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In my book it is said that in cold weather fog appears. This fog is nothing but water droplets. After a hot day if the night is cold then the moister in air starts to condense. What I do not understand is why does water condenses heavily on specific places like grass and iron objects only? I first thought that since iron is good conductor of heat so it becomes cold at night early which then captures nearby moisture more effectively than usual objects. But then grass is a poor conductor of heat. Another thought is that most of the objects absorb water. Those which do not absorb are metals and grass.

Please explain.

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  • $\begingroup$ Gravity takes colder air down. In the yard the "highest" thing to come in contact with the cold air is the grass blade. Also the grass cools faster than the dirt. $\endgroup$
    – MaxW
    Nov 5, 2015 at 17:12
  • $\begingroup$ @MaxW But then with the same logic colder air should come down on the concrete roofs of houses too -- but this doesn't happen. $\endgroup$
    – user31782
    Nov 5, 2015 at 17:15
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    $\begingroup$ A concrete roof has a lot of thermal mass. It doesn't cool off easily. $\endgroup$
    – MaxW
    Nov 5, 2015 at 17:29
  • $\begingroup$ In early morning good conductors will eventually lose heat and cool things like wood concrete will cause the condensation only because they are still cool. Hence the poor conductors like cement road should capture water droplets. $\endgroup$
    – user31782
    Nov 5, 2015 at 17:33
  • $\begingroup$ Well, in the bantering here I realize that there is a factor that you didn't consider. "Dew" does not necessarily come from settling "fog." The air can (usually?) becomes supersaturated with water, but lacks a sufficient nucleation site. The grass, iron grill or whatever provide such a site - if they are cold enough. $\endgroup$
    – MaxW
    Nov 5, 2015 at 17:43

2 Answers 2

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While grass IS a poor conductor of heat, it is super thin and will cool off really quickly because of of that alone. The increased surface area also means a greater area is exposed to the night air during the time when the moist air drops to the ground and will thus collect more dew than maybe a stone or a tree trunk.

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Temperature is directly related to the speed of particles. Colder temperature means that particles, on average, will be moving slower.

Slower means that any interaction of the particles (e.g. gaseous water molecules) can be prolonged in time and thus (in the case of water), hydrogen bonding is induced between molecules to form droplets. If it gets cold enough, the water molecules will be more intensely bonded (i.e. ice, solid phase water). Hydrogen bonding looks like this between water molecules:

enter image description here

The same interaction principle applies for interactions of water molecules with other stuff, like the molecules on grass blades and the metallic lattice on a grill. The electrostatic interactions aren't "stronger", there's just more probability of prolonged interaction and thus adhesion/cohesion because in colder environments the gas just moves slower. But -- the water has to get up in the air in the first place, which is why what you observe typically happens after a hot day (moving particles faster in the air) cools down substantially.

This is a principle described by Boltzmann called "Boltzmann's distibution of velocities":

enter image description here

Here you can consider a=1 cold, a=2 warmer, and a=5 hot. The x-axis is for velocities and the y-axis is for probabilities of respective velocities.

Metallic surfaces are favorable because they have high conductance -- i.e. susceptibility to electrostatic interaction (compared to, e.g. the paint on the walls outside). Grass blades "attract" the water mostly because, simply, many water molecules cohesively bind to make droplets, which have enough mass to be effected by earth's gravitational pull, and fall/collect to whatever is on the ground. There are also biochemical mechanisms that help certain grasses pull in water from the air -- I think.

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  • $\begingroup$ As an aside -- this is why the speed of sound is less in cold weather -- the gas molecules are what "carry" the sound waves, and if they move slightly slower, so does sound. No matter, no sound. $\endgroup$
    – khaverim
    Mar 5, 2016 at 3:34
  • $\begingroup$ 1. What kind of electrostatic interactions happen on metallic surface? Is it something like Hydrogen spillover where hydrogen atoms make electrostatic bonds with the metal surface? 2. Why doesn't water droplets, which are already cohesively bound and are pulled by gravity, fall over cement surface like road? $\endgroup$
    – user31782
    Mar 5, 2016 at 5:51
  • $\begingroup$ Coulombic interactions are the primary cause of atomic attraction -- the negative electronic charge and positive nuclear charges attract, as a principle. Spillover involves bond breaking/formation -- the interactions we're talking about are generally weaker. Why grass is "favored" over a roadway can be answered practically -- cars drive on it and move the molecules before they can collect/settle -- and chemically -- the interactions of grass/water are simply stronger than cement/water because of chemical properties. $\endgroup$
    – khaverim
    Mar 5, 2016 at 5:56
  • $\begingroup$ "cars drive on it and move the molecules before they can collect/settle" -- I don't think that's the reason. Because in early morning there is no dew on my house's roof(cement). Secondly what kind of chemical properties of a substance decide its surface's water holding capability? Does cement absorb water and grass doesn't? $\endgroup$
    – user31782
    Mar 5, 2016 at 6:01
  • $\begingroup$ Porosity of cement can be a partial explanation -- the bottom line here is that there are many cooperative and dynamic forces at work, the fundamental, quantitative answer to your question lies in understanding of quantum mechanics; i.e. the physical movements and distribution of particles at the atomic level. The most simplistic answer is that not all atoms/molecules interact in the same way -- some interactions are stronger than others. You seem curious -- put a few grams of salt on the ground outside and look at it the next day -- NaCl interacts with water very differently than cement. $\endgroup$
    – khaverim
    Mar 5, 2016 at 6:08

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