Why does rain drop and snow fall?
Your question might relate to the precipitation factors — how each form of moisture forms before dropping to earth — or it might refer to the difference in the effect of gravity on each form when it falls.
My understanding of the procedure of precipitation would indicate that the temperature of the air and related matters determine whether the moisture is frozen or not. As air currents move, temperature variations determine when and how, and to what extent, H2O gas collects and cools into a more solid form.
Freezing takes place is two formats, freezing rain (or sleet) and snow. Factors involved in the form that particular moisture will take has to do with the relationship of cool air masses to warm air masses, the atmospheric height of the moisture when it collects and cools, the speed of cooling and other technical factors that can be found in meteorological sources.
Concerning how the precipiatation falls, the shape as well as the weight of the individual pieces of cooled moisture as they precipitate determines how the individual particles of water, frozen or freezing water or snow fall.
The shape of individual bits of water are commonly called "drops." The verbal effect of drops falling is "dropping." But we don't actually say that rain drops (meaning to fall out of the sky. We say rain falls. But the general meaning of "drop" as a noun is still associated with "drop" as a verb. But we usually use the verb "drop" for solid things. "Oops, I dropped my lunch." "He dropped that brick on his toe."
But in other settings, another form of the word depicts the perception of how a single drop falls. Thus when water falls from a faucet a drop at a time we say it "drips." This is a form of the same word.
Drips and Drops
Incidentally, these vowel changes in the root of Germanic words for different grammatical meaning (drop-drip) is called umlaut. Various umlaut forms of the same word can take on a separate life. Thus we have two English verbs from this same root: drop, drip. "I dropped my pen." "The water is dripping." Idiomatically we also say "The faucet is dripping," or "My nose is dripping."
The faucet drips drops. Drops of water drip. But flakes of snow never drip. But then, flakes of snow can drift.
The observed behaviour of rain as it falls fits the event referred to by the common verb "fall." Rather than rain drops "dropping," we actually say rain falls. Since the "drops" are discrete bits of falling water, we call them rain drops, but we still say the rain falls. "Raindrops keep falling on my head." We don't say raindrops are dropping on my head.
Wafting and Drafting
Likewise, I believe you'll hear the same usage for frozen bits like sleet or freezing rain (which is a mix of frozen and unfrozen bits of H2O). As the snow "falls" it is a fluffy, irregularly shaped particle. The noun "drop" applies only to liquids.
Thus the action pictured by "dropping" is not appropriate for snow. The infinitely variable shape is referred to as a flake, due to the generally flat shape and individuality of the particles. Like a "flake" of something sliced off the larger part, like a flake of soap.
The effect of gravity on this is similar to that of a feather, which is caught by the air and wafted to and fro. So while a raindrop is concentrated and answers the downward call of gavity to fall straight through the air, a snowflake enjoys its meandering, liesurely trip from the troposhere (or somewhere up there) to its final random resting place below.
Upon arrival, each snowflake just nestles in among its mates, while waiting the arrival of more falling flakes. The snow flake, likewise, is caught by drafts of air, and thus gravity acts differently than upon a solid compact bit of water or frozen water. Think of a snowflake as a wind surfer that gradually comes to ground to rest.
Rainfall and Snowfall
Note that we do speak of rainfall and snowfall. "The rain is falling slowly now." "Snow has started falling." We conceive of rain falling and snow falling as well. Thus the effective verbal meaning covering the movement of rain under the effect of gravity is also "falling." We speak of rain failing. I don't think I have ever heard anyone speak of rain "dropping."
We don't say rain drops from the sky, but it falls. Note also that in the same way we do not say snow flakes from the sky, but the snow also falls. Thus the nouns for the physical form of the water in these cases, is not the same word as for the verb used in the action.
Thus we can say that both rain and snow fall. But only the rain is composed of drops. The snow is composed of flakes. We have rain drops but snow flakes. The terminology is directly expressive of the physical difference between the types of H2O involved.
Usages and Understandings
But keep in mind that all these usages, while related and understandable, are not based on any universal rule set somewhere out there in the universe. Language usages, forms and pronunciation arise totally out the exchanges of the community using the language. These forms and usages change due to the creativity and expressiveness of the individuals of those communities in their speaking and writing.
The final arbiter of meaning is usage. Often we can explain usages and see how they developed. But we cannot explain all usages, and we cannot predict just what new extensions or changes will occur to produce new usages!
Through Thick and Thin: The Role and the Rule of Usage
Core of the topic originally written in answer to an email query 15 November 2007
Expanded for this article and posted 15 November 2007.
Reviseded 3 December 2007
Copyright © 2007 Orville Boyd Jenkins
Permission granted for free download and transmission for personal or educational use. Other rights reserved.