The Hydrologic Cycle

    Once precipitation occurs, it is essential to understand that the water does not disappear, or for the most part change into some new chemical. Rain water that falls to the ground will some day come back as precipitate again, or at least still continue to do its part in the hydrologic cycle. There are several processes that take place to ensure that water, in whichever form, is still maintained in just enough of liquid, gas, and solid. Most of the Earth's precipitation originated before condensation from the oceans. More water is stored in polar ice caps that is even present in all our rivers. streams, and lakes combined. 
    There are a few main processes that help get water around to different spots in the cycle to keep it going. Precipitation is the most noticeable one, and it represents the water molecules that are moving from the atmosphere to the surface in a liquid or solid state (falling out of the clouds). Water then is either Intercepted by plants for use, or it Infiltrates into the ground. That water does a lot more than cause a bunch of mud. Eventually, it is either stored as groundwater, used as soil moisture by the plants, or most noticeably, becomes the Runoff that swells rivers and streams. The runoff collects in either oceans and lakes, and in large part becomes the vat of water for any storm system to come by and draft into use. Ground-based water molecules, when the time is right, can re-enter the atmosphere in gaseous form through either Evaporation from the surface when water is heated by the sun or Transpiration from plants in their normal processes of photosynthesis. Any airborne water vapor would then represent our discussion thus far, whereby it will, under the right conditions, through Condensation, form clouds and eventually Precipitation, and we're back to our starting point.
    Though there may be times when the atmospheric conditions aren't right for forming clouds or precipitation, it is never too cold or warm for it to happen. And, for the large majority of the time, there is at least some water vapor present in the atmosphere overhead, regardless of cloud cover or precipitation. Forming clouds will not guarantee precipitation, so a little bit more than condensation is needed to get the ball rolling.

The Precipitation Process

    Let's begin by assuming relative humidities of 100% and condensation. A normal cloud droplet is only about 0.002 centimeters in diameter. At an equilibrium state, the size of the droplet will be fixed at a size dictated by the balance between evaporation and condensation onto that droplet. But because of the curvature effect, more water molecules are always needed to keep a rounded surface at equilibrium than a flat surface. Thus, the relative humidities for any droplet need to be above 100% (supersaturated). The larger the droplet, the less of a problem the curvature effect is, so raindrops for condensation purposes exist as if it were a flat surface. In supersaturated air, water droplets are allowed to grow by condensation, but the air is not often at a supersaturation level necessary for pure water to condense enough onto water. What helps is that good 'ole dirty air and that condensation nuclei. When we go back to that, we find out that not only is it great for condensation, but it can also dilute with the water and form a new compound that doesn't need as high a humidity for this process to take off (this is also known as the solute effect). Eventually, as more water is condensing onto these particles, enough of them combine to so dilute the previous compound that it behaves as if it were pure water, and at the same time it exists as a larger droplet. To go from a cloud droplet to a raindrop is quite a task, however. It takes about 1 million cloud droplets combining to make one typical raindrop (the difference in volume is about 1000 times). This would take days to accomplish, yet we can obviously observe that clouds can form and rain can show up in hours. There has got to be another way...Of course there is.
    At temperatures above freezing, water droplets contained within a cloud can move, and always do so. Collisions between these droplets provide much of what is necessary to get these droplets to form precipitation. Depending upon the nuclei first used and such, some droplets naturally get a head start at growing than others, and thus are a bit larger during this process. The larger the droplet is, the more attractive it becomes for collision. When a droplet collides with another in a cloud and merge, that is called coalescence. It is no surprise then that this process is called Collision-Coalescence. 
    One would think that as soon as a droplet is formed, it would immediately fall from the sky, together with any of the clouds that are up there as well. Water is quite heavy, and the amount of water in a typical cloud can reach well into the tons. It's a good thing that is is spread out quite thin in comparison to a bucket-full of water. It is enough such that each water droplet is treated individually within a cloud system with respect to gravity. Earlier, we stated that the atmosphere and all its molecules are in balance with respect to each other and gravity because of the hydrostatic equilibrium. That is, any effects of gravity are balanced by the pressure gradient force in the atmosphere. That doesn't hold true for everything though, as the effects of gravity were obviated much sooner in history than anything dealing with pressure. Cloud droplets themselves must adhere to gravity principles like everything else, and water droplets always fall, but there's a catch (isn't there always?).
    Any falling object interacts with air molecules that could also be moving with the environment, or otherwise content to be where they are out of that hydrostatic balance. Each time a droplet has to push all that stuff out of the way to keep falling, there's an exchange of force, and it works against the growing speed of the droplet. Eventually, there's a balance reached (remember, everything in this atmosphere is about balance), and it begins to fall at a constant speed. For a human, this constant speed in the atmosphere is about 150 miles per hour, for a water droplet, it's a measly 0.00017 miles per hour. Likewise rising air motions that we talked about earlier, even the slightest nudge, will propel a water droplet upward. In either case, don't expect these clouds or droplets to be coming down anytime soon.

    So we can see that the longer a droplet is in a cloud, and the more it's moving around, the more water it attracts, and the larger it grows. Usually a water droplet will ride the initial updraft of a particular cloud, attracting more water molecules and smaller droplets as it goes until its weight is too much for the cloud to sustain. I then falls to the Earth, picking up some last enlarging bits of water on its falling journey through the cloud. Recent studies have suggested that using the 'opposites attract' principle, individual water droplets with differing charges have an added advantage of coalescing when they collide. The main factors of rain production from a cloud are:
    1. Water content of the cloud
    2. Relative droplet size
    3. Electric charge
    4. Cloud thickness
    5. Strength of the cloud updraft

The Ice-Crystal Process

    There is another way of forming precipitation in a cloud, and actually on  that's pretty important for places that see cloud temperatures well below freezing (Read: Most of the Earth). Even on the hottest summer day a large cumulonimbus cloud stretches into parts of the atmosphere that are well below freezing. Just as it is difficult to see water immediately freeze to ice when the air temperature reaches 32ºF, so it is with water droplets in a cloud below freezing. In fact, it is not until you get to about -10ºF that you will begin to see ice crystals matching liquid water droplet concentrations. Water droplets that exist at temperatures below freezing in liquid form are said to be supercooled. Supercooled water droplets exist every day in our atmosphere.
    At temperatures of around -40ºF, you're pretty much down to just ice crystals, as water droplets at this temperature will pretty much freeze spontaneously. The process in some cases is pretty much the same, whereby there is a nuclei (in this case a frozen one), to which other droplets immediately freeze onto. Sometimes they aid condensing of water droplets and then freezing, and sometimes supercooled droplets immediately freeze upon collision. In our atmosphere it's just a little bit easier for water vapor to condense and grow onto ice crystals than water droplets, hence it is often that ice crystals grow at the expense of water droplets. Since it is easier to for water molecules to collide and grow in a frozen state, and since much of the mid and high latitudes contain clouds that are well below freezing, a lot of our precipitation, even in the summer, begins as snow. 

Precipitation Types

Here are the precipitation types that you would experience through atmospheric processes:
Rain:            Most common form of precipitation, it's actually the largest classification of a water droplet. Formed by any type of precipitation melting through a large warm layer, and then falling to the ground. 
Drizzle:         Smaller classification of a water droplet. Very persistent precipitation that usually forms from low clouds that aren't too thick. Often present with fog.
Snow:            Perhaps the most advanced form of precipitation, most of what we notice as precipitation actually begins as snow in the high altitudes of cold regions of clouds. Forms through accretion or deposition of ice crystals or supercooled water droplets. If the layer of cold air extends to the surface, snow will be seen when it finally heavy enough.
Ice Crystals: Cold temperature precipitation in situations where it is too cold to get any supercooled droplets around (really cold), or in situations where the cold air is present, but clouds are not thick enough to accumulate enough onto ice crystals to get snowflakes. Perhaps the drizzle form of snow.
Sleet:            Now we're getting into some complicated precipitation. Sleet occurs when there is a warm layer of air that's right above the ground and right below the cloud, and everywhere else it's cold enough to snow. Snowflakes that fall through this layer begin to melt, but then re-encounter colder air before reaching the ground, and thus show up as quite a mess of a snowflake. If you get enough of this to accumulate, it would have the consistency of a slush-puppy. 
Freezing Rain: This form of precipitation, imagining our scenario from the "sleet" example, has a much larger warm layer that a falling snowflake would encounter. This warm layer would change the snow all the way to a raindrop, but then would encounter the cold layer right before, and right down to impact. This would show up as falling rain, but ice accumulating on the ground as each raindrop would freeze on contact. This is quite a dangerous form of precipitation, because ice would accumulate every where, to everything, and make whatever it's attached to heavier and more brittle. Anyone who's been in an ice-storm can vouch for this.
Freezing Drizzle: The same as freezing rain, but with the smaller droplets. Perhaps more common than freezing rain because of the thinner cloud requirements and more persistent nature of drizzle. Even the thinnest coat of ice can be hazardous, so this is one to be wary of. It can even fall in temperatures below freezing throughout the atmosphere if the clouds aren't thick enough to actually sustain snowfall.
Ice Pellets:     Similar to sleet, but they actually resemble what a raindrop would look like frozen in the air. These little spheres of ice would resemble fallen Styrofoam balls. They usually show up with sleet or snow, and don't accumulate to much depths.
Graupel:         This form of precipitation is not readily observed at the surface, but occurs all the time within a cloud.  Graupel is the sloppy accumulation of ice crystals that stick together using supercooled water droplets (accretion) that freeze to them upon collision. A very messy type of precipitation, it is often broken apart during further colliding with other supercooled water droplets or ice crystals. In a way it's the building block for snowflakes.
Hail:                 Perhaps the most complexly developing form of precipitation, it usually falls more into the severe weather category rather than garden-variety precipitation. It forms in violent cyclical updrafts and downdrafts of thunderstorms. As precipitation is thrust upward and freezes, and downward to accumulate another liquid coating, it begins to grow in rings from the center like a tree. When it is finally too heavy to be sustained in an updraft, it plummets to the ground, denting whatever it hits. Not something you want to be outside during, because it is quite unpredictable what hail size you will see by the clouds, and sometimes smaller hail will fall with larger hail in the core of the "hail shaft".

Liquid Equivalents

    All precipitation can be measured. Usually it's as easy as sticking out a can with a ruler in it to "gauge" the amount of rain that has fallen. Measuring snow is as easy as sticking a ruler into the depth and measuring it. The amount of water that has precipitated from a cloud, however, can be comparable through any type of precipitation. You would have to take snowfall or other types of frozen precipitation and melt them down to liquid to compare to rainwater. You will find that the amount of water composing snowflakes changes by temperature, with a reading of 10" of snow for every inch of liquid water at 32ºF, with drier snow the colder it is. Wet snow, sleet, graupel, etc. have a much higher water content, to the point that sometimes it only takes about 3" or 4" of it to equate to 1" of liquid water. It is important to note this, because rainwater will immediately flow out to the lowest point by gravity, while snowfall and sleet, etc., will just stay where it is in a nice blanket. If you have 1" of liquid water spread out over the top of a roof (about 10" of snow), the roof is now supporting thousands of pounds of snow. Quite a thought, and more of a concern with those who have flat roofs with no drainage. Also, the liquid equivalent water in the snow or other frozen precipitation will tend to stay in the snow until it melts, becoming slushier and slushier as there is less snow there to dilute the presence of water by volume. That water can still freeze however, as those who wait till morning to shovel the walk can attest to the brick-like ice that is now cemented to the pavement.