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There was an open dump yard a few miles away from where I used to live for an internship. It was not noticeable during the daytime, but once the sun sets, the dump yard reminded us of its presence through its stinking odor. The smell came every evening and left the following day as if on a schedule, and my colleagues shared the same experience. One of my friends said it might be because of the temperature, but that didn't make sense. Smells, or the gases that cause those smells, travel faster at higher temperatures, which is why food has a more intense smell when served hot, as opposed to when taken out of a fridge. That would mean that we would get the smell during the day, and not during the night, which is the exact opposite of what is happening in reality. What might be the reason then, why we can smell the dump yard at night, and not during the day?

P.S: There is no activity going on in the dump yard, apart from people throwing garbage during random intervals throughout the day. It used to be a barren land, and at some point in history became the nucleation site of the garbage of an entire city.

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    $\begingroup$ I think it is because of the humidity increases at nights; cat's litter box is always the most "intensive" when the air is humid. $\endgroup$
    – hyportnex
    Commented Jul 1, 2023 at 12:31
  • $\begingroup$ The wind frequently changes direction at sundown. $\endgroup$
    – mmesser314
    Commented Jul 1, 2023 at 18:17

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This effect could be related to the missing vertical air movement at night. During the day, typically the sun heats up the soil (or any dump). This leads to evaporation of water (and to transpiration of the plants) that creates an upward flow of water vapour in air. It can be measured with eddy-covariance towers, for instance (see figure below).

The vertical flows are also "carrying" the smells away, in the upward direction. In case of your dump yard, you may try an airborne fly-by above the yard to get a good smell of this effect ;-)

Try the same at night, and you would smell less above the yard. Without solar energy input, there is no energy exchange between ground and air during the night, so the smell can diffuse horizontally instead, eventually passing by your house.

Here is a basic plot of the evaporation processes and heat flux exchange during the day and night time (from fao.org): enter image description here

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  • $\begingroup$ That makes sense! Why did you bring up the point about water vapour though? Does smell has anything to do with water vapour? $\endgroup$
    – AlphaLife
    Commented Jul 2, 2023 at 14:35
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    $\begingroup$ Yes, for two reasons: (1) bacteria and other molecules are often bound to water droplets. That's why warm summer rain smells funny. (2) The water vapour gradient leads to a general upward flow of all air which drags all molecules in the same direction. $\endgroup$
    – Martin
    Commented Jul 2, 2023 at 15:52
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    $\begingroup$ For the same reason, some people suffer more from pollen allergies by night than by day. $\endgroup$
    – gerrit
    Commented Jul 3, 2023 at 9:14

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