Why does water condense on grass and iron grills in cold weather?

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.




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](https://i.stack.imgur.com/ALt09.jpg)](https://i.stack.imgur.com/ALt09.jpg)


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](https://en.wikipedia.org/wiki/Maxwell%E2%80%93Boltzmann_distribution)": 


[![enter image description here](https://i.stack.imgur.com/mdWe5.png)](https://i.stack.imgur.com/mdWe5.png)


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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