What's up with rain?
Rain is a type of weather in which water accumulates as a vapor, condenses, and then falls to earth. Many systems are totally reliant on rain, which makes sense, water being as valuable as it is combined with the abundance offered by rain and similar processes. This process does a great job moving water around the Earth, and that is much appreciated by many! But what's actually happening?
Humidity is simply a measure of how much water vapor is in the surrounding air - the amount of water that is vaporized in a body is dependent on: the energy levels of the water molecules, and the humidity level in the surrounding air - the evaporation of a molecule is positively related to the former and negatively related to the later - so if the energy is high enough / the surrounding humidity is low enough, the water molecule will evaporate. This is a similar process to sweating, and in fact, the reason that a fan blowing room temperature air will cool you when you are hot is because the fan's air will displace the humid air around your skin, replacing it with fresh air which is more conductive to evaporation/ sweat! Because of this relationship, the rate of evaporation is linked to a body's surface area exposed to air, not its volume.
So then the summary of evaporation is quite goofy - a liquid's molecules are constantly bouncing around at a high rate (comparatively faster than solids and slower than gasses), and some of these molecules are near the 'top' if you imagine a body of water, but generally a border between liquid and air. If that molecule has enough energy and bounces upwards, it will escape. This is just something that happens when water is exposed to air, and not at a slow rate! It's just that a molecule is really small (1017 molecules can be lain on a piece of letter paper, so ~1018 are on the surface of a full bathtub?), and so the rate of evaporation is hard to notice. Conveniently, the sun outputs quite a bit of energy, though, and so when a ray of sunshine falls on a body of water, the molecules in line of the ray are excited, meaning they move faster and are more prone to escaping in the form of water vapor.
Ok, so the water turns to vapor. Then what?
Oxygen has an atomic mass of 0, hydrogen 1, with nitrogen and carbon at 14 and 12 (the atomic mass is a dimensionless quantity, defined as the ratio of the mass of the subject vs an unimportant baseline). So, when the water evaporates, it is much lighter than the air around it which is full of nitrogen and carbon, among other things, and so it will rise - to aid it in this journey, the air around the point where the molecules evaporated will have heated up quite a bit, and will be similarly excited :D! This means that the air molecules around the water vapor will be moving faster and more, resulting in a lesser density, and so this will rise, too, taking the water vapors with it.
Now, as excited as we all are, when the water vapor reaches these new heights, it must cool down - for one, it is probably no longer under the influence of the sun, and the air around it is now much cooler - the water molecules slow down to a point where the vapor returns to a liquid (maybe even a solid, if cold enough). While it is a liquid, it actually does not drop and turn to rain at this point - this is because the mass- surface ratio barely beats out air resistance, and so the molecules fall at around 1cm/s!! So, you can effectivley model them as not falling while in this state, with lots of individual molecules of water being pushed around by the wind. When these molecules inevitably collide, they combine - so, a bunch of molecules pushed around, if they collide, they meet up - of course there will be groupings of them as time goes on, right? And so, as they build up, their mass increases faster than the surface area, and eventually you get to a rain droplet. That these often come from clouds is almost a logic necessity - the collisions would be much rarer without a dense area, and so most of the collisions happen in the dense area. When you look up at a cloud, you are seeing water in the middle of this journey - after it has evaporated (of course), then confessed, as evidenced by its being visible, and before the individual water droplets have grown large enough to fall to the ground (but more than large enough to effect light such that it looks white/ gray..).