I noticed than when I blow up on my spects it get blurr as water vapor condense onmy spects, but after some time it dissappear! How?

If water vapor at first place loose heat to my spects as my spects were cool , but it again vaporise by gaining heat from my spects!

So why at first place the condense on my lens if they can gain heat from.my lens to vaporise again?

and why only vapor come out if my mouth condense over my lens (spects) and NOT the vapor present in atmosphere condense over there

Also I observe that this phenomenon don't take place on hot surfaces so my another doubt is

do vapor also condense(irrespective of surface temperature) on hot surface once , but due to its temperature it SIMULTANEOUSLY again evaporate by absorbing heat without taking much time like in cold surface THEREFORE not becoming visible like blur on surface as condense water droplets immediately change into vapor within no time? OR something else happen?

Overall my doubt is

If water vapor condense on any surface say my spects for first time then why after some time the blurr cause due to condense vapor disappear I.e evaporate by taking surface heat?

And why not atmospheric moisture condense on my lens , why only vapor original from my mouth?

Why does condensation takes place (visible)between hot vapor and cool surface NOT between (hot vapor and hot surface)or (cool vapor and cool surface)?


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My glass window (that created by my mouth breath and after sometime it dissappear, why?)


1 Answer 1


That's incorrect: water vapour in the atmosphere does condense onto cold surfaces: it's called dew.

There are two things happening

  • water vapour is condensing on a cold surface

  • that water is evaporating from the surface

If the surface is cold, the condensation might be faster than the evaporation.

The warmer the air, the more water vapour it can hold. If you blow moisture-laden breath onto the surface, it cools and the excess vapour condenses onto the glass, perhaps faster than it can evaporate. When you stop blowing, the balance is restored.

It is to do with the dew point.

Another aspect, is that if you have just put on your glasses, the lenses will quickly warm up from the proximity to your face. So they might mist up when you put them on, but soon clear as the glass warms. This can be more noticeable when wearing a mask for Covid-19 protection, as more exhaled air might be directed around the edge of the mask towards the eyes.

And if you go outside wearing your glasses, they don't condense as readily as other outdoor surfaces, because they are warmer.


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