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I took off my steamy glasses to be able to see and this question came up to my mind. Which is the physical reason for our eyes not fogging up like glasses do?

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You can fog up the surface of the cornea.

But in order to condense water on a surface the surface temperature has to at least reach the dew point temperature for a particular barometric pressure and relative humidity.

Typically if you are inside a building for example with air conditioning and walk out into warmer, humid air, glasses can fog since they can be of lower temperature, un-warmed by the body.

But since your body generally keeps the surface temperature of the cornea warm (I would guess between 30 to 35 degC), you would have to have very warm temperatures and highly humid conditions for that to happen. But the physics say it can happen. Good thing we have built in windshield wipers (eyelids).

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  • $\begingroup$ actually this would be a very dangerous situation, since your body cooling wouldnt work. Also condensation releases energy, increasing body temperature even more. So if your eyes get foggy: get out (or in), fast. $\endgroup$ – lalala Mar 2 '17 at 16:21
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    $\begingroup$ Can you fog the liquid surface though? I just don't see how it would do anything like that unless your eyes were dry. $\endgroup$ – JMac Mar 2 '17 at 16:31
  • $\begingroup$ In an air conditioned building your eyes indeed can get drier than normal. Natural tears have a surfactant that causes the tear to spread evenly over the cornea. Not sure, but flooding the surface with condensing water may dilute, reduce the effectiveness of the surfactant causing beads to form. But in any case water can condense on any solid or liquid surface if the partial pressure of water in the air reaches saturation. And that's determined by temperature. $\endgroup$ – docscience Mar 2 '17 at 18:27
  • $\begingroup$ I have heard of a way where it could happen, but I don't know how true it is. Fogged eyes would be the least of your worries. In the Sahara desert, it gets very hot. There are few bodies of water, but some exist. In rare conditions, hot air blowing over the surface can get saturated with water and yet remain hotter than a human body. If you are standing downwind, you cool the air when you breathe. Water condenses inside your lungs. You drown. As @lalala says, water would also condense on your skin and eyes, which would overheat you. And make it hard to see. $\endgroup$ – mmesser314 Mar 2 '17 at 19:45
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Eyes do fog up, if the environmental conditions are right.

But since you can clear the fogging by blinking, and blinking is a reflex action, you don't often notice that it is happening.

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  • $\begingroup$ If you stay with your eyes wide-open while staring at a plate of hot noodles with steam coming out of it, they still won't fog up (I've tried it myself), but your glasses will. $\endgroup$ – Tendero Mar 6 '17 at 0:05

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