve got triple glazing and under certain conditions we get condensation on the outside of the glass (because the outer pane is well enough insulated that the warmth of the house doesn’t raise it above condensation point).
We get that regularly with normal double glazing, seems to require certain specific conditions, but quite often see (in winter) condensation on the outside.
Old glass isn’t uniform due to an old manufacturing process where it was rolled rather than, uh, whatever they do these days.
Really old glass was spun. Hence why you get those twee windows with a big glob in the middle. That was essentially the undesirable bit. The better bits were cut from the flatter, outer section.
but surely the temperature inside a car is the same as outside.
But when you were driving it the temp inside was higher, so could hold more moisture. When you leave the car to cool off the air inside cools and can no longer hold all that vapour, so it condenses out.<br /><br />
🎯👍🏼
Some of my windows get a strip of condensation along the bottom, but they’re cheap double glazing units put in by the council when it was still a council house, probably around 45-50 years ago. Some of them have never shut properly since new, either.
I had a surveyor in this afternoon to measure up for all my windows to be replaced with much better ones. Three-way venting, they can be locked slightly open to allow airflow through the house if unoccupied for a while, and the opening part can be swivelled out to allow cleaning from both sides. All these things are a revelation to me! No more condensation! 😁
Thankfully I can finally afford them. Replacing front and back doors too. Going to make a big difference.
The water condenses quickest from still air. The bottom panes are in a corner formed by the window sill, so the connection current bypasses that corner, leaving the air stiller.
It's probably colder, too, being at the bottom of the window, but the stillness is most important. We have Roman blinds which cover the top panes of the leaded lights when they're cinched up and there's just as much or more condensation behind the blinds as in the bottom. If we have the blinds down, there'll be condensation all over.
See also dew, frost and low-lying mist forming on still nights and not windy nights.
