Or: Why are raindrops falling?

In vacuum a heavy (big mass) object is attracted by gravity with the same speed as a light object as shown on the moon with the hamer and the feather. But on earth there is no vacuum because of the atmosphere so heavy objects would fall faster because of Archimedes's principle.

But for air molecules it looks like the atmosphere isn't there so for them it is a vacuum. That would implie that fe an H2O molecule has an equal speed due to gravity than a much heavier O2 or N2 molecule.

Nevertheless H2O molecules are forming raindrops high in the sky. Rain is caused by evaporated H2O molecules. Because of their capacity of forming hydrogenbonds around aerosols (dust) a raindrop can evolve.

But how is it possible that H2O are higher in the sky and could form raindrop while according to the equality of affection by gravity their diffusion between N2 and O2 should be equal?

Is the 'volume' of those molecules also involved? Where N2 and O2 has a diameter of 120 pm as H2O as a diameter of about 180 pm, although you would expect that this difference would make the H2O molecules exactly because of that be more likely to be at the bottom because of their size and not higher in the sky?

  • $\begingroup$ the acceleration caused by gravity is the same regardless of whether there is a vacuum or not. Buoyancy is related to the density of an object, not simply the mass. A boat is much heavier than a pebble but the pebble sinks and the boat does not. What you mean by "for air molecules it looks like the atmosphere isn't there so for them it is a vacuum" is really unclear. It's like saying "if you are in Beijing, Beijing does not seem densely populated to you." $\endgroup$
    – JimmyJames
    Commented Jun 20, 2016 at 15:29
  • $\begingroup$ of course the acceleration is equal caused by gravity but the acceleration is different whether there is air pressure or not. A heavy stone is faster sinking in water than a lighter stone. $\endgroup$
    – Marijn
    Commented Jun 20, 2016 at 15:39
  • $\begingroup$ Do you mean 'denser' when you write 'heavier'? They aren't the same thing though they are often used interchangeably in casual conversation. You are oversimplifying things here. The acceleration due to gravity doesn't change depending on air or water pressure. What changes is the resistance of the medium to that acceleration. The shape of an object is really crucial. A skydiver can alter the rate of descent even though his weight and density do not changed. Ultimately it's about how fast the molecules of the medium are able to get around the object to let it fall. $\endgroup$
    – JimmyJames
    Commented Jun 20, 2016 at 15:51
  • $\begingroup$ Questions on Rain: physics.stackexchange.com/questions/65199/… and a nice answer here: earthscience.stackexchange.com/questions/5047/… $\endgroup$
    – userLTK
    Commented Jun 20, 2016 at 15:57

1 Answer 1


Gas molecules move very fast and tend to mix more than they tend to settle due to gravity and density. Similar to what happens inside any bottle of liquor. Alcohol is lighter than water but it doesn't float on top, it stays mixed in. The water and the Alcohol mix naturally in part due to their shape and in larger part, due to their charges. Gases aren't quite the same as what happens with water mixing with alcohol, but the effect is similar. Water tends to mix into the atmosphere more than it wants to float above it and the high speed of atmospheric molecules tends to keep the atmosphere well mixed.

The more important factor regarding water vapor in atmosphere is temperature. Water can both evaporate into air and condense out of it and it tends to do both at the same time to some degree, leaning towards an equilibrium based on the specific circumstances. As air gets warmer it can hold more water vapor. Surprisingly more. Every 1 degree c, the atmosphere can hold as much as 6% more water vapor, so, ballpark, every 11 or 12 degrees in temperature change doubles the amount of water the atmosphere can hold. A typical summer day, provided it's humid heat, not dry heat, the air you're breathing is over 2% water.


The other thing that happens to warm air is that it's lighter than the cold air above it. This lighter air wants to rise and as it rises it cools, and as it cools it can no longer hold all that water vapor, so the water vapor tends to form tiny drops of water or bits of ice, which begin to fall towards the earth, but quite slowly, and remember, they are falling in an updraft of rising warm air, which creates the effect of clouds appearing to hover in the sky, when it's usually a combination of slowly falling ice crystals (which fall about as fast as very light fluffy feathers), and an updraft.

Pictures of warm air rising added

enter image description here

enter image description here

Water vapor in the atmosphere is transparent. It can only be seen as clouds as it condenses out of the atmosphere. (Same with fog). The mass of the individual molecules isn't very important.

As to the size of the gas molecules, that's not important either, not in a gaseous state. The standard atmospheric formula, P1V1/K1=P2V2/K2 doesn't take into account molecular size. Now for liquid or gas, molecular size matters.

You also seem to have the wrong size for your molecules, maybe you took an individual Nitrogen atom. A water molecule is one of the smaller molecules, because hydrogen is so small and tightly bound to the Oxygen. It's about 2.75 angstroms. A Nitrogen Molecule (N2) is about 3 angstroms.

Water is also polar, which helps it stay liquid at higher temperatures because the lightly negative charge on the O tends to bind with the H molecules on neighboring water molecules. Nitrogen doesn't do that, so it only becomes a liquid at very cold temperatures and despite being a heavier molecule than water (by a fair bit, 28 to 18), liquid nitrogen is about 81% as dense as liquid water. It doesn't fit as tightly together. (see 2 pictures below). The 2nd one doesn't have Nitrogen, but Nitrogen is actually a slightly larger molecule than Oxygen, which as you see, Oxygen is slightly larger than water. Water is both smaller and it fits together more neatly, so it's surprisingly dense compared to what you might expect looking at it's atomic weight.


enter image description here

All that said, to your question, are lighter gas molecules affected by gravity, The answer, I believe is yes, but it's a very very minor factor. Wind, mixing and chemical interactions like evaporating and condensing are larger factors. Ozone, for example is quite a heavy molecule, but it's formed and broken down high in the atmosphere long before it can fall to the earth due to gravity. CO2 is a heavy gas but the atmosphere mixes enough that it has no problem providing plants high on mountains all the CO2 they need.

  • $\begingroup$ But why wants lighter air to rise? And I know that in comparisson with the other causes like diffusion gravity is a tiny factor. But the comparisson is not between gravity and other causes but between the gravity of different molecules. $\endgroup$
    – Marijn
    Commented Jun 20, 2016 at 15:43
  • $\begingroup$ Gravity on individual gas molecules is mostly irrelevant. Individual molecules bounce off each other like billiard balls in every direction thousands of times per second. The rate they "fall" isn't a significant factor. Warmer air wants to rise because it's lighter, colder air wants to fall cause it's heavier. It tends to move in circles. I've added a picture above. $\endgroup$
    – userLTK
    Commented Jun 20, 2016 at 15:51
  • $\begingroup$ Thanks for your extra information, but actionally the question remains why lighter air wants to rise and heavy cold air wants to fall? I don't understand why ligther airmolecules wants to rise? $\endgroup$
    – Marijn
    Commented Jun 20, 2016 at 17:36
  • 1
    $\begingroup$ @Marijn lighter air doesn't mean lighter molecules, it's just less dense air. $\endgroup$ Commented Jun 20, 2016 at 17:43
  • $\begingroup$ Ok, so in lighter air the space between the molecules is bigger and so less dense. But it still are the molecules (of the light air) who are rising. And why should those molecules rise more than molecules who are close to each other? $\endgroup$
    – Marijn
    Commented Jun 20, 2016 at 18:01

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