There used to be a children's nursery rhyme which went:
Doctor Foster went to Gloucester
In a shower of rain;
He stepped in a puddle, up to his middle
And never went there again.
The verse, it is said, recalls a visit by Edward I of England to that famous city early in the 14th century, during a period of excessive and persistent rain. Apparently the king's horse became so deeply stuck in mud that planks had to be laid out on the street before the animal could regain its footing. Edward declared that he would never set foot in the town again - although why the tetchy monarch should be immortalised as Dr Foster in the rhyme remains, to me at any rate, a mystery.
Now nursery rhymes are often intended, it is said, to illustrate a moral, and in this instance the lesson may well be another version of the old saying "It never rains but it pours". Be that as it may for hoi polloi, however, regular readers of this column will be aware that meteorologists are fussy folk who would never allow a woolly word like "pours" to mar their ever-vigilant vocabulary - even when it comes to faith or morals. It never "pours" on weather people: the rain which wets them must be "heavy", "moderate" or "light".
They have, of course, reduced all these words to simple numbers and they have defined each rate of rainfall in bands of so-many millimetres wide. But, broadly speaking, moderate rain falls fast enough to form puddles rapidly, so we may reasonably conclude that it was moderate rain which the unfortunate Dr Foster encountered; light rain never results in puddles; and heavy rain is a downpour which makes a roaring noise on roofs and results in a fine spray as each drop splashes on the wet surface of a concrete road.
And then there is rain, as we distinguish it from drizzle. The distinction has nothing to do with the intensity of the precipitation; heavy drizzle may often be more wetting than a light rain. The difference between the two phenomena is based purely on the size of drops: drizzle drops typically range from a tenth of a millimetre to about 1 millimetre in diameter, while any water drop larger than 1mm is classified as rain. And, since even meteorologists do not go around with callipers to measure drop-size in their quest for accuracy, they have a rule of thumb: drizzle falls to the ground so gently that it will not disturb a water surface if it lands on it, but rain will cause a splash.