So, I noticed that in a closed plastic bottle (say only less than half full) little droplets of water were condensed on one side of the internal surface. That was the side exposed to sun rays.

Why should water condense were sun rays hit more directly?

I would expect that water would condense on the (sensibly) colder portion of the internal surface. I can see that water vapor inside the bottle maybe absorbs more energy from the sunrays and therefore tends to condense on the (colder) plastic surface.

But why on the side where sun hits more directly?

I think I observed the same with a 70 W lightbulb.

enter image description here

  • $\begingroup$ Could it be that it is condensing on the cold side as well, but that droplets on this side get big enough to slide back to the body of water, where the droplets on the hot side just grow to a certain point? $\endgroup$ Apr 9, 2019 at 22:09
  • $\begingroup$ No, I checked and the other side of the bottle was completely dry. Plus I observed for a while (so I would have had chances of catching a big drop on the cold side forming sliding down). $\endgroup$
    – AoZora
    Apr 9, 2019 at 22:15
  • $\begingroup$ What is the diameter of your bottle? $\endgroup$
    – Dlamini
    Apr 10, 2019 at 2:28
  • $\begingroup$ I remember a Q concerning a store window in which something counterintuictive happened, too. $\endgroup$
    – Alchimista
    Apr 10, 2019 at 11:14
  • 1
    $\begingroup$ This happens to me when I place a water bottle at a certain spot besides my computer. I have photos, may I edit your post to include them? You may delete them if you think they are not representative of your question. $\endgroup$ Apr 11, 2019 at 14:59

3 Answers 3


My theory is also about the focusing of the light, but is opposite your idea in many ways:

The bottle and water both transmit light fairly well, so I see no reason to presume that the side nearer the sun is the warmer side. I am guessing that the focus of the reflected light off the far side of the bottle is the most important thing here. That area will be about R/2 away from the side of the bottle AWAY from the sun, and I hypothesize that that is the warmest area and therefore the rising warm air side of a convection cell in the gas above the water. That leaves the cooler condensing gas to flow back down the side of the bottle nearer the sun.

  • $\begingroup$ Mmmm I did not think of reflection.. Anyways I'm thinking that maybe the bottle-induced focusing is more or less symmetric and what makes the real difference is that as soon as a small portion of water vapor condenses evenly around the bottle the micro droplets on the sun side start focusing for real in some points nearby themselves, resulting in a cascade effect $\endgroup$
    – AoZora
    Apr 10, 2019 at 21:36
  • $\begingroup$ That would be cool if true, but I am dubious. Small droplets would focus light on points in the gas very close to the wall. I don’t see how that strongly encourages further condensation in that area. $\endgroup$ Apr 11, 2019 at 11:26
  • $\begingroup$ that is actually the point: nearby the sun exposed wall is hotter, so vapor gets heated up there and consequently "criocaptured" by the wall right-away $\endgroup$
    – AoZora
    Apr 11, 2019 at 15:33

Up to now I came up with this possible explanation: the water vapor nearby the side directly exposed to the light rays is heated up more, and therefore condenses more on the colder plastic surface of the bottle.

I think the reason for this could be that the sunrays are more focused near the side they hit, due to the curvature of the plastic bottle which acts as a lens. This lensing effect could grow as the first (microscopic) droplets condense (maybe more or less evenly at this point) on the inside of the bottle due to the heating up of the vapor.


It would indeed appear that water heated on the sunny side of the bottle evaporates and rises. It does so at a rate grater than the non-sunny side.

The convection flow set up goes up on the sunny side and down on the opposite side. The air with vapour rising on the heated side soon meets the cool bottle air above and cools down. Condensate collects on the nearby bottle wall which remains relatively cool due to the influence of the liquid below and the unheated air in the bottle.

By the time the air flow descends on the non-sunny side, there is too little condensate left in it to result in non-sunny side bottle wall condensation.

  • $\begingroup$ mmm, sure convective motion will be part of the dynamics of the system. Even though we are still not very clear on the main reason why the sun-side of the bottle is hotter. It could be a lensing effect due to the initial condensation on the sun side wall, or maybe the fact that the liquid water on the sun side is heated up more and therefore the evaporation takes place mainly on the sun side.. Or a combination of the two.. Thanks for the input! $\endgroup$
    – AoZora
    Apr 11, 2019 at 10:39

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