# Condensation Question

Can somebody please explain to me how come condensation on a surface stops near (but not at) the edges? I’ve noticed this on mirrors in the bathroom before but it was the following example that really made me think…

At my local train station there are outdoor bike lockers. These have metal doors with a picture of a bike created by a series of holes punched in the door (are you picturing this?). The thing that really got my attention was that last night (cold weather) the whole area of the bike ‘picture’ was condensation free with a good inch wide border around it, while the rest of the door was uniformly covered with droplets.

Bonus question: Somebody must have asked this before, tell me how to use the search function without trailing through hundreds of results. Is there a secret i’m missing?

Often, the frame (particularly a metal one) will have a higher thermal conductivity than the glass (especially double-paned) it contains. As such, heat from inside the building will flow through the frame and slightly warm the parts of the glass in contact with it. There is also the matter of specific heat capacity; glass is fairly low compared to many other materials which means as the ambient air cools off, glass typically retains less of its latent heat than the materials of the frame so the frame will stay warmer for a longer period of time than the glass will. Again, the frame will serve to slightly warm up the glass in contact with it.

I don’t doubt the accuracy or relevance of the above, however most of the mirrors i have seen in bathrooms don’t have frames. At most they have decorative or simple bevelled edges and nothing else, many are just plain edges.

And of course it doesn’t explain the bike picture in the OP, clearly it is something to do with it simply being the edge of the material itself, nothing to do with other materials in contact with it, especially as the bike picture example wasn’t even an exterior edge but rather an interior one.

The reason should be clear – the edges are warmer than the rest of the surface.

I’m going to take a guess that this happens when the primary mode of heat loss from the surface is radiant heat transfer. For the majority of the surface area, the direction of radiant heat loss (the surface exposure to a colder body) is similar. At an edge, however, the surface of the adjacent side (the thin edge of the material) the exposure is different because it is facing a different direction and this difference in heat transfer results in the edge being warmer.

That’s possible, but as Q.E.D. suggests, there could also be other explanations with convection and conduction. If you were setting up the equations to model this system, you would need to consider that the edge represents a boundary condition and you wouldn’t be surprised to see something like a warmer or colder area as a result.

Temperature is probably the key. In the case of the mirror, if the frameless edge of the mirror extends beyond the casing for the medicine cabinet, it will have a very different response time to temperature changes than the center of the mirror. In the door example, if the temperature behind the doors is higher than the temperature outside the doors, the holes will increase surface area and airflow, increasing the temperature in that section of the door.

The holes will speed air flow, so as water collects it also has a chance to dissipate near the holes.

Also, there is radiant heat energy (far IR) from the nearby walls acting to warm up the edges very slightly, perhaps enough to keep condensation from forming there. I’ve never encountered a frameless mirror in a bathroom, as far as I can recollect, so I’ll have to take your word that condensation doesn’t occur at the edges.