When a chemist speaks of "precipitation" it concerns a process - a process whereby a solid substance is separated from a liquid in such a way that it settles, or tends to settle, under the force of gravity.
But to a meteorologist precipitation is the product, not the process; he or she, like Mr Blotton's linguistically eccentric friend, might be described as "using the word only in a Pickwickian sense".
Meteorological precipitation is sometimes defined as "particles of ice or water formed within a cloud and falling towards the ground". This implies, strictly speaking, that precipitation does not actually have to reach the ground; it may evaporate on the way down. Indeed, well-defined streamers of precipitation which evaporate in this way can sometimes be seen under a distant shower cloud. They are known as virga.
We are very familiar with at least four types of precipitation: rain, drizzle, snow and hail. Rain and drizzle both consist of drops of liquid water and differ only in the dimensions of the drops. Droplets of drizzle are so fine that no splash occurs when they fall on a surface of water.
Snowflakes, as we know, are loose aggregates of ice crystals, while hail consists of small translucent pellets of ice, roughly spherical in shape, and usually in these parts around a quarter of an inch in diameter. They can, however, measure two inches or even more.
But there are other less familiar forms of precipitation. Ice-pellets are very similar to hail except that the little lumps of ice are transparent; they are composed of frozen raindrops or melted and refrozen snowflakes. Granular snow, on the other hand, sometimes known as graupel, is quite opaque and consists of very small grains of white ice, usually flat or elongated in shape. And the most exotic of all is diamond dust, associated with very low temperatures. It consists of small sparkling ice crystals in the form of needles or tiny plates, often so tiny that they appear to be suspended in the air.
In chemistry, some of the products of precipitation - called precipitates - fall very rapidly; others settle more slowly, and some take forever and a day to separate. And so it is in meteorology: large hailstones fall with a speed which is sometimes very dangerous, while drizzle-drops just gently float to earth. And then, to provide the required exception to prove any rule, dew drops - despite what the poets say - do not really fall at all: they form in situ, but are none the less included by most meteorological Pickwicks as a form of precipitation.