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Last night was pretty chilly here in South East Queensland, Australia.

We woke this morning to condensation on the bedroom windows but noticed something curious.

Condensation on window

Where a small branch was touching the window there was no condensation. What caused this?

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The branch block outgoing infrared radiation and reflect some of it back to the window this will keep the temperature slightly above the condensation point.

Infrared radiation is the same as heat radiation.

Most objects will block and reflect infrared radiation and this is why dew does not form under a tree or under different types of roofs and objects partially covering the ground.

The CO2 in our atmosphere has a similar effect,absorbing and re emitting the infrared radiation keeping earth warm at night,had it not been for the different greenhouse gasses the temperature changes during the day night cycles had been a lot larger.

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    $\begingroup$ So effectively it is a form of insulation, right? $\endgroup$ – Steve Jul 16 at 5:15
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    $\begingroup$ yes in a way more like a mirror reflecting the heat from the object. $\endgroup$ – trond hansen Jul 16 at 5:17
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    $\begingroup$ The branch is insulating against conduction and convection - the branch itself will absorb most infrared radiation, causing it to warm up. It will not be a major reflector of IR radiation. $\endgroup$ – J... Jul 16 at 16:58
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    $\begingroup$ It shouldn't be too hard to find out (and post) about spectral properties of plants, however dry or healthy they may be ;-) $\endgroup$ – a_donda Jul 16 at 18:58
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The glass will be receiving heat from the inside and transferring it outside via several mechanisms, both radiant heat in the infrared and conduction/convection by its contact with the warm indoor air and cold outdoor air.

It's difficult to say for sure which loss mechanism is most important in this case, but the other answer only mentions radiation and as suggested in comments there it's hard to believe that the reduced motion of the air around the branch isn't contributing significantly.

It's also important to point out that lateral heat flow within the glass from the warmer to the colder areas of the glass is also important. If the window were made of the fictitious "transparent aluminum" with high thermal conductivity, it would be a uniform temperature. It's only because the conductivity of the glass is so low that it can support a thermal gradient.

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