Any conscientious meteorologist will readily distinguish for you between an umbrella weather word and a portmanteau term. In fact he or she will do it in the course of work a dozen times a day, but sometimes the subtle nuances are lost on hoi largely uncaring and often ignorant polloi. Let Weather Eye explain.
"Precipitation", for example, is a typical umbrella word. It "covers", as it were, a long and potentially tedious list of quite separate meteorological eventualities, each dependent on a special combination of temperature, altitude, or time of day. The phenomena range from the banal to the sublimely esoteric.
We are all 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. Snow-flakes, of course, are loose aggregates of ice crystals, while hail consists of small translucent pellets of ice, roughly spherical in shape, and usually 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. Icepellets are very similar to hail, except that the little lumps of ice are transparent; they are composed of frozen raindrops, or melted and re-frozen 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. 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.
To understand a portmanteau term, on the other hand, it is necessary to recall the little rhyme devised by HumptyDumpty in Alice Through the Looking Glass:
Twas brillig and the slithy toves Did gyre and gimble in the wabe . . . When Alice questions him on "slithy", Humpty explains it as a mixture of the two words "lithe" and "slimy". "You see it's like a portmanteau," he elaborates. "There are two meanings packed into one word."
Two examples come to mind from meteorology. The familiar "smog" is a combination of the two words "smoke" and "fog". Less widely know is the term "mizzle", a meteorological portmanteau holding "mist" and "drizzle" which aptly describes the kind of gentle precipitation wrung from a moist south-westerly flow of air in Kerry, Cork or Donegal.